Journal of Petrology

Journal of Petrology Advance Access originally published online on February 21, 2006
Journal of Petrology 2006 47(5):929-964; doi:10.1093/petrology/egi101

This Article Abstract FREE Full Text (PDF) All Versions of this Article: 47/5/929    most recent egi101v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (14) Request Permissions Google Scholar Articles by USUI, T. Articles by HELMSTAEDT, H. Search for Related Content GeoRef GeoRef Citation Social Bookmarking          What's this?

© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Petrology and Geochemistry of Eclogite Xenoliths from the Colorado Plateau: Implications for the Evolution of Subducted Oceanic Crust

TOMOHIRO USUI^1,*, EIZO NAKAMURA¹ and HERWART HELMSTAEDT²

¹ PHEASANT MEMORIAL LABORATORY FOR GEOCHEMISTRY AND COSMOCHEMISTRY, INSTITUTE FOR STUDY OF THE EARTH'S INTERIOR, OKAYAMA UNIVERSITY AT MISASA, TOTTORI, 682-0193, JAPAN
² DEPARTMENT OF GEOLOGICAL SCIENCES AND GEOLOGICAL ENGINEERING, QUEEN'S UNIVERSITY, KINGSTON, ONT., K7L 3N6, CANADA

RECEIVED DECEMBER 9, 2004; ACCEPTED DECEMBER 19, 2005

	ABSTRACT

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS APPENDIX REFERENCES

 
Eclogite xenoliths from the Colorado Plateau, interpreted as fragments of the subducted Farallon plate, are used to constrain the trace element and Sr–Nd–Pb isotopic compositions of oceanic crust subducted into the upper mantle. The xenoliths consist of almandine-rich garnet, Na-clinopyroxene, lawsonite and zoisite with minor amounts of phengite, rutile, pyrite and zircon. They have essentially basaltic bulk-rock major element compositions; their Na₂O contents are significantly elevated, but K₂O contents are similar to those of unaltered mid-ocean ridge basalt (MORB). These alkali element characteristics are explained by spilitization or albitization processes on the sea floor and during subduction-zone metasomatism in the fore-arc region. The whole-rock trace element abundances of the xenoliths are variable relative to sea-floor-altered MORB, except for the restricted Zr/Hf ratios (36·9–37·6). Whole-rock mass balances for two Colorado Plateau eclogite xenoliths are examined for 22 trace elements, Rb, Cs, Sr, Ba, Y, rare earth elements, Pb, Th and U. Mass balance considerations and mineralogical observations indicate that the whole-rock chemistries of the xenoliths were modified by near-surface processes after emplacement and limited interaction with their host rock, a serpentinized ultramafic microbreccia. To avoid these secondary effects, the Sr, Nd and Pb isotopic compositions of minerals separated from the xenoliths were measured, yielding 0·70453–0·70590 for ⁸⁷Sr/⁸⁶Sr, –3·1 to 0·5 for Nd and 18·928–19·063 for ²⁰⁶Pb/²⁰⁴Pb. These isotopic compositions are distinctly more radiogenic for Sr and Pb and less radiogenic for Nd than those of altered MORB. Our results suggest that the MORB-like protolith of the xenoliths was metasomatized by a fluid equilibrated with sediment in the fore-arc region of a subduction zone and that this metasomatic fluid produced continental crust-like isotopic compositions of the xenoliths.

KEY WORDS: Colorado Plateau; eclogite xenolith; geochemistry; subducted oceanic crust

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS APPENDIX REFERENCES

 
Material recycling through subduction zones is one of the most important processes controlling the chemical evolution of the Earth, and materials from the Earth's surface are likely to have been reintroduced into its interior throughout geological history. Although most of the subducting plate is made of mantle material returning to depths from which it originated, hydrated and altered upper oceanic crust, as well as overlying sediments, carry a record of low-temperature interactions with continents, oceans and the atmosphere into the deep mantle. Subduction of surface and near-surface components can change the volatile contents, trace element abundances, amounts of heat-producing elements and radiogenic isotope systematics in the mantle.

A number of geochemical models have been developed to estimate the integrated trace element and isotopic composition of a hypothetical slab-derived ‘component’ in arc and ocean island basalt (OIB) lavas (e.g. Zindler & Hart, 1986; McCulloch & Gamble, 1991; Ishikawa & Nakamura, 1994; Hofmann, 1997; Taylor & Nesbitt, 1998; Stracke et al., 2003). However, contributions from individual sources are difficult to assess from lava compositions alone, because element fractionations during slab dehydration and mass fluxes from the various subducted ‘lithologies’ are poorly constrained. Nevertheless, most models have used a geochemical ‘component’ estimated from rock compositions prior to subduction (e.g. samples from ocean-floor drilling). Therefore, understanding the trace element and isotopic compositions of individual rock types subducted into the sub-arc mantle is indispensable for constraining geochemical models for subduction-zone material recycling.

One approach to constrain material cycling involves high-pressure experimental studies that simulate processes such as dehydration of the subducting slab under controlled conditions (e.g. Schmidt & Poli, 1998; Okamoto & Maruyama, 1999; Poli & Schmidt, 2002). Such experiments provide important constraints on the water contents and metamorphic phase relations in the subducting slab as a function of temperature and pressure. Other experiments have investigated the solubility of elements in fluids at high pressures, as well as element partitioning between the fluids and the constituent minerals in potential slab materials such as eclogites, blueschists and serpentinites (e.g. Brenan et al., 1994, 1995, 1998; Iizuka & Nakamura, 1995; Ayers et al., 1997; Kogiso et al., 1997; Stalder et al., 1998). However, microanalytical and mass balance studies of subduction-related metamorphic rocks have shown that many so-called trace elements are preferentially partitioned into accessory minerals such as allanite, rutile, apatite, and zircon, and that elemental partitioning commonly does not reach equilibrium because of the relatively low-temperature environment envisaged in the subducting oceanic lithosphere (e.g. Sorensen & Grossman, 1989, 1993; Sorensen, 1991; Tribuzio et al., 1996; Zack et al., 2002a, 2002b; Spandler et al., 2003). This suggests that partition coefficient data for trace elements may not be appropriate for evaluating trace element behaviors in subducting oceanic crust.

Thermomechanical studies indicate that subducting oceanic crust in cold subduction zones passes through the blueschist to lawsonite eclogite transition, and that the most important dehydration process for inducing island arc volcanism occurs in the lawsonite eclogite facies (Peacock, 1993). Thus, lawsonite eclogites are critical to investigating trace element and isotopic behavior in subducted oceanic crust. However, data for trace element distributions among the mineral assemblages in the lawsonite eclogite facies are limited, because lawsonite eclogite terranes are rare (Zack et al., 2004), and commonly much overprinted by high-T/P retrogression.

Lawsonite-bearing eclogite xenoliths from the Colorado Plateau of the southwestern USA present an important opportunity to study the trace element and isotopic systematics of this lithology. Helmstaedt & Doig (1975) showed that these xenoliths resemble eclogites of the Franciscan Complex of the California Coast Ranges, western USA, in many aspects of their petrography and geochemistry. The Franciscan Complex represents fragments of a fossil subduction-zone plate boundary from the Jurassic to the Paleogene (e.g. Ernst, 1970; Maruyama & Liou, 1989). Offshore magnetic anomalies demonstrate that thousands of kilometers of oceanic crust have been subducted beneath the western US continental margin since the Late Cretaceous (Hamilton, 1969). Usui et al. (2003) used ion microprobe techniques to determine the U–Pb ages of zircons from the Colorado Plateau eclogite xenoliths, which yielded concordant ages from 81 to 33 Ma. Those workers interpreted the two extremes in age to reflect zircon crystallization during subduction-related metamorphism and zircon recrystallization during intrusion of the host rock. Combining geotectonic reasoning with the zircon geochronology and petrographic similarities to subduction-related eclogites led Usui et al. (2003) to conclude that the Colorado Plateau eclogite xenoliths originated as fragments of the subducted Farallon plate that had resided in the upper mantle since the Late Cretaceous. Owing to their rapid transport to the surface, the eclogite xenoliths show few effects of retrograde overprinting compared with crustal eclogites exhumed by tectonic processes. Thus, they may preserve important chemical information about their subduction-related metamorphic history.

Two problematic issues remain concerning the origin of the Colorado Plateau eclogite xenoliths. One is whether they represent fragments of relatively young Phanerozoic subducted oceanic crust (Helmstaedt & Doig, 1975; Usui et al., 2003) or remnants of the much older, Proterozoic basement of the Colorado Plateau (Roden et al., 1990; Wendlandt et al., 1993; Smith et al., 2004). The contrasting hypotheses for their ages may be considered to result, in part, from the application of different geochronological methods to date them. U–Pb zircon ages by ion microprobe analysis have yielded Phanerozoic ages (Usui et al., 2003), whereas Sm–Nd isotopic data (Roden et al., 1990; Wendlandt et al., 1993) give Proterozoic model ages in the range of 1500–1800 Ma. In addition, U–Pb analyses of multigrain zircon fragments by the isotope dilution method with thermal ionization mass spectrometry (ID-TIMS) resulted in a linear discordant line that intersected the concordia at 1514 Ma (Smith et al., 2004).

The other debated issue is whether the Colorado Plateau eclogite xenoliths were formed by prograde metamorphism during subduction of oceanic lithosphere or by magmatic recrystallization processes in the upper mantle. Based on mineral textures and compositional zoning of garnet and clinopyroxene, Helmstaedt & Schulze (1988, 1991) concluded the former. They estimated two metamorphic temperatures for the eclogite xenoliths, corresponding to crystallization of garnet cores and peak metamorphic conditions, respectively, and concluded that the garnet rims formed at 100°C higher temperatures than the garnet cores. However, these estimates are unlikely to be conclusive because: (1) they depend on calculating the Fe²⁺/Fe³⁺ of clinopyroxene, a parameter that cannot be determined by electron microprobe; (2) it is difficult to determine the clinopyroxene core–garnet core temperature by using clinopyroxene with an oscillatory zoned structure. Smith & Zientek (1979) suggested that the oscillatory zoning of clinopyroxene may represent disequilibrium growth in the presence of fluid phases, and that the Colorado Plateau eclogite xenoliths formed coexisting with a fluid phase during cooling and metasomatism of basaltic dikes in a cool upper mantle.

In this paper, we report whole-rock and mineral data for major, minor, and trace elements, and Sr, Nd, and Pb isotopic data for a suite of Colorado Plateau eclogite xenoliths. The geochemical data are complemented by observations of the modes of constituent minerals, their textures and chemical zoning structures. We reassess the contrasting hypotheses for the origin of the Colorado Plateau eclogite xenoliths and demonstrate that these rocks can be used as proxies to understand the geochemical evolution of subducted oceanic crust. These data allow us to develop a model for the trace element and isotopic (Sr, Nd and Pb) composition of subducted oceanic crust in the sub-arc region.

	GEOLOGICAL SETTING

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS APPENDIX REFERENCES

 
The Colorado Plateau is a broadly elevated region, 1500 m above sea level, in the southwestern interior of the USA. Unmetamorphosed and little deformed Paleozoic to Cenozoic sedimentary rocks are well exposed on Precambrian basement (1·4–1·8 Ga; Condie, 1982). The plateau is surrounded by the Basin and Range, Rocky Mountains, and the Rio Grande Rift tectonic provinces, all of which have experienced intense Cenozoic orogenic activity and extensional tectonics (Fig. 1a). The structure of the Colorado Plateau consists of broad basins, uplifts and platforms, which are locally bordered by monoclines. Tectonic reconstruction of the history of the western margin of North America suggests that during the Late Cretaceous and Early Tertiary, the Farallon plate was subducting eastward (Atwater, 1970). The Late Cretaceous and Tertiary records of arc magmatism in the southwestern USA constrain the slab geometry and its evolution, suggesting that the migration of arc magmatism was probably caused by progressive flattening of a subducting slab (Coney & Reynolds, 1977). Intra-plate magmatism, which occurred extensively around the Colorado Plateau, may be associated with the formation of a slab window and/or slab tearing (Dickinson, 1997).

View larger version (17K): [in this window] [in a new window]   Fig. 1. (a) Sketch map of the southwestern interior of the USA and outline of the Colorado Plateau (dotted line). (b) Sketch map of the Navajo Volcanic Field in the Colorado Plateau near the Four Corners shown as the gray shaded area in (a), modified after Smith & Levy (1976). •, minettes; , serpentinized ultramafic microbreccia diatremes. Major monoclines with dip directions are also shown. ME, Mule Ear; MR, Moses Rock; CN, Cane Valley; GR, Garnet Ridge; RM, Red Mesa; BP, Buell Park; GK, Green Knobs; SF, San Francisco; LA, Los Angeles.

