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Journal of Petrology | Volume 44 | Number 7 | Pages 1237-1246 | 2003
© Oxford University Press 2003
Evolution of Low-Al Orthopyroxene in the Horoman Peridotite, Japan: an Unusual Indicator of Metasomatizing Fluids
1 RESEARCH SCHOOL OF EARTH SCIENCES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
2 GRADUATE SCHOOL OF NATURAL SCIENCE AND TECHNOLOGY, KANAZAWA UNIVERSITY, KAKUMA, KANAZAWA 920-1192, JAPAN
3 DEPARTMENT OF EARTH SCIENCES, KANAZAWA UNIVERSITY, KAKUMA, KANAZAWA 920-1192, JAPAN
* Corresponding author. Telephone: +81-76-264-5723. Fax: +81-76-264-5746. E-mail: moripta{at}kenroku.kanazawa-u.ac.jp
RECEIVED MAY 1, 2001; ACCEPTED FEBRUARY 20, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONSREFERENCES |
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Unusually alumina-poor orthopyroxene is found in a spinel peridotite from the Horoman Peridotite Complex, Japan. Al2O3, Cr2O3 and CaO contents in the low-Al orthopyroxene (named Low-Al OPX hereafter) are <0·25 wt %, <0·04 wt % and <0·3 wt %, respectively, and are distinctively lower than those in orthopyroxene porphyroclasts. The Low-Al OPX occurs in two modes, both at the margin of olivine. The first mode of occurrence is as the rim of a large orthopyroxene porphyroclast in contact with olivine. This type of Low-Al OPX occurs only locally (15 µm x 45 µm), and the orthopyroxene rim in contact with olivine more commonly has normal Al2O3 contents (>2 wt %). In the second mode of occurrence, the Low-Al OPX occurs as a thin film, 5 µm x 50 µm in dimension, at a grain boundary between olivine and clinopyroxene. Trace element compositions of porphyroclast clinopyroxene in the sample indicate that the sample having the Low-Al OPX underwent metasomatism although there are no hydrous minerals around the Low-Al OPX. Petrographic observations and trace element compositions of clinopyroxene combined with an inferred P–T history of the Horoman peridotite suggest that the Low-Al OPX was formed through a very local reaction between peridotite and invasive fluids, probably formed by dehydration of a subducted slab, in a late stage of the history of the Horoman peridotite. Crystallization of orthopyroxene, representing addition of silica to mantle lherzolite via a CO2 + H2O-bearing fluid phase, is a mechanism for metasomatic alteration of mantle wedge peridotite.
KEY WORDS: Horoman Peridotite Complex; low-Al orthopyroxene; metasomatism; mantle wedge
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONSREFERENCES |
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The chemical compositions of pyroxene in peridotite provide strong constraints on mantle processes such as partial melting, metasomatism, deformation and P–T trajectories. Unusually alumina-poor orthopyroxene (called Low-Al OPX hereafter) is found in a spinel peridotite from the Horoman Peridotite Complex, Japan (Morishita & Arai, 2001). Low-Al orthopyroxene in peridotite can be formed in several ways: (1) by an extremely high degree of partial melting (e.g. Jaques & Chappell, 1980); (2) high-pressure and moderate temperature equilibration with garnet (e.g. Zhang et al., 1995); (3) low-temperature equilibration with hydrous minerals (e.g. Smith, 1995; Smith & Riter, 1997); (4) dehydration of serpentinite (e.g. Arai, 1974; Trommsdorff et al., 1998); (5) extremely low-temperature equilibration within a spinel peridotite mineral assemblage; (6) fluid–peridotite interaction (e.g. Smith et al., 1999).