 
Eight diatremes of serpentinized ultramafic microbreccia and numerous minette diatremes, the products of intra-plate magmatism, are exposed in the Navajo Volcanic Field of the Colorado Plateau (Fig. 1b). The microbreccia diatremes occur as two structurally controlled clusters (McGetchin et al., 1977). One, related to the Comb Ridge monocline on the eastern margin of Monument uplift, includes the Mule Ear, Moses Rock, Cane Valley, and Garnet Ridge diatremes. The other, at the eastern margin of the Defiance uplift, includes the Buell Park and Green Knobs diatremes. A single diatreme, Red Mesa, lies between the two fields and is located about 30 km east of the Comb Ridge monocline.

The diatreme rocks that host the eclogite xenoliths lack primary igneous textures and consist of finely crushed fragments of strongly serpentinized ultramafic rocks. Although initially described as kimberlites (e.g. McGetchin et al., 1977), these rocks are not ‘true’ kimberlites (e.g. Clement et al., 1984) but serpentinized ultramafic microbreccias (SUM; Roden, 1981). The microbreccia diatremes were emplaced between 25 and 35 Ma (Naeser, 1971; Helmstaedt & Doig, 1975; Roden et al., 1979), contemporaneous with the numerous minette diatremes of the Navajo Volcanic Field (Ehrenberg, 1982). The diatremes contain several types of eclogite, as well as a broad spectrum of possible upper mantle rocks that include variously hydrated spinel- and garnet-bearing peridotites, pyroxenites and serpentinites. The eclogite xenoliths are found mainly in three microbreccia diatremes in the Comb Ridge cluster (Garnet Ridge, Moses Rock, Mule Ear), and are absent from the Green Knobs and Buell Park pipes near the Defiance uplift (McGetchin et al., 1977; Selverstone et al., 1999). The samples used for this study were collected from the Moses Rock and Garnet Ridge diatremes.

	ANALYTICAL METHODS

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS APPENDIX REFERENCES

 
Major element compositions of minerals were determined using a JEOL JXA-8800 electron microprobe at ISEI (Institute for Study of the Earth's Interior). Analyses were carried out at 15 kV and 20 nA sample current for all silicate minerals. Counting times of 30 s were used, and matrix corrections were performed by ZAF methods. The standards are synthetic oxides and natural minerals. The modal abundance of zircon was determined by digital image analysis of 25 randomly selected back-scattered electron images (each 1000 µm) per sample.

Trace element analyses of garnet, clinopyroxene, lawsonite and zoisite were performed using a CAMECA ims-5f ion microprobe at the PML (Pheasant Memorial Laboratory) in the ISEI, using techniques described by Nakamura & Kushiro (1998). Clinopyroxene from mantle xenoliths and basaltic natural glass were used as standard minerals for trace element calibrations. Trace element concentrations in these standards were chemically determined by inductively coupled plasma mass spectrometry (ICP-MS; Makishima & Nakamura, 1997). The homogeneity of these standards was confirmed by optical microscope, electron microprobe and ion microprobe analysis. Trace element contents of garnet and clinopyroxene were calibrated on the basis of the clinopyroxene standard, and those of lawsonite and zoisite were calibrated using the basaltic glass standards. Minerals in thin sections were sputtered with an O^– primary beam of 10–20 nA intensity, resulting in 10–20 µm beam diameter. Clinopyroxene and lawsonite inclusions <30 µm in diameter were measured by an O^– primary beam of 6 nA intensity, which produced a beam <10 µm in diameter. Positive secondary ions were collected by ion counting using an energy offset of –60 V from 4500 V acceleration with an energy bandpass of ±10 V. These operating conditions resulted in (1–1·5) x 10⁵ c.p.s. for ³⁰Si secondary ions. The secondary ion intensities of atomic masses of interest were normalized to ³⁰Si. For each analysis spot, 15 trace elements were determined in a run that took 40 min, including presputtering. The analytical reproducibility (relative standard deviation % for n = 10) for trace element measurements is generally <10% for the clinopyroxene standard and <5% for the basaltic glass standard, except for Er and Lu (7–10%).

To prepare whole-rock powders for analysis, eclogite samples were broken into chips 2 mm in diameter. Fresh pieces were hand-picked and washed with deionized water in an ultrasonic bath for 30 min. After drying at 60°C overnight, the chips were pulverized using a silicon nitride triturator to a grain size <400 mesh. To separate minerals, rock powders obtained using the silicon nitride triturator were sieved to capture grain sizes between 80 and 100 mesh (an average grain size of 200 µm). This sieved fraction was processed with an isodynamic separator, and the resulting mineral fractions were hand-picked.

Whole-rock major element compositions, along with Cr and Ni, were determined by X-ray fluorescence (XRF) spectrometry at the PML, on glass beads fluxed from mixing 100 mg of powdered sample and 5 g of lithium tetraborate (Takei, 2002). Trace element compositions were measured by ICP-MS at the PML, based on the methods of Makishima & Nakamura (1997) for Y, Cs, Ba, rare earth elements (REE), Pb, Th and U, those of Makishima et al. (1999) for Zr, Hf and Nb, and of Moriguti et al. (2004) for Li. Powdered samples (20 mg) for ICP-MS analysis were decomposed in 1·0 ml of HF and 0·1 ml of HNO₃ in Teflon bombs at 210°C for 2 days, because acid-resistant minerals such as zircon and rutile are present in the eclogite samples. Analytical errors for trace element were generally 3% for Zr, Hf, and Li, and 5% for other elements.

The analytical procedures for mass spectrometry followed Yoshikawa & Nakamura (1993) for Sr isotopic ratios and abundances of Rb and Sr, Makishima & Nakamura (1991) for Nd isotopic ratios and abundances of Sm and Nd, Kuritani & Nakamura (2002, 2003) for Pb isotopic ratios and Pb abundances, and Yokoyama et al. (1999, 2003) for abundances of U and Th, employing TIMS in static multi-collection mode [modified Finnigan MAT261: ‘Kiji’ (Nakano & Nakamura, 1998), MAT262: ‘Taro’ and MAT262: ‘INU’]. All of these elements (Rb, Sr, Sm, Nd, U, Th and Pb) were successively separated from each of the whole-rock and mineral samples, using a multi-ion exchange column chemistry approach modified after Nakamura et al. (2002).

Basaltic standard JB3 from the Geological Survey of Japan (GSJ) yields typical analytical reproducibility of 0·005%, 0·005% and 0·02% for Sr, Nd and Pb isotopic ratios, respectively, and <1% for Rb, Sr, Sm, Nd, U, Th and Pb abundances. Accuracies of the isotopic and concentration analyses were confirmed by repeated measurement of JB3. We obtained data for this standard consistent with the previously published data within the analytical reproducibility (see Appendix, Table A1). Isotopic fractionation during analysis was corrected using ⁸⁶Sr/⁸⁸Sr = 0·1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0·7219 as normalizing factors. Pb isotopic fractionation was corrected by the ‘two-double spikes method’ with ²⁰⁵Pb–²⁰⁴Pb and ²⁰⁷Pb–²⁰⁴Pb enriched spikes following the technique of Kuritani & Nakamura (2003). The composition of the spike was calibrated by assuming that the ²⁰⁸Pb/²⁰⁶Pb ratio of NBS982 is 1·00016 (Catanzaro et al., 1968). Instrumental mass discrimination of TIMS analyses was corrected by the following values of standards: ⁸⁷Sr/⁸⁶Sr = 0·71024 (NIST987), ¹⁴³Nd/¹⁴⁴Nd = 0·511839 (La Jolla), and ²⁰⁶Pb/²⁰⁴Pb = 16·9424, ²⁰⁷Pb/²⁰⁴Pb = 15·5003, ²⁰⁸Pb/²⁰⁴Pb = 36·7266 (NBS981).

View this table: [in this window] [in a new window]   Table 1: Mineral assemblages of Colorado Plateau xenoliths

 

	RESULTS

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS APPENDIX REFERENCES

 
Mineral assemblage and classification of eclogite xenoliths
The Colorado Plateau eclogite xenoliths investigated in this study were previously described and referred to as metabasic eclogite and jadeite-clinopyroxenite, based not only on their mineral assemblages and basaltic bulk compositions, but also on their textural similarity to crustal examples of eclogites (Helmstaedt & Schulze, 1988). In this paper, ‘jadeitic’-clinopyroxenite is used instead of jadeite-clinopyroxenite, because clinopyroxene in this xenolith type is not pure jadeite, but omphacite or impure jadeite as described later. The mineral assemblages of the xenoliths and their modal proportions are given in Tables 1 and 2.

View this table: [in this window] [in a new window]   Table 2: Modal proportions (vol. %) in the Colorado Plateau xenoliths

 
Metabasic eclogites consist mainly of various proportions of almandine-rich garnet and omphacitic clinopyroxene with some lawsonite and zoisite (Fig. 2). They are either lawsonite-eclogite or zoisite-eclogite, according to the major constituent mineral assemblages (Table 1). Zoisite-eclogite is distinguished from lawsonite-eclogite only by its lack of lawsonite, although trace amounts of lawsonite crystals occur as small (10 µm) inclusions in garnet in zoisite-eclogites (Fig. 3c). With a decrease in garnet content (approximately <10 vol. %) and an increase of the jadeite component in the clinopyroxene (approximately >70 mol %), the metabasic eclogites grade continuously into jadeitic-clinopyroxenites. Other minerals that occur in trace amounts in both xenolith types are phengite, pyrite, rutile, zircon and apatite. Rare chlorite, albite and barite occur as <5 µm crystals around garnet and clinopyroxene. In both eclogite types, coesite occurs as <20 µm inclusions in garnet crystals, but not in the matrix (Fig. 3a; Usui et al., 2002, 2003).

View larger version (113K): [in this window] [in a new window]   Fig. 2. (a), (b) Back-scattered electron (BSE) image of lawsonite-eclogite GR1. (c) BSE image of included clinopyroxene and lawsonite in GR1. A crack in the lower part of the garnet connects the lawsonite pseudomorphs and an included clinopyroxene to the exterior of the garnet. (d) Photomicrograph (plane-polarized light) of GR1 showing curved trails of inclusions (rutile and coesite) in garnet. Abbreviations are as in Table 1.

 

View larger version (65K): [in this window] [in a new window]   Fig. 3. (a) BSE image of zoisite-eclogite MR7 showing that garnet includes various kinds of inclusions, such as clinopyroxene, coesite, rutile and zircon. (b) High-contrast BSE image of annealed cracks (shown by small arrows) in garnet in zoisite-eclogite MR4. Major element compositions of annealed garnet are similar to that of the reaction rim (see text for details). (c) BSE image of zoisite-eclogite MR7 showing that lawsonite can still survive as inclusions in garnet. (d) BSE image of jadeitic-clinopyroxenite MR3B. Garnet (shown in Fig. 5) includes lawsonite pseudomorphs that are replaced by fibrous zoisite aggregates along with rutile. Cracks connect these lawsonite pseudomorphs. Ion-microprobe pits are indicated by small arrows. Abbreviations are as in Table 1.

 
Occurrence, textures and compositions of minerals
Garnet
Garnet commonly occurs in small clusters or in thin layers accompanied by lawsonite and zoisite; these generally have euhedral to subhedral outlines with maximum diameters of 0·1–1·0 mm (Figs 2a and 3a). They are commonly color-zoned, from pink cores to reddish brown rims under plane-polarized light (Fig. 2d). Some grains exhibit rotation textures and contain fine-grained curved inclusion trails of coesite and rutile, indicating that deformation continued after the development of a planar tectonic fabric (Fig. 2d) (see also Helmstaedt & Schulze, 1991).

Garnets have compositions of Alm (Almandine)_50–70, Sps (Spessartine)_<1, Pyr (Pyrope)_10–30, Grs (Grossular)₂₀, except for locally developed pyrope-rich rims (Table 3 and Fig. 4). These pyrope-rich rims have distinctively different compositions from the other parts of the garnet, indicating a sharp compositional boundary (Fig. 2a). The compositions of these pyrope-rich rims are about Alm₄₀Sps_<1Pyr₅₀Grs₁₀ for the lawsonite-eclogite and Alm₄₀Sps_<1Pyr₄₀Grs₂₀ for the zoisite-eclogite. The rims may have formed under higher temperature conditions when the xenoliths were entrained in the microbreccia host rock, as will be discussed later, and are called ‘reaction rims’, to distinguish them from ‘normal rims’ the composition of which is almost identical to those of the core and mantle (Table 4).

View larger version (26K): [in this window] [in a new window]   Fig. 4. Composition of garnets in (a) lawsonite-eclogite GR1, (b) zoisite-eclogite MR4 and (c) jadeitic-clinopyroxenite MR3B plotted in the system (Alm + Sps)–Pyr–Grs. The shaded part of the full compositional triangle (d) is enlarged in (a), (b) and (c). Stippled pattern indicates garnets in eclogite xenoliths from diamondiferous kimberlites; cross-hatched pattern, garnets in eclogites from subduction-related metamorphic suites.

 

View this table: [in this window] [in a new window]   Table 3: Representative electron microprobe analyses of garnet

 

View this table: [in this window] [in a new window]   Table 4: Representative electron microprobe analyses of clinopyroxene

 
In some particularly zoisite-rich eclogites (e.g. MR4, see Table 2), wide reaction rims are present around most garnet crystals, and also penetrate into the garnet cores along cracks (Fig. 3b). The chemical compositions of these garnets in the ‘zoisite-rich’ eclogites trend toward those of the reaction rims in the (Alm + Sps)–Pyr–Grs diagram (Fig. 4b). In contrast, no reaction rims were observed in jadeitic-clinopyroxenites, which have little or no lawsonite and zoisite, although some garnets have slightly pyrope-rich compositions (Figs 4c and 5a).

View larger version (82K): [in this window] [in a new window]   Fig. 5. (a) BSE image of garnet in jadeitic-clinopyroxenite MR3B. The garnet does not have any reaction rims. Lawsonite pseudomorphs preserve sharp and euhedral crystal boundaries of the original lawsonite, which are connected by cracks. (b) Electron microprobe X-ray map for Mn in the garnet shown in (a). White and red colors indicate higher concentration than green and blue.

 
The major element compositions of the garnet, except for the reaction rims, are more similar to those from eclogites in subduction-related metamorphic terranes (Low-Temperature eclogite) than to those from diamondiferous kimberlite pipes (High-Temperature eclogite) (Carswell, 1990). The garnets show faint zoning in their Alm, Pyr and Grs components but strong and euhedral zoning in their Sps component, which is much lower than the other three components (Fig. 5). The core is characteristically enriched in Mn relative to the rim.

Clinopyroxene
Clinopyroxene is generally prismatic, and its orientation defines the rock fabric. In thin section it varies in color from colorless to pale green under plane-polarized light. The grain size varies as a result of deformation and synkinematic recrystallization (Helmstaedt & Schulze, 1988). Clinopyroxene grains in the eclogites have compositions near Jd (Jadeite)_40–50, Aug (Augite)_50–60, Ac (Acmite)_0–8, falling into the range of omphacite compositions (Table 4, Fig. 6a and b). On the other hand, clinopyroxenes in the jadeitic-clinopyroxenites have compositions of Jd₆₅Aug₂₀Ac₁₅ to Jd₇₅Aug₂₀Ac₅. This variation suggests relatively linear jadeite to acmite substitution (Fig. 6c). Clinopyroxene occurs both as inclusions in garnet (referred as ‘included clinopyroxene’)and in the matrix (referred to as ‘matrix clinopyroxene’; Figs 2c and 3a). Included clinopyroxene grains are richer in Ac and poorer in Jd components than matrix clinopyroxene grains, and their composition varies linearly between Jd₃₅Aug₅₀Ac₁₅ and Jd₄₀Aug₅₅Ac₅ (Fig. 6a and b).This trend is also a jadeite to acmite substitution but it differs from the compositional variations of the matrix clinopyroxene.

View larger version (27K): [in this window] [in a new window]   Fig. 6. Compositions of clinopyroxene in (a) lawsonite-eclogite GR1, (b) zoisite-eclogites, MR4 and MR7, and (c) jadeitic-clinopyroxenite MR3B plotted in the system Jd–Ac–(Aug + Cats). The shaded part of the full compositional triangle (d) is enlarged in (a), (b) and (c). Broken arrow in (d) shows the compositional change of clinopyroxene in basic rocks from the Franciscan metamorphic suite with increasing metamorphic pressure and temperature through to blueschist facies (Maruyama & Liou, 1988). The classification and the nomenclature of clinopyroxene follow Essene & Fyfe (1967).

 
Lawsonite and zoisite
Lawsonite is transparent in plane-polarized light. Except for minute inclusions in garnet crystals in the zoisite-eclogite (Fig. 3c), lawsonite is restricted to lawsonite-eclogite. It is invariably rimmed by fine-grained zoisite aggregates (Fig. 2b). Lawsonite grains display relatively uniform compositions close to the ideal formula of CaAlSi₂O₇(OH)₂·H₂O, with limited substitution of Fe³⁺ for Al and (Na + K) for Ca (Table 5; e.g. Moore & Liou, 1979). Like clinopyroxene, lawsonite occurs both as inclusions in garnet (referred to as ‘included lawsonite’) and as matrix grains (referred to as ‘matrix lawsonite’) (Fig. 2b and c). Included lawsonite grains in unfractured garnet crystals lack any rims of zoisite aggregates (Fig. 3c). Most included lawsonites have higher Fe₂O₃ and lower Al₂O₃ contents than matrix lawsonite (Table 5 and Fig. 7).

View larger version (17K): [in this window] [in a new window]   Fig. 7. XFe3+ of lawsonite in lawsonite-eclogite GR1 plotted against XAl. XFe3+ and XAl are the cation numbers of Fe3+ and Al in lawsonite, respectively, when the oxygen number is eight.

 

View this table: [in this window] [in a new window]   Table 5: Representative electron microprobe analyses of lawsonite and zoisite

 
Zoisite invariably occurs as fine-grained, fibrous and radiating crystal aggregates in eclogite and some jadeitic-clinopyroxenites (Figs 2 and 3). Whereas the zoisite crystals themselves are transparent, the aggregates are dark brown in plane-polarized light. The grain size of zoisite is generally <10 µm along the major axis and <1 µm along the minor axis, and is much smaller than that of the other constituent minerals, garnet, clinopyroxene, lawsonite, phengite and rutile. It contains 39·7–40·2 wt % SiO₂, 32·6–32·3 wt % Al₂O₃, 23·6–24·2 wt % CaO, and trace amounts of Fe₂O₃, MgO, TiO₂, Na₂O and Cr₂O₃ (Table 5). The textures and major element composition of zoisite are almost identical in lawsonite-eclogite, zoisite-eclogite and jadeitic-clinopyroxenite. Zoisite aggregates also occur as inclusions in fractured garnet preserving the sharp crystal boundaries of euhedral lawsonite pseudomorphs (Fig. 3d), suggesting that the zoisite aggregates originated by replacement of lawsonite.

Other minerals
Phengite occurs in one eclogite (GR1) and two jadeitic-clinopyroxenite (MR21 and MR3B) samples. Grains are platy and up to 0·5 mm in diameter, have relatively homogeneous major element compositions and are characterized by Si values of 7·5 p.f.u. (per formula unit, O = 22; Table 6). In jadeitic-clinopyroxenite samples, albite occurs as a secondary phase along clinopyroxene grain boundaries and rarely along cleavages. The mode of albite is <0·1 vol. %, and it contains <1 % of Or and An components. Orthoclase is present only as inclusions in the core of a garnet from eclogite MR7. Its composition is close to the ideal formula of KAlSi₃O₈ (Table 6). Amphibole does not occur in any of our metabasic eclogite xenoliths.

View this table: [in this window] [in a new window]   Table 6: Representative electron microprobe analyses of other constituent minerals

 
Trace element compositions of minerals
Garnets from eclogites and jadeitic-clinopyroxenites are characterized by extreme LREE (light rare earth element) depletion and HREE (heavy rare earth element) enrichment, exceeding a chondrite-normalized Yb/Ce ratio of 10⁴ (Fig. 8). Garnets have REE patterns that appear to increase progressively from La to Dy and are convex-upward from Dy to Lu, except for the reaction rim. The REE patterns of the reaction rims differ in shape from those in other parts of the garnet; they are characteristically convex-upward from La to Dy and concave-downward from Dy to Lu. LREE (La, Ce, Pr and Nd) and MREE (middle rare earth elements: Sm, Eu, Gd and Dy) contents are mostly uniform throughout the crystal, whereas HREE (Er, Yb and Lu) contents are markedly zoned (Fig. 8). The HREE are more abundant in the core relative to the rim, and show a strong correlation with Mn contents (Fig. 9). Garnets in all of the xenoliths are rich in Y, which decreases from core to rim, similar to the HREE (Table 7). Other trace element data for garnet measured by ICP-MS are listed in Table 10.

View larger version (26K): [in this window] [in a new window]   Fig. 8. Chondrite-normalized rare earth element patterns of garnet, clinopyroxene, lawsonite and zoisite in eclogites (GR1, MR4 and MR7) and jadeitic-clinopyroxenites (M26 and MR47). The rare earth element data are normalized to the C1 chondrite value of Sun & McDonough (1989).

 

View larger version (17K): [in this window] [in a new window]   Fig. 9. Zoning profiles along traverse X–Y in Fig. 2a for HREE, Y and Mn (XSps) in the garnet. Concentrations of Dy, Er, Yb, Lu and Y were analyzed simultaneously by ion microprobe, and are normalized to the primitive mantle value of Sun & McDonough (1989). The Mn content of the garnet next to the ion microprobe pits was analyzed by electron microprobe.

 

View this table: [in this window] [in a new window]   Table 7: Representative ion microprobe trace element analyses of garnet (ppm)

 
The clinopyroxenes show two groups of trace element compositions (Fig. 8). The LREE, MREE and Sr contents of clinopyroxenes are higher in eclogite than jadeitic-clinopyroxenite. On the other hand, the HREE and Y contents of clinopyroxenes in eclogite are slightly lower than those of counterparts in jadeitic-clinopyroxenite (Table 8 and Fig. 8). Clinopyroxene in eclogite has REE patterns that are convex-upward from La to Dy and concave-downward from Dy to Lu. Such HREE characteristics are only weakly developed in clinopyroxene from the jadeitic-clinopyroxenites. In particular, no HREE depletion is observed in jadeitic-clinopyroxenite MR47 that contains <0·1 vol. % of garnet. Clinopyroxenes in both eclogite and jadeitic-clinopyroxenite are poor in HFSE (Zr <1 ppm and Nb <0·001 ppm; Table 8). Other trace element data for clinopyroxene measured by ICP-MS are listed in Table 10.

View this table: [in this window] [in a new window]   Table 8: Representative ion microprobe trace element analyses of clinopyroxene (ppm)

 
Lawsonite and zoisite are rich in incompatible trace elements, especially U, Th, Pb, LREE and Sr. They are poor in high field strength elements (HFSE), similar to clinopyroxene (Tables 9 and 10). Matrix lawsonite from the lawsonite-eclogite sample has trace element abundances similar to those of matrix zoisite (Fig. 8e). The matrix lawsonites appear unzoned in trace elements, and the trace element abundances are almost identical among the matrix lawsonite grains in each sample. However, matrix and included lawsonites display different REE patterns. Matrix lawsonites exhibit gently convex-upward LREE-enriched patterns from La to Er and relatively unfractionated patterns from Er to Lu. Included lawsonite grains have fairly flat REE patterns that decease slightly from La to Lu (Fig. 8). The trace element patterns of zoisite grains in eclogite and jadeitic-clinopyroxenite vary from sample to sample. For example, zoisite from apatite-bearing zoisite-eclogite MR7 contains less LREE than other zoisites (Fig. 8). This may reflect equilibration with apatite that is REE-rich (Usui, 2004). Other trace element data for zoisite measured by ICP-MS are listed in Table 10.

View this table: [in this window] [in a new window]   Table 9: Representative ion microprobe analyses of trace elements in lawsonite and zoisite (ppm)

 

View this table: [in this window] [in a new window]   Table 10: Trace element compositions of separated minerals measured by ICP-MS

 
Whole-rock chemistry
Major element whole-rock compositions (Table 11) are plotted against SiO₂ contents in Fig. 10. Compositions of fresh and altered mid-ocean ridge basalt (MORB) from ocean-floor drilling holes 504B and 417D (Tual et al., 1985; Zuleger et al., 1995), and subduction-related metamorphic rocks thought to originate from MORB-type oceanic crust (Becker et al., 1999; Bröcker & Enders, 2001; Gao & Klemd, 2001), are plotted for comparison. The whole-rock major element compositions of the eclogite and jadeitic-clinopyroxenite xenoliths do not lie in the ranges of either altered or fresh MORB. Instead, they are within the ranges of the subduction-related metamorphic rocks. In particular, the xenoliths are characterized by distinctively higher Na₂O contents with increasing SiO₂ contents than those of altered MORB, resulting in a continuous and near-linear trend on the SiO₂ vs Na₂O Harker diagram (Fig. 10b). In contrast, K₂O, FeO^*, Al₂O₃, and TiO₂ in both eclogites and jadeitic-clinopyroxenites are almost identical to those of altered MORB. Such major element characteristics are commonly observed in metabasic rocks from subduction-related metamorphic terranes, and they have been explained by spilitization during high-temperature hydrothermal alteration at mid-ocean ridges and subduction-related metasomatism at fore-arc depths (e.g. Zack et al., 2003). In particular, high Na₂O contents that are decoupled from K₂O contents are best explained by spilitization–albitization (Fig. 11). The jadeitic-clinopyroxenites are richer in Na₂O and SiO₂ than the eclogites, although there appears to be a continuous transition and overlap between the two groups. This reflects the differences in mineralogical composition and modal proportions of the major constituent minerals: Jd components in the clinopyroxenes are richer in the jadeitic-clinopyroxenites than those in the eclogites, and the jadeitic-clinopyroxenites contain more clinopyroxene but less garnet, lawsonite and zoisite than the eclogites (compare Fig. 6 and Table 2 with Fig. 10).