In the Horoman Peridotite Complex, Ozawa & Takahashi (1995), Takazawa et al. (1996) and Yoshikawa & Nakamura (2000) found compositional zoning of pyroxene (and plagioclase) in peridotite and inferred a decompressional P–T path from the garnet peridotite to the plagioclase peridotite stability fields. Ozawa (1997) proposed two stages of decompressional history to explain the observed differences in Al zoning profiles in orthopyroxene. The presence of low-alumina cores and higher alumina outer zones in large pyroxene porphyroclasts was ascribed to decompression with little temperature drop within the garnet-lherzolite to spinel-lherzolite transition (Ozawa & Takahashi, 1995; Takazawa et al., 1996). Rapid cooling of the complex at low pressure was recorded as a sharp compositional zoning at the rims of pyroxene (Ozawa & Takahashi, 1995; Takazawa et al., 1996). These processes, however, can not explain the occurrence of distinctive Low-Al OPX in the Horoman peridotite (Morishita & Arai, 2001). In this paper, these occurrences are linked to other evidence for a role for aqueous fluids in a late stage of the history of the Horoman peridotite (Hirai & Arai, 1987; Arai & Takahashi, 1989; Yoshikawa et al., 1993; Yoshikawa & Nakamura, 2000; Kaneoka et al., 2001; Matsumoto et al., 2001).
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GEOLOGICAL BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONSREFERENCES |
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The Horoman Peridotite Complex is located at the southern end of the Main Zone of the Hidaka metamorphic belt (e.g. Niida, 1984; Komatsu et al., 1986), and is 8 km x 10 km in plan and more than 3 km in thickness (e.g. Igi, 1953; Komatsu & Nochi, 1966; Niida, 1984; Sawaguchi, 1999). The complex consists of various kinds of layered peridotite with small amounts of mafic rocks (e.g. Igi, 1953; Komatsu & Nochi, 1966; Niida, 1984). On the basis of petrography and mineralogy, the Horoman peridotites have been divided into three suites (Takahashi, 1991, 1992). The first is the Main Harzburgite–Lherzolite suite (MHL), which has typical residual characteristics in major elements resulting from various degrees of magma extraction (Obata & Nagahara, 1987; Frey et al., 1991; Takahashi, 1991, 1992, 1997; Yoshida & Takahashi, 1997; Takazawa et al., 2000; Yoshikawa & Nakamura, 2000). The second is the Spinel-rich Dunite–Wehrlite suite (SDW), which is a cumulate from magma (Takahashi, 1991, 1992, 1997; Yoshida & Takahashi, 1997). The third is the Banded Dunite–Harzburgite suite (BDH), which is also a cumulate, but from a high-Mg, high-Cr magma such as high-Mg andesite (Takahashi, 1991, 1992). Komatsu & Nochi (1966) and Niida (1984) divided the complex into two zones, the Upper and Lower Zones. The Upper Zone is characterized by an abundance of mafic layers and by sharp lithological boundaries. On the other hand, the Lower Zone is characterized by gradational lithological boundaries. Ozawa & Takahashi (1995) and Ozawa (1997) suggested that a difference exists in P–T trajectory between the Upper and Lower Zones. Ozawa & Takahashi (1995) proposed that the Horoman peridotite ascended as a mantle diapir from the garnet peridotite stability field, taking a higher-temperature decompression path (T = 1150°C and P > 2·0 GPa) for the Upper Zone and a lower-temperature path (T = 900°C and P > 2·0 GPa) for the Lower Zone. Thus a high-temperature decompression path provides an explanation for pyroxene + spinel symplectites after garnet and zonation of pyroxene porphyroclasts from lower-Al2O3 cores to higher-Al2O3 outer zones to rims of decreasing Al2O3 (equated with entry into the plagioclase-lherzolite stability field and cooling at low pressure) (Ozawa & Takahashi, 1995; Takazawa et al., 1996).
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PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONSREFERENCES |
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The studied sample is a symplectite-bearing spinel lherzolite of the MHL suite from the Upper Zone (Fig. 1). Relatively depleted peridotites ranging from spinel lherzolite to harzburgite occur as a thin layer (<1 m in thickness) in a plagioclase-bearing peridotite, although mineral chemical compositions gradually change from plagioclase lherzolite to harzburgite through spinel lherzolite. The studied sample has a weak porphyroclastic texture. Orthopyroxene, clinopyroxene and spinel occur as porphyroclasts. A very small extent of serpentinization occurs along grain boundaries. The symplectite is a spherical aggregate of very fine-grained orthopyroxene, clinopyroxene and spinel, and it occurs within a lenticular fine-grained aggregate of the same minerals (e.g. Takahashi & Arai, 1989). In the sample, symplectite-bearing fine-grained aggregate forms a thin layer, 7 mm in thickness, parallel to lithological boundaries between plagioclase-bearing lherzolite and spinel lherzolite. The mineral mode and whole-rock compositions of the studied sample would be, therefore, strongly changed depending on the propotion of the symplectite-bearing layer in the analysis. Olivine (<1 mm across) has kink bands, and clinopyroxene (<0·5 mm across) exhibits exsolution of orthopyroxene. Discrete spinel (<1 mm) is anhedral.
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The Low-Al OPX has two modes of occurrence, both at the margin of olivine (Fig. 2). The first is as the rim of a large orthopyroxene porphyroclast (6 mm across) in contact with olivine. This type of Low-Al OPX occurs only locally and most of the orthopyroxene rim in contact with olivine usually has normal Al2O3 contents (>2 wt %). The Low-Al OPX of this mode of occurrence (the low-Al zone hereafter) is 15 µm x 45 µm in dimension but it cannot be recognized either in the backscattered electron image (Fig. 2a) or under the microscope. The long axis of the orthopyroxene porphyroclast is not concordant with the foliation of the peridotite. There are no exsolution lamellae of clinopyroxene or spinel within or around the low-Al zone. Other parts of the porphyroclast have prominent clinopyroxene exsolution lamellae. In the second mode of occurrence, the Low-Al OPX occurs as a thin film, 5 µm x 50 µm in dimension, at the grain boundary between olivine and clinopyroxene (film-shaped opx hereafter) (Fig. 2c). We found two grains of the film-shaped opx around the same clinopyroxene grain, both in contact with olivine. It should be noted that we cannot find any hydrous minerals where the Low-Al OPX occurs.
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MINERAL COMPOSITIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONSREFERENCES |
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Most of the constituent minerals in the studied sample were analyzed using a JEOL 6400 SEM fitted with Link Energy Dispersive Detector at the Electron Microscopy Unit, Australian National University (ANU). The analyses were performed under an accelerating voltage of 15 kV and a beam current of 1 nA, using a spot mode (
2–3 µm diameter beam). Total energy-dispersive spectrometry (EDS) live-time was 100 s. JEOL software using ZAF corrections was employed. Chemical compositions of the Low-Al OPXs and olivine were analyzed using a JEOL JXA-8800 at the Cooperative Center of Kanazawa University. The analyses were performed under an accelerating voltage of 15 kV and a beam current of 15 nA using 3 µm diameter beam; X-ray peaks of Ti, Al, Cr, Mn, Ca, Na, K and Ni were counted for 50 s (other elements for 10 s). JEOL software using ZAF corrections was also employed. Representative analyses of the rock-forming minerals are shown in Table 1. Trace element compositions [rare earth elements (REE), Sr, Zr, Ti and Y] of both core and rim of a porphyroclast clinopyroxene in the same sample were determined by laser ablation (193 nm ArF excimer)–inductively coupled plasma mass spectrometry (LA–ICP-MS) using an Agilent 7500S system at the Research School of Earth Sciences, ANU, according to the method of Eggins et al. (1998) (Table 2). Each analysis was performed by ablating a large-diameter spot (230 µm) at 5 pulses/s to produce an ablation rate of 0·1–0·2 µm/pulse. The NIST 612 glass was used as the primary calibration standard and was analyzed at the beginning and end of each batch of more than seven unknowns, with a linear drift correction applied between each calibration. Data reduction was facilitated using Ca as the internal standard, based on CaO contents obtained by electron probe microanalysis (EPMA). Eggins et al. (1998) reported agreement between LA–ICP MS and solution ICP-MS data at ANU for orthopyroxene and clinopyroxene, ranging over eight orders of magnitude. Accuracy of the solution ICP-MS system at ANU was also reported by Eggins et al. (1997).