View larger version (25K): [in this window] [in a new window]   Fig. 10. Whole-rock major element compositions (wt % oxide) of the eclogite and jadeitic-clinopyroxenite xenoliths as a function of SiO2 content (wt %). , eclogite; , jadeitic-clinopyroxenite. Compositional fields of fresh MORB and altered MORB are based on data from Tual et al. (1985) and Zuleger et al. (1995), and that of subduction-related metamorphic rocks, which represent MORB-type oceanic crust, is from Becker et al. (1999), Bröcker & Enders (2001) and Gao & Klemd (2001).

 

View larger version (20K): [in this window] [in a new window]   Fig. 11. Whole-rock Na2O and K2O contents (wt %) in eclogite and jadeitic-clinopyroxenite xenoliths. The field of fresh MORB (data sources as in Fig. 10) and trends produced by spilitization–albitization and low-T seawater alteration (see text for explanation) are also shown.

 

View this table: [in this window] [in a new window]   Table 11: Major and trace element compositions of whole-rock samples

 
Whole-rock chondrite-normalized REE patterns of eclogite and jadeitic-clinopyroxenite xenoliths are shown in Fig. 12; altered MORB from ocean-floor drilling hole 504B is shown as a shaded field for comparison. Eclogites have LREE-enriched patterns that decrease progressively from La to Lu, resulting in chondrite-normalized abundance ratios [La/Lu]_N = 6·0–11 and [La/Sm]_N = 2·5–3·3 that are distinctively greater than those of altered MORB (Table 11). In addition to the higher abundance ratios, LREE concentrations are higher in the eclogite than in altered MORB. Jadeitic-clinopyroxenites display V-shaped REE patterns with the lowest normalized abundances at Eu. Although jadeitic-clinopyroxenites exhibit variable LREE contents, which are lower than those of the eclogites, their chondrite-normalized [La/Sm]_N ratios are relatively constant and similar to those of the eclogite xenoliths. The REE patterns of eclogite and jadeitic-clinopyroxenite xenoliths are probably controlled by the modes of garnet and zoisite (or lawsonite), which have extremely different REE characteristics (Fig. 8). HFSE abundances are almost identical between eclogite and jadeitic-clinopyroxenite. The Zr/Hf and Nb/Ta ratios of the xenoliths vary from 36·9 to 37·6 and from 10·9 to 15·0, respectively. Both fall in the range of altered MORB, although the contents of these elements in the xenoliths are slightly higher than those of altered MORB (David et al., 2000). The abundances of other trace elements, especially Pb, Sr and U, vary considerably; they are generally more abundant in eclogite than jadeitic-clinopyroxenite (Table 11).

View larger version (26K): [in this window] [in a new window]   Fig. 12. Whole-rock rare earth element patterns of (a) eclogite and (b) jadeitic-clinopyroxenite xenoliths normalized to the C1 chondrite values of Sun & McDonough (1989). Shaded area shows the compositional field of altered MORB after Bach et al. (2003).

 
Sr–Nd–Pb isotopic compositions
Whole-rock Sr, Nd and Pb isotopic data were age-corrected to 30 Ma (Table 12), the emplacement age of the host rock at Moses Rocks and Garnet Ridge (Naeser, 1971; Helmstaedt & Doig, 1975; Usui et al., 2003), and are plotted on ⁸⁷Sr/⁸⁶Sr–Nd, Nd–²⁰⁶Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb–²⁰⁸Pb/²⁰⁴ diagrams (Fig. 13). The isotopic compositions of Juan de Fuca MORB and Phanerozoic Cordilleran crust, which represent Farallon MORB and its sedimentary cover, respectively, are shown for comparison.

View larger version (27K): [in this window] [in a new window]   Fig. 13. Sr–Nd–Pb isotopic compositions of whole-rocks and the constituent minerals in eclogite and jadeitic-clinopyroxenite xenoliths plotted in (a) Nd vs 87Sr/86Sr, (b) 206Pb/204Pb vs Nd, (c) 207Pb/204Pb vs 206Pb/204Pb, and (d) 208Pb/204Pb vs 206Pb/204Pb diagrams. All measured data were age-corrected to 30 Ma. Compositional fields of Juan de Fuca MORB and Phanerozoic Cordilleran crust are based on data from Church & Tatsumoto (1975), Aleinikoff (1987), Hegner & Tatsumoto (1987), White et al. (1987) and Farmer et al. (1993). Northern Hemisphere Reference Line (NHRL; Hart, 1984) is also shown.

 

View this table: [in this window] [in a new window]   Table 12: Sr, Nd and Pb isotopic compositions of whole-rock samples

 
The Sr isotopic ratios of the eclogites and jadeitic-clinopyroxenites vary (⁸⁷Sr/⁸⁶Sr = 0·7050–0·7084) and are greater than those of MORB. The jadeitic-clinopyroxenites have much higher Sr isotopic ratios than the eclogites. In contrast, the Nd isotopic compositions are similar in both eclogites and jadeitic-clinopyroxenites; Nd values vary from –3·1 to –1·2, much less than those of MORB, but similar to those of crustally derived sediments. Eclogites and jadeitic-clinopyroxenites have Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18·91–19·38, ²⁰⁷Pb/²⁰⁴Pb = 15·62–15·68, and ²⁰⁶Pb/²⁰⁴Pb = 38·68–38·92) that fall in the compositional field of sedimentary rocks in ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb–²⁰⁸Pb/²⁰⁴Pb diagrams (Fig. 13).

In addition to the whole-rock data, Rb–Sr, Sm–Nd and U–Th–Pb isotopic studies were also performed on garnet, clinopyroxene and zoisite mineral separates from the xenoliths (Table 13). The Pb isotopic compositions of garnet were not measured because of its extremely low Pb contents (0·038–0·087 ppm; see Table 10). Reference isochrons at 30 Ma were calculated to pass through each mineral in the ⁸⁷Sr/⁸⁶Sr–⁸⁷Rb/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd–¹⁴⁷Sm/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb–²³⁸U/²⁰⁴Pb isotopic systems to distinguish isotopic equilibrium and disequilibrium at 30 Ma (Fig. 14). If one mineral was isotopically in equilibrium with another when the host diatremes were emplaced, the reference isochrons for the two minerals should be coincident within error. The slope of the reference isochron for the Rb–Sr system is almost horizontal because of the small variation of Rb/Sr ratio.

View larger version (17K): [in this window] [in a new window]   Fig. 14. Rb–Sr, Sm–Nd and U–Pb isochron diagrams for constituent minerals in eclogite MR19 (a, c, and e) and jadeitic-clinopyroxenite MR26 (b, d and f) xenoliths. The lines marked in each figure are 30 Ma reference isochrons, which were calculated to pass through the three constituent minerals: G, garnet; C, clinopyroxene; Z, zoisite; W, whole-rock. Continuous line, zoisite; dotted line, clinopyroxene; dashed line, garnet. Vertical bar shows standard deviation (2) in Table A1.

 

View this table: [in this window] [in a new window]   Table 13: Sr, Nd and Pb isotopic compositions of minerals

 
Figure 14a and b shows Rb–Sr isochron diagrams for representative eclogite sample MR19 and jadeitic-clinopyroxenite sample MR26; the diagrams include both mineral and whole-rock data. Both whole-rock values of ⁸⁷Sr/⁸⁶Sr are greater than those of the constituent minerals, although the minerals in the eclogite xenolith have almost identical Sr isotopic compositions. Individual minerals in the jadeitic-clinopyroxenite have different isotopic compositions, suggesting that they were not in equilibrium with each other at 30 Ma.

The reference Sm–Nd isochrons for all minerals and the whole-rock of eclogite are coincident within error (Fig. 14c), but garnet from the jadeitic-clinopyroxenite has a different Nd isotopic composition from the other minerals and the whole-rock (Fig. 14d). Zoisites display almost identical Sm/Nd abundance ratios and Nd isotopic compositions to those of the whole-rocks because they are the dominant reservoir for LREE in both eclogite and jadeitic-clinopyroxenite (as discussed later; see Fig. 15).

View larger version (37K): [in this window] [in a new window]   Fig. 15. Mass balance calculations for (a) eclogite MR19 and (b) jadeitic-clinopyroxenite MR26. Density-weighted modal abundances of minerals are calculated by multiplying the mode in vol. % in Table 2 by densities. The densities are interpolated from values of Deer et al. (1992): 4·1 for Alm70Pyr30 garnet; 3·3 for omphacitic clinopyroxene; 3·15 for zoisite. Bar length corresponds to the amount of the trace element in each phase in relative proportion to the whole-rock concentration. If the total amount of the relative proportions of the minerals falls in the range 80–120%, mass balances are judged to be achieved, considering uncertainties in modal proportions of minerals and heterogeneity in their trace element concentrations; outside this range, either the uptake of trace elements in an unanalyzed phase or overestimation of modal proportions is suspected.

 
The Pb isotopic compositions of clinopyroxene in the eclogite xenoliths are almost in equilibrium with zoisite at 30 Ma (Fig. 14e). In contrast, the Pb isotopic compositions of clinopyroxene and zoisite in jadeitic-clinopyroxenite are not in equilibrium with each other at 30 Ma. Such features are weakly observed in the other U-series isochron diagrams (²⁰⁷Pb/²⁰⁴Pb–²³⁵U/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb–²³²Th/²⁰⁴Pb). Although zoisite is the dominant host of U and Pb, it has distinctively different Pb isotopic compositions and U/Pb abundance ratios from those of the whole-rocks. This might be explained by the presence of zircons with extremely high ²³⁸U/²⁰⁴Pb (>10⁶) and ²⁰⁶Pb/²⁰⁴Pb (>10³) (Usui, 2004).

	DISCUSSION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS APPENDIX REFERENCES

 
Evidence for high-P/T prograde metamorphism
The compositional zoning structure of garnet and the inclusion assemblages within garnet grains in eclogites and jadeitic-clinopyroxenites preserve information about a certain range of the P–T history of the xenoliths. The Sps component of garnet in both xenolith types is markedly zoned and preserves a fine euhedral structure (Fig. 5). Mn zoning of this type is not observed in other mantle-derived eclogite xenoliths; however, it is common in garnet-bearing regional metamorphic rocks that have undergone prograde metamorphism (e.g. Atherton & Edmunds, 1966; Zack et al., 2004). The Mn zoning of the garnet in the xenoliths is thus interpreted to reflect growth during prograde metamorphism.

Included clinopyroxenes are richer in Ac and poorer in Jd contents than matrix clinopyroxene (Fig. 6). This probably reflects jadeite–acmite substitution. Clinopyroxene generally becomes more jadeitic and less acmitic with increasing pressure and temperature in the upper blueschist to eclogite facies (Maruyama & Liou, 1988). Most included lawsonite grains have higher Fe₂O₃ and lower Al₂O₃ contents than matrix lawsonite (Fig. 7). Maruyama & Liu (1988) reported negative correlations between Fe³⁺ and Al molar abundances in lawsonite during progressive metamorphism at blueschist facies: Fe³⁺ content decreases with increasing pressure and temperature. Thus, included clinopyroxene and lawsonite probably formed in a lower pressure and temperature metamorphic environment than matrix clinopyroxene and lawsonite.

Garnet grains from the Colorado Plateau eclogites contain curved trails of fine-grained coesite and rutile inclusions (Fig. 2d), indicating the former presence of a planar tectonic fabric (see also Helmstaedt & Schulze, 1988, 1991). Such textures do not commonly form by magmatic processes, but are commonly found in regional metamorphic suites, such as those of Corsica and the Franciscan metamorphic complex of the western USA, which are known for their occurrences of high-pressure and low-temperature eclogites (Caron & Pèquignot, 1986; Maruyama & Liou, 1988). Thus, the inclusions indicate that the immediate protolith of the eclogite xenoliths was a fine-grained foliated rock, probably a blueschist.

In summary, garnet crystals grew during prograde metamorphism, and successively captured minerals such as clinopyroxene and lawsonite, reaching peak metamorphic conditions ranging from 560 to 700°C at 3 GPa and to 600 to 760°C at 5 GPa (Usui et al., 2003). Such high-P–low-T environments, which are colder than typical mantle geotherms, can be achieved only in subducted oceanic lithosphere. Therefore, we propose that the Colorado Plateau eclogite xenoliths formed by prograde metamorphic recrystallization of subducted oceanic crust.

‘Retrogression’ of eclogite xenoliths during emplacement
The Colorado Plateau eclogites were entrained within and rapidly transported to the surface by the host ultramafic microbreccia diatremes during the Tertiary (30 Ma). The eclogite xenoliths exhibit less retrograde overprinting than subduction-related eclogites from high-pressure metamorphic terranes, as indicated by the absence of amphibole. However, some modifications resulting from xenolith–host-rock interaction are observed. In the lawsonite-eclogite xenoliths, the lawsonite grains are invariably rimmed by zoisite, and, in the zoisite-eclogite xenoliths, the lawsonite is totally replaced by zoisite. Although similar replacement reactions of lawsonite are commonly observed in lawsonite-bearing eclogites from regional metamorphic belts (e.g. Zack et al., 2004), the mineral products and their textures in the regional metamorphic eclogites are different from those in the Colorado Plateau eclogite xenoliths. Whereas lawsonite pseudomorphs in the Colorado Plateau eclogite xenoliths contain only fibrous and radiating zoisite aggregates, those in regional metamorphic eclogites are replaced by well-crystallized multiple mineral phases, such as epidote, paragonite, phengite and albite (Zack et al., 2004). Because lawsonite is stable at lower-T and higher-P conditions than these replacing minerals (e.g. Schmidt & Poli, 1998; Poli & Schmidt, 2002), breakdown of lawsonite could occur as a result of increasing temperature and/or decreasing pressure during a retrogression event. The petrographical difference in lawsonite pseudomorphs between the Colorado Plateau eclogite xenoliths and regional metamorphic eclogites may reflect the nature of the P–T path of retrograde metamorphism. The radiating and fibrous textures of zoisite and preservation of lawsonite in the Colorado Plateau eclogite xenoliths suggest that the zoisite crystallized rapidly by replacement of lawsonite under disequilibrium conditions, probably during the transport of the lawsonite-eclogites in the host rocks.

The textures and major element compositions of zoisite are almost identical in both lawsonite- and zoisite-eclogite xenoliths (Figs 2 and 3 and Table 5). Moreover, zoisite-eclogite xenoliths lack matrix lawsonite but contain included lawsonite in unfractured garnet (Fig. 3c). Thus, the textural and mineralogical differences between the lawsonite- and zoisite-eclogites could reflect varying extents of recrystallization induced by the host rock. Both types of eclogite xenoliths could, therefore, have been originally lawsonite-eclogites.

Garnet compositions are almost uniform within individual mineral grains, except for variations in the Mg/Fe ratio in some reaction rims (Figs 2a and 5). Such rims display extremely pyrope-rich compositions relative to other parts of the garnet, suggesting that they crystallized at higher temperature conditions than the rest of the grain. The reaction rims are well developed in zoisite-rich eclogite. Moreover, the pyrope-rich compositions are also observed along cracks within the garnet grains that connect to lawsonite pseudomorphs (Fig. 3b). On the other hand, such reaction rims are not found in jadeitic-clinopyroxenite xenoliths, which have little or no zoisite aggregate after lawsonite. Formation of the reaction rim was probably related to the alteration of lawsonite to zoisite, which liberated fluid that catalyzed the reaction.

Heterogeneous trace element distributions in the subducted oceanic crust
Colorado Plateau eclogite xenoliths have been interpreted to represent fragments of the subducted Farallon plate (probably altered oceanic crust) that underwent high-P/T metamorphism in the lawsonite eclogite facies (Usui et al., 2003). Therefore, the compositions of the xenoliths should carry a record of the trace element characteristics of the subducted oceanic crust in the Farallon plate. In this study, we have determined element distributions of Rb, Sr, Sm, Nd, U, Th and Pb between the constituent minerals, garnet, clinopyroxene and zoisite. Rb–Sr, Sm–Nd and U–Pb isotopic systematics of both constituent minerals and whole-rocks allows the degree of isotopic equilibrium and disequilibrium to be evaluated.

Mass balance calculations were performed by comparing the measured whole-rock data with calculated whole-rock data. The calculated whole-rock data were obtained by multiplying density-weighted modal abundances of the constituent minerals by their trace element concentrations, and summing each mineral's contribution. The ICP-MS analyses for separated minerals were used as average values for the trace element contents of each mineral (Table 10). Differences between measured and calculated whole-rock compositions may point to undetected accessory phases. The calculations also indicate the dominant carriers of each trace element. Figure 15 shows the results of the calculations for zoisite-eclogite MR19 and jadeitic-clinopyroxenite xenolith MR26.

Mass balance can be achieved only for HREE; garnet controls the HREE and Y budgets of the whole-rock. Although zoisite hosts the other REE (especially LREE), Sr, Th and Pb, the calculated whole-rock compositions of these elements are more than twice as much as those of the measured whole-rock compositions. These overestimates of calculated compositions against measured compositions might be caused by overestimates of the modal proportions of zoisite. For modal proportion determinations by optical microscope, areas of lawsonite pseudomorphs that were totally replaced by zoisite aggregates were defined as zoisite. Because fine-grained and radiating zoisite aggregates cannot be completely embedded in the lawsonite pseudomorphs owing to their morphology (Figs 2 and 3), the modal proportion of zoisite is likely to be overestimated. The obvious deficits of Cs, Rb and Ba can be explained by the lack of analyses of phengite for Cs, Rb and Ba, and barite for Ba. These deficits are observed in the jadeitic-clinopyroxenite (Fig. 15). Barite could have been formed by surface weathering after exhumation of the xenoliths to the surface by the host-rock emplacement, because it has not been found as part of the primary mineral assemblage of the eclogites and occurs only as fine-grained crystals along the grain boundaries of the other primary minerals, such as garnet and clinopyroxene. The deficit for U cannot be explained and might be attributed to the presence of an unknown U-enriched phase.

Our results for trace element distributions in the Colorado Plateau eclogite xenoliths are generally consistent with previous studies regarding eclogite samples from regional metamorphic belts (Tribuzio et al., 1996; Zack et al., 2002a, 2002b; Spandler et al., 2003). In some regional metamorphic eclogites and garnet-amphibolites, allanite dominates the whole-rock REE, U and Th budgets, instead of lawsonite and zoisite (Sorensen & Grossman, 1989, 1993; Tribuzio et al., 1996). This reflects the difference in metamorphic pressure and temperature conditions between these regional metamorphic eclogites and the Colorado Plateau eclogite xenoliths. The regional metamorphic eclogites underwent lower-P and/or higher-T metamorphic conditions than our xenoliths, in which neither lawsonite nor zoisite was stable.

We tested for isotopic equilibrium among the constituent minerals by examining the Rb–Sr, Sm–Nd and U–Th–Pb systematics of both minerals and whole-rocks. The mass balance calculations suggest that, except for U, these elements are concentrated almost exclusively in the zoisite that crystallized during xenolith emplacement (Fig. 15). This means that zoisite and the whole-rock should plot close together in the isochron diagrams or, if this is not the case, another phase with extremely different isotopic compositions and element ratios, not accounted for in the mass balance, must be suspected in the rocks. The former case was observed in the Sm–Nd isochron diagram, and the latter in both the Rb–Sr and U–Pb diagrams.

As seen in the Rb–Sr isochron diagrams (Fig. 14a and b), the whole-rock ⁸⁷Sr/⁸⁶Sr isotopic ratios are distinctly higher than those of zoisite. This discrepancy could be explained by the presence of barite crystals, which contain abundant Sr (up to 20 wt % as Sr oxide). Differences in ⁸⁷Sr/⁸⁶Sr between whole-rock and zoisite are much larger in the jadeitic-clinopyroxenite. This may reflect differences in the contributions of barite to the whole-rock Sr budget between eclogite and jadeitic-clinopyroxenite, because eclogite contains more zoisite, which dominates the whole-rock Sr, than jadeitic-clinopyroxenite.

For the ²³⁸U–²⁰⁶Pb isochron diagrams, the whole-rocks of both zoisite-eclogite and jadeitic-clinopyroxenite plot far from the zoisites and their reference isochrons (Fig. 14e and f). This discrepancy is explained by the presence of young zircons (33–81 Ma) with high ²³⁸U/²⁰⁴Pb (>10⁶) and ²⁰⁶Pb/²⁰⁴Pb (>10³) ratios (Usui, 2004). Although these zircons should also affect the whole-rock data for ²³⁵U–²⁰⁷Pb and ²³²Th–²⁰⁸Pb systematics, their isotopic signatures are not clearly observed in the ²³⁵U–²⁰⁷Pb and ²³²Th–²⁰⁸Pb isochron diagrams, because the natural abundance ratio of ²³⁵U/²³⁸U is low (1/137·88) and contents of Th are lower than those of U.

The zoisite-eclogite xenolith MR19 consists of garnet, clinopyroxene and zoisite that equilibrated at 30 Ma, the age of emplacement of the host diatreme, although the Sr isotopic composition of the zoisite is slightly higher than those of the other minerals (Fig. 14a, c and e). The mineral isochron ages for zoisite-eclogite xenoliths are 39 ± 11 Ma for the ¹⁴⁷Sm/¹⁴⁴Nd–¹⁴³Nd/¹⁴⁴Nd isochron diagram, and 33 ± 20 Ma for the ²³⁸U/²⁰⁶Pb–²⁰⁷Pb/²⁰⁶Pb isochron diagram. Despite large errors, both of the mineral isochron ages are consistent with the U–Pb ages of zircons in the zoisite-eclogites (Usui et al., 2003), and with the emplacement ages of the host diatremes (Naeser, 1971; Helmstaedt & Doig, 1975; Roden et al., 1979). On the other hand, it is difficult to obtain meaningful isotopic ages from the ⁸⁷Rb/⁸⁶Sr–⁸⁷Sr/⁸⁶Sr, ¹⁴⁷Sm/¹⁴⁴Nd–¹⁴³Nd/¹⁴⁴Nd and ²³⁸U/²⁰⁶Pb–²⁰⁷Pb/²⁰⁶Pb isochron diagrams for jadeitic-clinopyroxenite xenolith MR26 (Fig. 14b, d and f). If clinopyroxene and garnet in MR26 were isotopically homogenized, the pair should yield identical isochron ages among the different isotopic systems. However, this is not the case, and the ages obtained for ⁸⁷Rb/⁸⁶Sr–⁸⁷Sr/⁸⁶Sr and for ¹⁴⁷Sm/¹⁴⁴Nd–¹⁴³Nd/¹⁴⁴Nd are 492 ± 27 Ma and 42·4 ± 6·7 Ma, respectively. Moreover, mineral isochron ages obtained for zoisite combined with garnet and clinopyroxene are geologically unreasonable (for example, the zoisite–clinopyroxene mineral isochron age in the ⁸⁷Rb/⁸⁶Sr–⁸⁷Sr/⁸⁶Sr system is 13 400 ± 700 Ma).

The differences in mineral isochron ages between the zoisite-eclogite and the jadeitic-clinopyroxenite are related to the amount of fluid that was generated by lawsonite breakdown to zoisite aggregates during diatreme emplacement. Fluid not only catalyzes metamorphic reactions, but it also facilitates rapid mass transport. In the zoisite-eclogite xenoliths, the availability of fluids evidently allowed the recrystallization of zircons, which yield U–Pb concordia ages almost identical to those of microbreccia intrusion (Usui et al., 2003). Because the zoisite-eclogite contains abundant zoisite aggregates that were originally lawsonite, isotopic homogenization was probably facilitated by the presence of fluids, which evidently induced recrystallization of parts of garnet and clinopyroxene and caused diffusive isotopic resetting preferentially along cracks and grain boundaries. Petrographic observations indicate that almandine-rich garnet in the zoisite-eclogites recrystallized pyrope-rich compositions not only at the rim, but also along cracks into the garnet cores (Fig. 3b). Thus, all the mineral isochron ages and the U–Pb zircon ages in the zoisite-eclogite represent the emplacement age of the microbreccia diatreme.

In contrast, the jadeitic-clinopyroxenites contain little or no zoisite, reflecting a lawsonite-poor precursor. This lack of prograde lawsonite resulted in little or no fluid production during retrogression associated with the microbreccia emplacement. The experimentally determined volume diffusion rates for Pb and Nd in clinopyroxene (Sneeringer et al., 1984; van Orman et al., 1998; Cherniak, 2001) are too slow to have changed the isotopic ratios during diatreme emplacement without a fluid phase. In the absence of fluids, minerals have been shown to preserve their isotopic signatures far above their commonly accepted closure temperatures, even for time scales of entire orogenic metamorphic episodes (Farquhar et al., 1996; Tilton et al., 1997; Kelley & Wartho, 2000). Therefore, the constituent minerals in the jadeitic-clinopyroxenite xenoliths were probably not in equilibrium, and their mineral isochron ages are, thus, meaningless. Fortunately, however, the isotopic compositions of these minerals allow us to infer the pristine isotopic compositions of subducted oceanic crust prior to isotopic homogenization by diatreme emplacement.

Infiltration of metasomatizing fluids derived from sedimentary rocks
Although the chemical compositions of lithologies likely to be subducted (i.e. variably altered oceanic crust and overlying sedimentary cover) have been well investigated by ocean-floor drilling (e.g. Alt, 1995), chemical characterization of the subducted lithologies remains relatively poorly studied. In particular, there are few reliable datasets for the Sr, Nd and Pb isotopic compositions of subducted materials (e.g. Bernard-Griffiths & Cornichet, 1985; Thöni & Jagoutz, 1992; Becker et al., 2000). Such data could be used to constrain whole-mantle and individual subduction-zone chemical recycling models, and to explain the enriched isotopic signatures of OIB and island arc magmas (e.g. Hofmann, 1997; Shibata & Nakamura, 1997; Taylor & Nesbitt, 1998; van Keken et al., 2002). In this section, we discuss the isotopic and trace element compositions of the subducted materials, based on the geochemical datasets obtained from the Colorado Plateau eclogite xenoliths.