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The forsterite content and NiO wt % of olivine are 91·5 and 0·4, respectively (Fig. 2). The Cr-number [= Cr/(Cr + Al) atomic ratios] is 0·29 in the core of a discrete spinel (1 mm) and 0·23 in symplectite spinel. Clinopyroxene porphyroclasts are very low in TiO2 and Na2O. These features suggest that the peridotite is a refractory lherzolite in the MHL suite. Pyroxene porphyroclasts are chemically zoned from core to rim (Table 1). For example, the Al2O3 content in a large clinopyroxene porphyroclast decreases from core (3·7 wt %) to rim (2·8 wt %) (Fig. 3). The Al2O3 content in symplectite clinopyroxene is 2·8 wt %. The Al2O3, Cr2O3 and CaO contents of symplectite orthopyroxene are 2·4, 0·5 and 0·4 wt %, respectively.
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The Al2O3, Cr2O3 and CaO contents of the Low-Al OPX are lower than 0·25 wt % and are distinctively lower than those in orthopyroxene porphyroclast (Fig. 3 and Table 1). The Al2O3 content of the orthopyroxene porphyroclast that has the low-Al zone systematically decreases from core (3 wt %) to rim (>1·8 wt %), but abruptly decreases from >2 wt % to <0·3 wt % within a few micrometres of the low-Al zone (Figs 2b and 3a). Cr2O3 and CaO contents change in the same way, decreasing gradually from core to rim and dropping abruptly adjacent to the low-Al zone (Fig. 3b). The Al2O3 content of the clinopyroxene in contact with film-shaped orthopyroxene is >3·7 wt % (Fig. 2).
No significant differences in trace element compositions are found between core and rim of the cpx porphyroclast (Fig. 4). The primitive mantle-normalized pattern (PM pattern hereafter) for the cpx porphyroclast shows slight depletion of light REE (LREE) relative to heavy REE (HREE), but an abrupt change of slope at Zr leading to a high La/Yb ratio. It shows slightly positive anomalies of Ti and Sr.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONSREFERENCES |
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Genesis of the low-Al orthopyroxene
The bulk composition of the peridotite with the low-Al orthopyroxene is sufficiently ‘fertile’ to have the four-phase lherzolite assemblage of olivine + orthopyroxene + clinopyroxene + aluminous spinel. It is, however, refractory or residual in terms of the low content of TiO2 and Na2O in the pyroxene solid solutions. The localized occurrence and extremely low Al2O3 and CaO contents of the orthopyroxene described above, together with the sharp contacts with the major orthopyroxene cores, suggest that an explanation is required other than by continuous reaction along an upwelling P–T path. Previous work has identified localized metasomatism by aqueous fluids as a late event in the history of the Horoman peridotite (Hirai & Arai, 1987; Arai & Takahashi, 1989; Takahashi et al., 1989; Yoshikawa et al., 1993; Yoshikawa & Nakamura, 2000; Kaneoka et al., 2001; Matsumoto et al., 2001). In the following section, the local occurrence of extremely Low-Al OPX is interpreted to be the consequence of penetration of aqueous fluids.