Both the eclogite and jadeitic-clinopyroxenite xenoliths from the Colorado Plateau, which are interpreted to represent subducted oceanic crust, exhibit ‘geochemicallyenriched’ Sr, Nd and Pb isotopic compositions and whole-rock trace element patterns (Figs 12 and 13).Because the whole-rock trace element abundances of the xenoliths are likely to have been modified by hydrothermal alteration at the mid-ocean ridge, where the MORB protolith formed, and by metasomatism during subduction (Becker et al., 1999, 2000), they cannot be used to constrain their protoliths, e.g. normal-type MORB (N-MORB) or enriched-MORB (E-MORB). In addition, we cannot discount the possibility that the xenoliths represent fragments of OIB, similar to Hawaiian tholeiites and associated subalkaline rocks, which probably occurred sporadically on the Farallon plate. However, the eclogite and the jadeitic-clinopyroxenite xenoliths have almost constant Zr/Hf ratios ranging from 36·9 to 37·6 (Table 11). Such values are consistent with all types of MORB, but not with OIB (David et al., 2000). The MORB-like Zr/Hf ratios of the xenoliths are evidence against a seamount origin, because Zr and Hf are relatively immobile elements, and the Zr/Hf ratios should not change during subduction. Even if any of the different MORB-types are protoliths for the xenoliths, their Sr, Nd and Pb isotopic signatures reflect the addition of an enriched geochemical component to the xenoliths.

The Sr, Nd and Pb isotopic compositions of the constituent minerals of the Colorado Plateau eclogite xenoliths are used to identify the source of the enriched component, because the whole-rock isotopic data were probably contaminated by surface weathering and/or interaction with the microbreccia host rock. A surface weathering effect is seen in the difference between the Sr isotopic compositions of the whole-rocks and those of the constituent minerals (Fig. 13a) and is best explained by the presence of late barite along grain boundaries. Differences in Pb isotopic compositions between whole-rocks and minerals were also observed (e.g. Fig. 13c). If abundant geochemical components originating in the microbreccia transport medium were added to the xenoliths, the mineral isotopic compositions of the eclogite xenoliths would be unusable, because, in contrast to the jadeitic-clinopyroxenite xenoliths, the eclogite minerals reached isotopic equilibrium during diatreme emplacement. However, judging from the REE element patterns of garnet and clinopyroxene (Fig. 8), such magmatic influences on the geochemistry of the constituent minerals of the xenoliths were probably small. Therefore, the mineral isotopic signatures observed not only the jadeitic-clinopyroxenite but also in the eclogite xenolith could represent the Sr, Nd and Pb isotopic compositions of the subducted basaltic oceanic crust.

The Sr, Nd and Pb isotopic compositions of the constituent minerals in the xenoliths are thought to reflect mixing between an ‘altered MORB component’ and a ‘sedimentary component’, based on considerations of the possible lithologies subducted with the protolith of the xenoliths in the Farallon plate (Fig. 13). The isotopic variations could have resulted from combining heterogeneous sedimentary components with variably altered MORB in variable mixing ratios. Such mixing cannot be explained by mechanical mixing between metasedimentary rocks and altered MORB crust, because a mechanical mixing model is inconsistent with the basaltic bulk-rock major element composition of the xenoliths. Instead, it requires metasomatic alteration induced by migration of a fluid from sedimentary layers to the basaltic protolith. Because the quantity of fluids and fluid mobility are limited in high-grade metamorphic conditions (e.g. Scambelluri & Philippot, 2001) metasomatism probably occurred at an early stage of subduction. At shallow levels (<10 km depth), large volumes of predominantly sedimentary rock and detritus accumulate in accretionary wedges, occurring in many subduction complexes as mélange-like material between the slab and the hanging wall (Shreve & Cloos, 1986). Such subduction mélanges contain mafic and ultramafic blocks showing varying extents of metasomatic alteration as a result of infiltrating aqueous fluids (Sorensen, 1988; Sorensen & Grossman, 1989, 1993). Although the Colorado Plateau eclogite xenoliths might not have been derived from mafic blocks in a mélange unit, studies of mélanges yield insights into the metasomatic mixing process. Detailed isotopic studies of a subduction mélange unit (Bebout, 1991) have suggested that large amounts of aqueous fluid, previously equilibrated with metasedimentary rocks, enter the mafic–ultramafic mélange, and produce large-scale stable isotopic homogenization and pervasive metasomatism. Therefore, in the fore-arc region, the protolith of the eclogite and jadeitic-clinopyroxenite xenoliths could have been metasomatized by large volumes of fluid derived from associated metasedimentary units.

Reassessment of Precambrian model ages
The Colorado Plateau eclogite xenoliths have distinctively lower Nd values than MORB at any time in the Phanerozoic (Roden et al., 1990; Wendlandt et al., 1993; this study). Wendlandt et al. (1993) applied Sm–Nd isotope systematics to determine the age of oceanic lithosphere that could have contributed to the protolith of the Colorado Plateau eclogite xenoliths. The Nd isotopic compositions and Sm/Nd ratios of the xenolith samples were used to calculate the time at which the xenolith would have the Nd value of the depleted mantle evolution curve proposed by DePaolo et al. (1991). However, the Nd isotopic compositions as well as the Sm/Nd ratios of MORB can be modified by subduction-related metasomatic events after extraction of the MORB melts from the depleted mantle at a mid-ocean ridge (e.g. Jahn et al., 1996). As discussed above, the Colorado Plateau eclogite xenoliths were metasomatized by large amounts of fluid that were isotopically equilibrated with metasedimentary rocks. To obtain accurate Nd model ages, Wendlandt et al. (1993) selected ‘less’ altered eclogite xenoliths based on their whole-rock chemistries. Nevertheless, these eclogite xenoliths have weak V-shaped REE patterns and distinctively higher [La/Sm]_N ratios (0·97–1·29) than those of MORB, suggesting that even they have undergone metasomatism. Thus, Nd model ages are an unsatisfactory measure of the protolith age for the Colorado Plateau eclogite xenoliths.

Smith et al. (2004) reported Proterozoic U–Pb zircon ages from the Colorado Plateau eclogite xenoliths. Nine multigrain zircon fractions were analyzed with TIMS following the chemical separation of Pb and U. Four of the nine multigrain zircon fractions almost plot on the concordia growth curve and yield ages from 35 to 70 Ma, consistent with the age range reported by Usui et al. (2003). The other five multigrain zircon fractions were discordant. Smith et al. (2004) selected seven multigrain zircon fractions from the nine fractions, and then obtained a poorly fit discordia (MSWD = 92) between 34·9 ± 3·3 and 1514 ± 74 Ma. The discordant zircon fractions were interpreted to establish that Colorado Plateau eclogites contained inherited mid-Proterozoic zircons. From this Smith et al. (2004) concluded that the Colorado Plateau eclogites represent fragments of Proterozoic lithosphere, and that the concordant young ages between 35 and 70 Ma record episodic zircon growth as a result of Mesozoic or Cenozoic eclogite-facies recrystallization of Proterozoic mantle. However, the reliability of the Proterozoic discordant U–Pb age must be further investigated, because all discordant multigrain zircon fractions plot close to the lower intercept (<10% of concordance from the upper intercept). The Smith et al. (2004) model is also inconsistent with the euhedral zoning structure of Mn in garnet and the garnet inclusion assemblages described in this paper. These petrographic features are incompatible with the postulated eclogite-facies recrystallization processes in the upper mantle but are better explained by subduction-related prograde metamorphism.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS APPENDIX REFERENCES

 
Eclogite and jadeitic-clinopyroxenite xenoliths from the Colorado Plateau consist of garnet, clinopyroxene, lawsonite and zoisite with minor amounts of phengite, rutile, pyrite and zircon. The xenoliths have basaltic whole-rock major element compositions, but significantly higher Na₂O with increasing SiO₂ contents compared with those of altered MORB. These major element characteristics are interpreted to reflect spilitization during hydrothermal alteration in a mid-ocean ridge environment, followed by subduction-related metasomatism in the fore-arc. Euhedral garnet zoning structures and the major element compositions of included clinopyroxene and lawsonite indicate that the xenoliths underwent subduction-related prograde metamorphism. Fine-grained zoisite aggregates crystallized rapidly, formed by replacement of lawsonite during the transport of lawsonite-eclogite to the surface in the microbreccia host. These petrographic results are consistent with those of Usui et al. (2003), who proposed that the Colorado Plateau eclogite xenoliths originated as fragments of subducted oceanic crust comprising part of the Farallon plate, which has resided in the upper mantle since the Late Cretaceous.

The whole-rock chemistry of the xenoliths has variable trace element enrichment compared with MORB or altered MORB, except for the Zr/Hf ratio, which ranges from 36·9 to 37·6. Whole-rock mass balance analysis demonstrates the trace element distributions among the constituent minerals in the xenoliths. Garnet is the principal residence site for HREE and Y, and zoisite hosts the other REE (especially LREE), Sr, Th and Pb. In the eclogite xenoliths, garnet, clinopyroxene and zoisite were in approximate isotopic equilibrium at the time of emplacement (30 Ma). These minerals yield ¹⁴⁷Sm–¹⁴³Nd mineral isochron ages of 39 ± 11 Ma and ²³⁸U–²⁰⁶Pb ages of 33 ± 20 Ma. In contrast, the same minerals in the jadeitic-clinopyroxenite xenoliths were not in isotopic equilibrium at the time of diatreme emplacement. The fluid released by the breakdown of lawsonite during the entrainment by the microbreccia host may have facilitated isotopic equilibration in the eclogite xenoliths.

Based on the Sr–Nd–Pb isotope systematics of their constituent minerals, the xenoliths underwent metasomatism by a fluid in equilibrium with sediments in the fore-arc region of the subducting Farallon plate. The metasomatism resulted in the xenoliths acquiring distinctively different isotopic compositions from those of altered MORB despite their altered MORB protoliths; Sr and Pb have more radiogenic and Nd less radiogenic isotopic compositions than those of altered MORB. This suggests that basaltic oceanic crust with an isotopic composition reflecting metasomatism by a fluid in equilibrium with a sedimentary component can be introduced into the sub-arc mantle through subduction zones and could, therefore, explain the sediment-like isotopic signatures of some island arc basalt and ocean island basalts.

	APPENDIX

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS APPENDIX REFERENCES

 

View this table: [in this window] [in a new window]   Table A1: Repeat analyses for Sr–Nd–Pb isotopic compositions and abundances of Rb, Sr, Sm, Nd, U, Th and Pb (ppm) in standard rock (JB3)

 

	ACKNOWLEDGEMENTS

 
We are very grateful to S. Maruyama, G. E. Bebout and all of the PML members for their scientific guidance, technical support and valuable discussion. I. Buick and R. L. King are also acknowledged for improving the quality of this paper. We thank J. Selverstone, S. S. Sorensen and T. Zack for constructive reviews of this manuscript, and M. Wilson for editorial handling. This study was financially supported in part by grants from the Ministry of Education, Science, Sports and Culture of Japan (E.N.) and by the program ‘Center of Excellence for the 21st Century in Japan’ (E.N.).

---

^* Corresponding author. Present address: Department of Earth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, TN 37996, USA. Telephone: +1-865-974-3874. Fax: +1-865-974-2368. E-mail: tusui{at}utk.edu

	REFERENCES

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS APPENDIX REFERENCES

 
Aleinikoff, J. N., Dusel-Bacon, C., Foster, H. L. & Nokleberg, W. J. (1987). Lead isotopic fingerprinting of tectono-stratigraphic terranes, east–central Alaska. Canadian Journal of Earth Sciences 24, 2089–2098.[Web of Science]

Alt, J. C. (1995). Subseafloor processes in mid-ocean ridge hydrothermal systems. In: Humphris, S. E., Zierenberg, R. A., Mulineaux, L. S. & Thomson, R. E. (eds) Seafloor Hydrothermal Systems: Physical, Chemical, Biological, and Geological Interactions. Geophysical Monograph, American Geophysical Union 91, 85–114.

Atherton, M. P. & Edmunds, W. M. (1966). An electron microprobe study of some zoned garnets from metamorphic rocks. Earth and Planetary Science Letters 1, 185–193.[CrossRef]

Atwater, T. (1970). Implications of plate tectonics for the Cenozoic tectonic evolution of Western North America. Geological Society of America Bulletin 81, 3513–3536.[Abstract/Free Full Text]

Ayers, J. C., Dittmer, S. K. & Layne, G. D. (1997). Partitioning of elements between peridotite and H₂O at 2·0–3·0 GPa and 900–1000°C, and application to models of subduction zone processes. Earth and Planetary Science Letters 150, 381–398.[CrossRef][Web of Science]

Bach, W., Peucker-Ehrenbrink, B., Hart, S. R. & Blusztajn, J. S. (2003). Geochemistry of hydrothermally altered oceanic crust: DSDP/ODP Hole 504B—implications for seawater–crust exchange budgets and Sr- and Pb-isotopic evolution of the mantle. Geochemistry, Geophysics, Geosystems 4, doi:10.1029/2002GC000419.

Bebout, G. E. (1991). Field-based evidence for devolatilization in subduction zones: implications for arc magmatism. Science 251, 413–416.[Abstract/Free Full Text]

Becker, H., Jochum, K. P. & Carlson, R. W. (1999). Constraints from high-pressure veins in eclogites on the composition of hydrous fluids in subduction zones. Chemical Geology 160, 291–308.[CrossRef][Web of Science]

Becker, H., Jochum, K. P. & Carlson, R. W. (2000). Trace element fractionation during dehydration of eclogites from high-pressure terranes and the implications for element fluxes in subduction zones. Chemical Geology 163, 65–99.[CrossRef][Web of Science]

Bernard-Griffiths, J. & Cornichet, J. (1985). Origin of eclogites from south Brittany, France: a Sm–Nd isotopic and REE study. Chemical Geology 52, 185–201.[CrossRef][Web of Science]

Brenan, J. M., Shaw, H. F., Phinney, D. L. & Ryerson, F. J. (1994). Rutile–aqueous fluid partitioning of Nb, Ta, Hf, Zr, U and Th: implications for high field strength element depletions in island-arc basalts. Earth and Planetary Science Letters 128, 327–339.[CrossRef][Web of Science]

Brenan, J. M., Shaw, H. F., Ryerson, F. J. & Phinney, D. L. (1995). Mineral–aqueous fluid partitioning of trace elements at 900°C and 2·0 GPa: constraints on the trace element chemistry of mantle and deep crustal fluids. Geochimica et Cosmochimica Acta 59, 3331–3350.[CrossRef][Web of Science]

Brenan, J. M., Ryerson, F. J. & Shaw, H. F. (1998). The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium, and lithium during subduction: experiments and models. Geochimica et Cosmochimica Acta 62, 3337–3343.[CrossRef][Web of Science]

Bröcker, M. & Enders, M. (2001). Unusual bulk-rock compositions in eclogite-facies rocks from Syros and Tinos (Cyclades, Greece): implications for U–Pb zircon geochronology. Chemical Geology 175, 581–603.[CrossRef][Web of Science]

Caron, J.-M. & Pèquignot, G. (1986). The transition between blueschists and lawsonite-bearing eclogite based on observations from Corsican metabasalts. Lithos 19, 205–218.[CrossRef][Web of Science]

Carswell, D. A. (1990). Eclogite and the eclogite facies: definitions and classification. In: Carswell, D. A. (ed.) Eclogite Facies Rocks. New York: Chapman and Hall, 396 pp.

Catanzaro, E. J., Murphy, T. J., Shields, W. R. & Garner, E. L. (1968). Absolute isotopic abundance ratios of common, equal-atom, and radiogenic lead isotopic standards. Journal of Research of the National Bureau of Standards 72A, 261–267.

Cherniak, D. J. (2001). Pb diffusion in Cr diopside, augite, and enstatite, and consideration of the dependence of cation diffusion in pyroxene on oxygen fugacity. Chemical Geology 177, 381–397.[CrossRef][Web of Science]

Church, S. E. & Tatsumoto, M. (1975). Lead isotope relations in oceanic ridges basalts from the Juan de Fuca–Gorda Ridge Area, N. E. Pacific Ocean. Contributions to Mineralogy and Petrology 53, 253–279.[CrossRef][Web of Science]

Clement, C. R., Skinner, E. M. W. & Smith, B. H. S. (1984). Kimberlite redefined. Journal of Geology 92, 223–228.[Web of Science]

Condie, K. C. (1982). Plate-tectonics model for Proterozoic continental accretion in the southwestern United States. Geology 10, 37–42.[Abstract/Free Full Text]

Coney, P. J. & Reynolds, S. (1977). Cordilleran Benioff zones. Nature 270, 403–406.[CrossRef]

David, K., Schiano, P. & Allègre, C. J. (2000). Assessment of the Zr/Hf fractionation in oceanic basalts and continental materials during petrogenetic processes. Earth and Planetary Science Letters 178, 285–301.[CrossRef][Web of Science]

Deer, W. A., Howie, R. A. & Zussman, J. (1992). An Introduction to the Rock-Forming Minerals. Harlow: Longman.

DePaolo, D. J., Linn, A. M. & Schubert, G. (1991). The continental age distribution: methods of determining mantle separation ages from Sm–Nd isotopic data and application to the southwestern U.S. Journal of Geophysical Research 96, 2071–2088.

Dickinson, W. R. (1997). Tectonic implications of Cenozoic volcanism in coastal California. Geological Society of America Bulletin 109, 936–954.[Abstract/Free Full Text]

Ehrenberg, S. N. (1982). Petrogenesis of garnet lherzolite and megacrystalline nodules from the Thumb, Navajo volcanic field. Journal of Petrology 23, 507–547.[Abstract/Free Full Text]

Ernst, W. G. (1970). Tectonic contact between the Franciscan mélange and the Great Valley Sequence—crustal expression of a late Mesozoic Benioff Zone. Journal of Geophysical Research 75, 886–901.

Essene, E. J. & Fyfe, W. S. (1967). Omphacite in California metamorphic rocks. Contributions to Mineralogy and Petrology 15, 1–23.[CrossRef]

Farmer, G. L., Ayuso, R. & Plafker, G. (1993). A Coast Mountains provenance for the Valdez and Orca groups, southern Alaska, based on Nd, Sr, and Pb isotopic evidence. Earth and Planetary Science Letters 116, 9–21.[CrossRef][Web of Science]

Farquhar, J., Chacko, T. & Ellis, D. J. (1996). Preservation of oxygen isotope compositions in granulites from Northwestern Canada and Enderby Land, Antarctica: implications for high-temperature isotopic thermometry. Contributions to Mineralogy and Petrology 125, 213–224.[CrossRef][Web of Science]

Gao, J. & Klemd, R. (2001). Primary fluids entrapped at blueschist to eclogite transition: evidence from the Tianshan meta-subduction complex in northwestern China. Contributions to Mineralogy and Petrology 142, 1–14.[Web of Science]

Hamilton, W. (1969). Mesozoic California and the underflow of Pacific mantle. Geological Society of America Bulletin 80, 2409–2430.[Abstract/Free Full Text]

Hart, S. (1984). A large-scale isotope anomaly in the Southern Hemisphere mantle. Nature 309, 753–757.[CrossRef]

Hegner, E. & Tatsumoto, M. (1987). Pb, Sr, and Nd isotopes in basalts and sulfides from the Juan de Fuca ridge. Journal of Geophysical Research 92, 11380–11386.[CrossRef]

Helmstaedt, H. & Doig, R. (1975). Eclogite nodules from kimberlite pipes of the Colorado Plateau—samples of subducted Franciscan-type oceanic lithosphere. Physics and Chemistry of the Earth 9, 95–112.[CrossRef]

Helmstaedt, H. & Schulze, D. J. (1988). Eclogite-facies ultramafic xenolith from Colorado Plateau diatreme breccias: composition with eclogites in crustal environments, evaluation of the subduction hypothesis, and implications for eclogite xenoliths from diamondiferous kimberlites. In: Smith, D. C. (ed.) Eclogites and Eclogite-Facies Rocks. New York: Elsevier, pp. 387–450.

Helmstaedt, H. H. & Schulze, D. J. (1991). Early to Mid-Tertiary inverted metamorphic gradient under the Colorado Plateau: evidence from eclogite xenoliths in ultramafic microbreccias, Navajo volcanic field. Journal of Geophysical Research 96, 13225–13235.

Hofmann, A. W. (1997). Mantle geochemistry: the message from oceanic volcanism. Nature 385, 219–229.[CrossRef]

Iizuka, Y. & Nakamura, E. (1995). Experimentally study of the slab–mantle interaction and implications for the formation of titanoclinohumite at deep subduction zone. Proceedings of the Japan Academy 71, 159–164.[CrossRef]

Ishikawa, T. & Nakamura, E. (1994). Origin of the slab component in arc lavas from across-arc variation of B and Pb isotopes. Nature 370, 205–208.[CrossRef]

Jahn, B.-M., Cornichet, J., Cong, B. & Yui, T.-F. (1996). Ultrahigh-Nd eclogites from an ultrahigh-pressure metamorphic terrane of China. Chemical Geology 127, 61–79.[CrossRef][Web of Science]

Kelley, S. P. & Wartho, J. A. (2000). Rapid kimberlite ascent and the significance of Ar–Ar ages in xenolith phlogopites. Science 289, 609–611.[Abstract/Free Full Text]

Kogiso, T., Tatsumi, Y. & Nakano, S. (1997). Trace element transport during dehydration process in the subducted oceanic crust: 1. Experiments and implications for the origin of ocean island basalts. Earth and Planetary Science Letters 148, 193–205.[CrossRef][Web of Science]

Kuritani, T. & Nakamura, E. (2002). Precise isotope analysis of nanogram-level Pb for natural rock samples without use of double spikes. Chemical Geology 186, 31–43.[CrossRef][Web of Science]

Kuritani, T. & Nakamura, E. (2003). High precise and accurate isotopic analysis of small amounts of Pb using ²⁰⁵Pb–²⁰⁴Pb and ²⁰⁷Pb–²⁰⁴Pb, two double spikes. Journal of Analytical Atomic Spectrometry 18, 1464–1470.[CrossRef][Web of Science]

Makishima, A. & Nakamura, E. (1991). Precise measurement of cerium isotope composition in rock samples. Chemical Geology 94, 1–11.[Web of Science]

Makishima, A. & Nakamura, E. (1997). Suppression of matrix effect in ICP-MS by high power operation of ICP: application to precise determination of Rb, Sr, Y, Cs, Ba, REE, Pb, Th, and U at ng g^–1 levels in milligram silicate samples. Geostandards Newsletter.

Makishima, A., Nakamura, E. & Nakano, T. (1999). Determination of zirconium, niobium, hafnium and tantalum at ng g^–1 levels in geological materials by direct nebulisation of sample HF solution into Fl-ICP-MS. Geostandards Newsletter 23, 7–20.[Web of Science]

Maruyama, S. & Liou, J. G. (1988). Petrology of Franciscan metabasites along the jadeite–glaucophane type facies series, Cazadero, California. Journal of Petrology 29, 1–37.[Abstract/Free Full Text]

Maruyama, S. & Liou, J. G. (1989). Possible depth limit for underplating by a seamount. Tectonophysics 160, 327–337.[CrossRef][Web of Science]

McCulloch, M. T. & Gamble, J. A. (1991). Geochemical and geodynamical constraints on subduction zone magmatism. Earth and Planetary Science Letters 102, 358–374.[CrossRef][Web of Science]

McGetchin, T. R., Smith, D. C., Ehrenberg, S. N., Roden, M. & Wilshire, H. G. (1977). Navajo Kimberlite and Minette Guide. Second International Kimberlite Conference Field Excursion Guide. Santafe, NM. Washington, DC: American Geophysical Union, 40 pp.

Moore, D. E. & Liou, J. G. (1979). Mineral chemistry of some Franciscan blueschist metasediments from the Diablo Range, California Part I. Geological Society of America Bulletin 90, 1089–1091.[Abstract/Free Full Text]

Moriguti, T., Makishima, A. & Nakamura, E. (2004). Determination of lithium contents in silicates by isotope dilution ICP-MS and its evaluation by isotope dilution thermal ionisation mass spectrometry. Geostandards Newsletter 28, 371–382.[CrossRef]

Naeser, C. W. (1971). Geochronology of the Navajo–Hopi Diatremes, Four Corners Area. Journal of Geophysical Research 76, 4978–4985.

Nakamura, E. & Kushiro, I. (1998). Trace element diffusion in jadeite and diopside melt at high pressure and its geochemical implication. Geochimica et Cosmochimica Acta 62, 3151–3160.[CrossRef][Web of Science]

Nakamura, E., Makishima, A., Moriguti, T., Kobayashi, K., Sakaguchi, C., Yokoyama, T., Tanaka, R., Kuritani, T. & Takei, H. (2002). Comprehensive geochemical analyses of small amounts (<100 mg) of extraterrestrial samples for the analytical competition related to the sample-return mission, MUSES-C. The Institute of Space and Astronautical Science Report 16.

Nakano, T. & Nakamura, E. (1998). Static multicollection of Cs₂BO₂⁺ ions for precise boron isotope analysis with positive thermal ionization mass spectrometry. International Journal of Mass Spectrometry and Ion Processes 176, 13–21.

Okamoto, K. & Maruyama, S. (1999). The high-pressure synthesis of lawsonite in MORB + H₂O system. American Mineralogist 94, 362–373.