Evidence for aqueous fluid metasomatism in the Horoman peridotite and evolution of the fluid
The PM trace element patterns of a clinopyroxene porphyroclast in the sample show high La/Yb and a positive Sr anomaly. The enrichment of highly incompatible elements has been commonly found in whole-rock compositions as well as clinopyroxene porphyroclasts in the Horoman peridotites (Takazawa et al., 1992, 1996; Yoshikawa & Nakamura, 2000). Yoshikawa & Nakamura (2000) reported positive anomalies of Sr and Pb for whole-rock compositions in some Horoman peridotites and suggested that this was caused by mantle metasomatism involving aqueous fluid derived from crustal materials, because Sr and Pb are relatively more mobile in aqueous fluid than other elements of similar incompatibility during magmatic processes (Kogiso et al., 1997; Shibata & Nakamura, 1997). Petrographic evidence for hydrous fluid access is generally provided by the local occurrence of phlogopite and more widespread occurrence of pargasitic amphibole (Niida, 1975; Arai & Takahashi, 1989; Takahashi et al., 1989). The trace element compositions of the clinopyroxene porphyroclast suggest that the studied sample also underwent metasomatism by aqueous melt or fluid during its upwelling (decompression path) even though no hydrous metasomatic minerals, such as phlogopite and amphibole, crystallized in this particular sample.
Yoshikawa et al. (1993) examined Rb/Sr systematics in a phlogopite-bearing lherzolite from the Horoman Complex and inferred a higher 87Sr/86Sr (0·7039) of the fluid, relative to phlogopite-free lherzolite with 87Sr/86Sr <0·7028. The differences in Sr-isotope ratios were attributed to a subducted oceanic crust component. This hypothesis was further supported by more recent trace element geochemical data (Yoshikawa & Nakamura, 2000) and noble gas data (Matsumoto et al., 2001). Hirai & Arai (1987) presented direct evidence for the former presence of fluid inclusions in the Horoman peridotite by their descriptions of serpentine + brucite + magnesite inclusions in olivine. These were interpreted to result from the original entrapment of H2O + CO2 fluids, rich in dissolved solutes.
A complementary approach to the problem is to seek the composition of fluids that might be derived from subducted oceanic crust by dehydration reactions. High-pressure veins in eclogite, inferred to be precipitated from such fluids, are rich in SiO2, Al2O3 and Na2O relative to CaO, MgO and FeO (Becker et al., 1999). An experimental approach (Brenan et al., 1995) shows that solute in fluids equilibrated with garnet and clinopyroxene at 900°C, 2 GPa are high in SiO2 and Al2O3. Experimental studies aimed at characterizing such fluids as they equilibrated with and moved through peridotite show that the fluids have high SiO2 contents (Nakamura & Kushiro, 1974; Schneider & Eggler, 1986; Ayers et al., 1997) although it must be assumed that the activity of silica in the fluids is buffered by the coexisting olivine + orthopyroxene of the peridotite host.
The evidence of localized fluid access to the Horoman peridotite suggests that fluid compositions may evolve in a local situation where amphibole or phlogopite are precipitating with loss of K2O and Al2O3 from the fluid (Fluid I), leading to enrichment of SiO2 in the residual fluid (Fluid II). The SiO2 activity of the fluid will be, however, maintained at an approximately constant value on the olivine + orthopyroxene buffer by addition of silica to olivine to form orthopyroxene or dissolution of orthopyroxene to form olivine + SiO2-bearing fluid. We attribute the presence of the extremely low-Al orthopyroxene in the spinel lherzolite to reaction with and precipitation from a SiO2-rich aqueous fluid. It is not possible to make a quantitative estimate of the amount of silica added, given the variability of compositions in the finely layered rocks.