Peacock, S. M. (1993). The importance of blueschist–eclogite dehydration reactions in subducting oceanic crust. Geological Society of America Bulletin 105, 684–694.[Abstract/Free Full Text]

Poli, S. & Schmidt, M. W. (2002). Petrology of subducted slabs. Annual Review of Earth and Planetary Sciences 30, 207–235.[CrossRef][Web of Science]

Roden, M. F. (1981). Origin of coexisting minette and ultramafic breccia, Navajo Volcanic Field. Contributions to Mineralogy and Petrology 77, 195–206.[CrossRef][Web of Science]

Roden, M. F., Smith, D. & McDowell, F. W. (1979). Age and extent of potassic volcanism on the Colorado Plateau. Earth and Planetary Science Letters 43, 279–284.[CrossRef][Web of Science]

Roden, M. F., Smith, D. & Murthy, V. R. (1990). Chemical constraints on lithosphere composition and evolution beneath the Colorado Plateau. Journal of Geophysical Research 95, 2811–2831.

Scambelluri, M. & Philippot, P. (2001). Deep fluids in subduction zones. Lithos 55, 213–227.[CrossRef][Web of Science]

Schmidt, M. W. & Poli, S. (1998). Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation. Earth and Planetary Science Letters 163, 361–379.[CrossRef][Web of Science]

Selverstone, J., Pun, A. & Condie, K. C. (1999). Xenolithic evidence for Proterozoic crustal evolution beneath the Colorado Plateau. Geological Society of America Bulletin 111, 590–606.[Abstract/Free Full Text]

Shibata, T. & Nakamura, E. (1997). Across arc variation of isotope and trace element compositions from Quaternary basaltic volcanic rocks in northeastern Japan: implication for interaction between subducted oceanic slab and mantle wedge. Journal of Geophysical Research 102, 8051–8064.[CrossRef]

Shreve, R. L. & Cloos, M. (1986). Dynamics of sediment subduction, mélange formation, and prism accretion. Journal of Geophysical Research 91, 10229–10245.[CrossRef]

Smith, D. & Levy, S. (1976). Petrology of the Green Knobs diatreme and implications for the upper mantle below the Colorado Plateau. Earth and Planetary Science Letters 29, 107–125.[CrossRef][Web of Science]

Smith, D. & Zientek, M. (1979). Mineral chemistry and zoning in eclogite inclusions from Colorado Plateau diatremes. Contributions to Mineralogy and Petrology 69, 119–131.[CrossRef][Web of Science]

Smith, D., Connelly, J. N., Manser, K., Moser, D. E., Housh, T. B., McDowell, F. W. & Mack, L. E. (2004). Evolution of Navajo eclogites and hydration of the mantle wedge below the Colorado Plateau, southwestern United States. Geochemistry, Geophysics, Geosystems 5, 2003GC000675.

Sneeringer, M., Hart, S. R. & Shimizu, N. (1984). Strontium and samarium diffusion in diopside. Geochimica et Cosmochimica Acta 48, 1589–1609.[CrossRef][Web of Science]

Sorensen, S. S. (1988). Petrology of amphibolite-facies mafic and ultramafic rocks from the Catalina Schist, southern California: metasomatism and migmatization in a subduction zone metamorphic setting. Journal of Metamorphic Geology 6, 405–435.[Web of Science]

Sorensen, S. S. (1991). Petrogenetic significance of zoned allanite in garnet amphibolites from a paleo-subduction zone: Catalina Schist, southern California. American Mineralogist 76, 589–601.[Abstract]

Sorensen, S. S. & Grossman, J. N. (1989). Enrichment of trace elements in garnet amphibolites from a paleo-subduction zone: Catalina Schist, southern California. Geochimica et Cosmochimica Acta 53, 3155–3177.[CrossRef][Web of Science]

Sorensen, S. S. & Grossman, J. N. (1993). Accessory minerals and subduction zone metasomatism: a geochemical comparison of two mélanges (Washington and California, U.S.A.). Chemical Geology 110, 269–297.[CrossRef][Web of Science]

Spandler, C., Hermann, J., Richard, A. & Mavrogenes, J. (2003). Redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies; implications for deep subduction-zone processes. Contributions to Mineralogy and Petrology 146, 205–222.[CrossRef][Web of Science]

Stalder, R., Foley, S. F., Brey, G. P. & Horn, I. (1998). Mineral–aqueous fluid partitioning of trace elements at 900–1200°C and 3·0–5·7 GPa: new experimental data for garnet, clinopyroxene, and rutile, and implications for mantle metasomatism. Geochimica et Cosmochimica Acta 62, 1781–1801.[CrossRef][Web of Science]

Stracke, A., Bizimis, M. & Salters, V. J. M. (2003). Recycling oceanic crust: quantitative constraints. Geochemistry, Geophysics, Geosystems 4, doi: 10.1029/2001GC000223.

Sun, S. S. & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalt: implications for mantle composition and process. In: Sun, S. S. & McDonough, W. F. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 313–345.

Takei, H. (2002). Development of precise analytical techniques for major and trace element concentrations in rock samples and their applications to the Hishikari Gold Mine, southern Kyushu, Japan. PhD thesis, Okayama University.

Taylor, R. N. & Nesbitt, R. W. (1998). Isotopic characteristics of subduction fluids in intra-oceanic setting, Izu–Bonin Arc, Japan. Earth and Planetary Science Letters 164, 79–98.[CrossRef][Web of Science]

Thöni, M. & Jagoutz, E. (1992). Some new aspects of dating eclogites in orogenic belts: Sm–Nd, Rb–Sr, and Pb–Pb isotopic results from the Austroalpine Saualpe and Koralpe type-locality (Carinthia/Styria, southeastern Austria). Geochimica et Cosmochimica Acta 56, 347–368.[CrossRef][Web of Science]

Tilton, G., Ames, L., Schertl, H. P. & Schreyer, W. (1997). Reconaissance isotopic investigations on rocks of an undeformed granite contact within the coesite-bearing unit of the Dora Maira Massif. Lithos 41, 25–36.[CrossRef][Web of Science]

Tribuzio, R., Messiga, B., Vannucci, R. & Bottazzi, P. (1996). Rare earth element redistribution during high-pressure–low-temperature metamorphism in ophiolitic Fe-gabbros (Liguria, northwestern Italy): implications for light REE mobility in subduction zones. Geology 24, 711–714.[Abstract/Free Full Text]

Tual, E., Jahn, B. M., Bougault, H. & Joron, J. L. (1985). Geochemistry of basalts from hole 504B, leg 83, Costa Rica rift. In: Anderson, R. N., Honnorez, J., et al. (eds) Initial Reports of the Deep Sea Drilling Project, 83. Washington, DC: US Government Printing Office, pp. 201–214.

Usui, T. (2004). A geochemical study of the origin of eclogite xenoliths from the Colorado Plateau, southwestern United States: implications for the evolution of subducted oceanic crust. PhD thesis, Okayama University.

Usui, T., Kobayashi, K. & Nakamura, E. (2002). U–Pb isotope systematics of micro-zircon inclusions: implications for the age and origin of eclogite xenolith from the Colorado Plateau. Proceedings of the Japan Academy 78, 51–56.[CrossRef]

Usui, T., Nakamura, E., Kobayashi, K., Maruyama, S. & Helmstaedt, H. (2003). Fate of the subducted Farallon plate inferred from eclogite xenoliths in the Colorado Plateau. Geology 31, 589–592.[Abstract/Free Full Text]

van Keken, P. E., Hauri, E. H. & Ballentine, C. J. (2002). Mantle mixing: the generation, preservation, and destruction of chemical heterogeneity. Annual Review of Earth and Planetary Sciences 30, 493–525.[CrossRef][Web of Science]

van Orman, J. A., Grove, T. L. & Shimizu, N. (1998). Rare earth element diffusion in diopside and disequilibrium melting in the mantle. EOS Transactions, American Geophysical Union 79, S371.

Wasserburg, G. J., Jacobsen, S. B., DePaolo, D. J., McCulloch, M. T. & Wen, T. (1981). Precise determination of Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochimica et Cosmochimica Acta, 45, 2311–2323.[CrossRef][Web of Science]

Wendlandt, E., DePaolo, D. J. & Baldridge, S. W. (1993). Nd and Sr isotope chronostratigraphy of Colorado Plateau lithosphere: implications for magmatic and tectonic underplating of the continental crust. Earth and Planetary Science Letters 116, 23–43.[CrossRef][Web of Science]

White, W. M., Hofmann, A. W. & Puchelt, H. (1987). Isotope geochemistry of Pacific mid-ocean ridge basalt. Journal of Geophysical Research 92, 4881–4893.

Yokoyama, T., Makishima, A. & Nakamura, E. (1999). Separation of thorium and uranium from silicate rock samples using two commercial extraction chromatographic resins. Analytical Chemistry 71, 135–141.

Yokoyama, T., Kobayashi, K., Kuritani, T. & Nakamura, E. (2003). Mantle metasomatism and rapid ascent of slab components beneath island arcs: evidence from ²³⁸U–²³⁰Th–²²⁶Ra disequilibria of Miyakejima volcano, Izu arc, Japan. Journal of Geophysical Research 108(B7), doi: 10.1029/2002JB002103.

Yoshikawa, M. & Nakamura, E. (1993). Precise isotope determination of trace amounts of Sr in magnesium-rich samples. Journal of Mineralogy, Petrology and Economic Geology 88, 548–561.

Zack, T., Foley, S. F. & Rivers, T. (2002a). Equilibrium and disequilibrium trace element partitioning in hydrous eclogites (Trescolmen, Central Alps). Journal of Petrology 43, 1947–1974.[Abstract/Free Full Text]

Zack, T., Kronz, A., Foley, S. F. & Rivers, T. (2002b). Trace element abundances in rutiles from eclogites and associated garnet mica schists. Chemical Geology 184, 97–122.[CrossRef][Web of Science]

Zack, T., Tomascak, P. B., Rudnick, R. L., Dalpé, C. & McDonough, W. F. (2003). Extremely light Li in orogenic eclogites: the role of isotope fractionation during dehydration in subducted oceanic crust. Earth and Planetary Science Letters 208, 279–290.[CrossRef][Web of Science]

Zack, T., Rivers, T., Brumm, R. & Kronz, A. (2004). Cold subduction of oceanic crust: implications from a lawsonite eclogite from Dominican Republic. European Journal of Mineralogy 16, 909–916.[Abstract/Free Full Text]

Zindler, A. & Hart, S. (1986). Chemical geodynamics. Annual Review of Earth and Planetary Sciences 14, 493–571.[CrossRef][Web of Science]

Zuleger, E., Alt, J. C. & Erzinger, J. (1995). Primary and secondary variations in major and trace element geochemistry of the lower sheeted dike complex: hole 504B, leg 140. In: Becker, K., Graham, A. G., et al. (eds) Proceedings of the Ocean Drilling Program, Scientific Results, 137/140. College Station, TX: Ocean Drilling Program, pp. 65–80.

CiteULike    Connotea    Del.icio.us    What's this?

This article has been cited by other articles:

				M. Brown Metamorphic patterns in orogenic systems and the geological record Geological Society, London, Special Publications, January 1, 2009; 318(1): 37 - 74. [Abstract] [Full Text] [PDF]

				T. Kuritani, T. Yokoyama, and E. Nakamura Generation of Rear-arc Magmas Induced by Influx of Slab-derived Supercritical Liquids: Implications from Alkali Basalt Lavas from Rishiri Volcano, Kurile Arc J. Petrology, July 1, 2008; 49(7): 1319 - 1342. [Abstract] [Full Text] [PDF]

				H. Cheng, E. Nakamura, K. Kobayashi, and Z. Zhou Origin of atoll garnets in eclogites and implications for the redistribution of trace elements during slab exhumation in a continental subduction zone American Mineralogist, July 1, 2007; 92(7): 1119 - 1129. [Abstract] [Full Text] [PDF]

				C. G. Mattinson, C. G. Mattinson, J. L. Wooden, J. G. Liou, D. K. Bird, and C. L. Wu Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, Western China Am J Sci, November 1, 2006; 306(9): 683 - 711. [Abstract] [Full Text] [PDF]

This Article Abstract FREE Full Text (PDF) All Versions of this Article: 47/5/929    most recent egi101v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (14) Request Permissions Google Scholar Articles by USUI, T. Articles by HELMSTAEDT, H. Search for Related Content GeoRef GeoRef Citation Social Bookmarking          What's this?

Online ISSN 1460-2415 - Print ISSN 0022-3530

Copyright © 2009 Oxford University Press

Oxford Journals Oxford University Press

• Site Map
• Privacy Policy
• Frequently Asked Questions

Other Oxford University Press sites: Oxford University Press Oxford Journals China Oxford Journals Japan American National Biography Booksellers' Information Service Children's Fiction and Poetry Children's Reference Corporate & Special Sales Dictionaries Dictionary of National Biography Digital Reference English Language Teaching Higher Education Textbooks Humanities International Education Unit Law Medicine Music Online Products Oxford English Dictionary Reference Rights and Permissions Science School Books Social Sciences Very Short Introductions World's Classics