Stage of the fluid supply
The dominant orthopyroxene and clinopyroxene compositions are those approaching equilibration at T = 950°C and probably P > 1 GPa (spinel-lherzolite stability field) for the core compositions (Wells, 1977). We deduced that the temperature for the process was <900°C based on the very low CaO content of orthopyroxene juxtaposed with clinopyroxene (Wells, 1977) and on the clear evidence that the new orthopyroxene remains in sharp compositional contrast with no evidence for adjustment of neighboring orthopyroxene, clinopyroxene or spinel compositions. The extremely Low-Al OPX is inferred to have formed from olivine in both cases described. It grew with compositional discontinuity where orthopyroxene was in contact with olivine. The presence of the Low-Al OPX in mantle peridotite suggests that quenching of the geochemical signatures of the Horoman peridotite occurred after formation of the Low-Al OPX, possibly as a result of rapid cooling. Otherwise, the chemical characteristics of the Low-Al OPX would have been erased and equilibrated with the Al-rich porphyroclast at mantle conditions (Smith & Riter, 1997). Rapid cooling of the complex has been recorded as a sharp compositional zoning at the rim of pyroxene in plagioclase lherzolite (Ozawa & Takahashi, 1995; Takazawa et al., 1996). Furthermore, Yoshikawa et al. (1993) and Kaneoka et al. (2001) obtained 23 Ma (Rb–Sr method) and 21 Ma (Ar–Ar method), respectively, as an age of the metasomatic event in the Horoman Complex. Thus, we conclude that the Low-Al OPX was also formed as a late event in the history of the Horoman Complex (20 Ma), probably within the plagioclase-lherzolite stability field (Fig. 5).
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If we are correct in our inference that local orthopyroxene precipitation is from reaction involving aqueous fluid, then some explanation must be given for the absence of pargasitic or even tremolitic amphibole in rocks that clearly contain orthopyroxene, clinopyroxene, aluminous spinel + olivine. The stability field of pargasitic–edenitic hornblende in refractory peridotite extends to
1000°C at 1 GPa (Fig. 5) (Wallace & Green, 1991; Niida & Green, 1999) so that the absence of pargasite must be attributed to low H2O activity in the fluid phase rather than aspects of the bulk-rock composition. Conversely, if CO2 is a major component of the fluid (Hirai & Arai, 1987), then the P–T conditions and fluid compositions must be on the low-pressure side of the carbonation reaction (Fig. 5) (Wyllie, 1978; Wallace & Green, 1988). The reaction moves to higher pressure with decreasing CO2 activity (Wyllie, 1978), providing a window or P–T field in which the low-Al2O3 orthopyroxene is in equilibrium with olivine + clinopyroxene + Al–Cr spinel and a fluid phase. Neither amphibole, phlogopite nor dolomite is found in the reaction producing the Low-Al OPX in the sample studied, and it may be inferred that fluid compositions (solutes) other than CO2 + H2O were high enough to lower H2O and CO2 activities.
Possible significance of Low-Al orthopyroxene in the mantle wedge
The observation of a process occurring in the Horoman peridotite may be applied more generally to speculate on mantle behavior in convergent margins, as the late-stage evolution and crustal emplacement of the Horoman peridotite are inferred to have occurred in a mantle wedge setting (Yoshikawa & Nakamura, 2000). We have inferred migration and metasomatism by fluids that may contain CO2 + H2O but with H2O activity and CO2 activity both lowered by other components. Smith et al. (1999) interpreted an orthopyroxenite xenolith from a diatreme in the Colorado Plateau to be the product of reaction between peridotite and fluid, possibly derived from a subducted slab. The orthopyroxenite is characterized by low contents of Al2O3, Cr2O3, TiO2 and CaO. Peridotite xenoliths from Lihir, near Papua New Guinea, are postulated to be samples of sub-arc mantle (McInnes et al., 2001) and contain orthopyroxenite veins with fibrous, radiating orthopyroxene (0·3 wt % Al2O3). These samples provide evidence of hydraulic fracturing and metasomatism involving slab-derived hydrous fluids (McInnes et al., 2001). Blatter & Carmichael (1998) described orthopyroxene with low Al2O3 contents in hornblende peridotite xenoliths from Central Mexico. In these samples, amphibole is common and the mineralogy reflects high oxygen fugacities, and Blatter & Carmichael argued for alteration of sub-arc mantle by slab-derived fluids.
Arai & Kida (2000) described sub-arc mantle xenoliths from Iraya volcano (Philippines), and classified them into ‘coarse-grained’ (C-type) and ‘fine-grained’ (F-type). Orthopyroxene in the F-type occurs as radial aggregations of acicular to prismatic crystals, and is markedly lower in Al2O3, Cr2O3 and CaO than that in the C-type. Arai & Kida suggested that the C-type xenoliths are products of either dehydration of subducted serpentinite or metasomatic transformation of C-type peridotite by SiO2-rich fluid.
Orthopyroxene characterized by low Al2O3 contents has also been reported from apatite-bearing domains in the Finero phlogopite-peridotite massif, by Zanetti et al. (1999). Those workers interpreted the Finero phlogopite-peridotite to be highly metasomatized by slab-derived H2O–CO2-bearing silicate melts, and the apatite-bearing domain is thought to be the result of carbonate fluids resulting from increased CO2/H2O by crystallization of hydrous phases (phlogopite and amphibole) (Zanetti et al., 1999). Yaxley et al. (1991) reported secondary orthopyroxene in carbonatite-metasomatized mantle peridotite xenoliths in western Victoria, Australia. The secondary orthopyroxene occurs as thin rims on apatite and olivine within the lherzolite mineralogy and was interpreted to result from reaction of olivine with a late-stage fluid evolved from carbonatites through decarbonation reactions such as Enstatite + (dolomite)melt = Forsterite + Diopside + (CO2)fluid. The Victorian example is in an intra-plate rather than mantle-wedge–subduction zone setting, but has the common element of aqueous fluid-present replacement of olivine together with high CO2 and high inferred solute contents.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONSREFERENCES |
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We have observed orthopyroxene of unusual, very low Al2O3 content as a minor and petrographically distinct phase within spinel lherzolite. The major mineralogy of the spinel lherzolite records a high-pressure, high-temperature decompression path yielding olivine + orthopyroxene (2–3 wt % Al2O3) + clinopyroxene (2–3 wt % Al2O3) + aluminous spinel with evidence of more rapid late-stage cooling at low pressures in thin rims to pyroxene porphyroclasts. The unusual orthopyroxene (<0·25 wt % Al2O3) is juxtaposed between olivine and orthopyroxene (>2 wt % Al2O3) or between olivine and clinopyroxene. It has a sharp chemical discontinuity against the orthopyroxene and is inferred to be a replacement of olivine, reflecting addition of silica from a fluid phase.
The trace element compositions of the clinopyroxene in the studied sample indicate that it underwent metasomatism, possibly caused by slab-derived fluids, during its decompression path. We infer that the unusually low-Al2O3 orthopyroxene is a consequence of fluid migration and metasomatism at temperatures <900°C and low pressure. The fluid contained CO2 and H2O but with a high solute content reducing both CO2 activity and H2O activity. Comparison with other data from the literature suggests that low-temperature crystallization of orthopyroxene, representing addition of silica to mantle lherzolite via a CO2 + H2O-bearing fluid phase, is a mechanism for metasomatic alteration of mantle wedge peridotite. The metasomatism is cryptic in that there may be no introduction of a new phase to the lherzolite (i.e. pargasite, phlogopite, talc, carbonate, apatite) and little or no retention of the carrier fluid as fluid inclusions.
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ACKNOWLEDGEMENTS |
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The authors are grateful to Frank Brink for technical assistance in EPMA analyses at Electron Microscopy Unit, ANU, and the Board of Education of Samani Town for preparing useful accommodation during our stay for field work. The constructive reviews by Douglas Smith and Ian Parkinson improved the manuscript. This research was supported by the JSPS Fellowships for Japanese Junior Scientists to T.M.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONSREFERENCES |
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