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Journal of Petrology Volume 42 Number 1 Pages 25-37 2001
© Oxford University Press 2001
Re–Os Isotopes in the Horoman Peridotite: Evidence for Refertilization?
1DEPARTMENT OF GEOLOGY AND GEOPHYSICS, WOODS HOLE OCEANOGRAPHIC INSTITUTION, WOODS HOLE, MA 02543, USA
2DEPARTMENT OF GEOLOGY, FACULTY OF SCIENCE, NIIGATA UNIVERSITY, 2-8050 IKARASHI, NIIGATA 950-2181, JAPAN
3DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
Received November 22, 1999; Revised typescript accepted June 27, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Re–Os isotopic data for 20 samples from a well-characterized 140 m section across a layered sequence, ranging from plagioclase lherzolite through lherzolite to harzburgite, of the Horoman peridotite show: (1) a range in 187Os/188Os ratios (from 0·1158 to 0·1283) similar to that reported for other peridotitic massifs, thereby suggesting that the processes responsible for the Re–Os isotopic variation at the meter-scale and the whole-massif scale are similar; (2) that the Os isotopic ratio is controlled by the Re content through radiogenic ingrowth over a period of
0·9 Gy. The ultramafic and some of the mafic rocks (Type I layers) from the Horoman massif define an ‘apparent age’ of 1·12 ± 0·24 Ga in the Re–Os isochron diagram, within error of the previously reported age of 833 ± 78 Ma based on Sm–Nd isotopes. Although the Re–Os isotopic data do not define an isochron, the consistency of the 900 Ma age defined by both isotopic systems suggests that this age has a geologic meaning and that mafic (Type I layers) and ultramafic rocks are genetically related. A plausible explanation for the genetic relationship between the mafic and ultramafic rocks, the meter-scale compositional variations from lherzolite to plagioclase lherzolite, the suprachondritic 187Re/188Os ratios in some fertile peridotites, and the oldest Re depletion model age of 1·86 Ga obtained for Horoman rocks is a refertilization process involving reaction of a mid-ocean ridge basalt-like magma with depleted lithospheric mantle at 900 Ma. KEY WORDS: Re–Os isotopes; Horoman peridotite
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Re–Os isotope system provides new insights into understanding mantle processes. Parent and daughter elements for other isotopic systems (Rb–Sr, Sm–Nd and U–Pb) behave incompatibly during mantle melting, being mainly controlled by silicate phases. In contrast, Re behaves as a moderately incompatible element, whereas Os is highly compatible during melting, and both Re and Os have chalcophile and siderophile affinities (Hart & Ravizza, 1996
; Burton et al., 1998, 1999; Shirey & Walker, 1998). Consequently, mantle peridotites have higher Os concentrations than their partial melts, and the Os isotope system is more resistant to young metasomatic disturbances than are the other isotopic systems (Luck & Allègre, 1991; Reisberg et al., 1991; Reisberg & Lorand, 1995; Hart & Ravizza, 1996; Roy-Barman et al., 1996; Burton et al., 1998, 1999; Shirey & Walker, 1998).
In this paper, we report Re–Os isotope data for the Horoman massif (Fig. 1). We selected a well-characterized 140 m section (the Bozu section) across this layered peridotite with rock types ranging from plagioclase lherzolite through lherzolite to harzburgite; two mafic layers from another part of the Horoman peridotite were also studied. Takazawa and co-workers have studied this section and mafic layers from the Horoman massif, reporting major, trace element and isotopic compositions of whole rocks and mineral separates (Takazawa et al., 1992, 1994, 1996, 1999, 2000; Takazawa, 1996). The main objectives of this Re–Os study are to determine the origin of the plagioclase lherzolites (do they represent unmelted fertile peridotite or a region of melt impregnation and accumulation?), to evaluate the role of mafic layers in the formation of the layered peridotites, and to constrain the ages of magmatic processes that affected this peridotitic massif.
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Geological background
The Horoman peridotite (Fig. 1) is a fault-bounded mantle slice, 8 km x 10 km x 3 km, emplaced 23 ± 1·2 My ago (Rb–Sr isotopes on a phlogopite-bearing spinel lherzolite, Yoshikawa et al., 1993) in the southern end of the high-temperature low-pressure type Hidaka Metamorphic Belt, Hokkaido, Japan (Niida, 1974, 1984). The peridotite consists of several lithological sequences of plagioclase lherzolite–lherzolite–harzburgite ± dunite–harzburgite–lherzolite–plagioclase lherzolite (Komatsu & Nochi, 1966; Niida, 1974; Obata & Nagahara, 1987; Takahashi, 1991; Takazawa et al., 2000). From plagioclase lherzolite through lherzolite to harzburgite, the peridotites are increasingly depleted in a basaltic component. The boundary between the compositional layers is sharp on a megascopic scale, but transitional over a few centimeters on a microscopic scale (Takazawa et al., 2000).
The massif has been divided into two main zones, the Upper and Lower Zones. Between these zones there is a continuous transition with no evidence for a thrust or shear zone (Niida, 1974, 1984). The Lower Zone, 2 km thick, consists of cyclic layers of plagioclase lherzolite through lherzolite to harzburgite, with subordinate dunite; the cycle repeats every 100–500 m. The abundance of mafic layers in the Lower Zone is sparse, with the exception of thin seams in the plagioclase lherzolite. In these seams plagioclase occurs in a fine-grained aggregate of plagioclase, olivine, spinel, minor orthopyroxene and very rare clinopyroxene. This assemblage is interpreted to have formed by a decompression reaction between two pyroxenes and spinel, which in turn were derived from a reaction between garnet and olivine (e.g. Ozawa & Takahashi, 1995; Takahashi, 1997). The Upper Zone, 1 km thick, is characterized by a centimeter-scale layering of plagioclase lherzolite and harzburgite as the dominant rock types, with subordinate dunite. In the Upper Zone, plagioclase lherzolite and mafic layers are more abundant than in the Lower Zone (Takazawa et al., 1999). On the basis of core compositions of pyroxenes, the plagioclase lherzolite equilibrated at 20 kbar and temperatures of 900–950°C in the Lower Zone and 1100–1150°C in the Upper Zone (Ozawa & Takahashi, 1995).
Several hypotheses have been proposed to explain the formation of the lithologically layered peridotites:
- formation of cumulates through fractional crystallization (Niida, 1984);
- melting of lherzolite to create residual harzburgite and complementary plagioclase lherzolite, which represents a region of melt accumulation (Obata & Nagahara, 1987);
- melt segregation processes governed by suction associated with local melting along a fracture, during continuous ascent of a mantle peridotite diapir (Takahashi, 1992);
- polybaric melting of an upwelling fertile mantle (plagioclase lherzolite), followed by subsolidus thinning and folding of the residual harzburgite, lherzolite and plagioclase lherzolite (Takazawa et al., 2000; Yoshikawa & Nakamura, 2000).
In the Horoman peridotite, there are two dominant types of mafic layers (Niida, 1984; Takazawa et al., 1999):
- Type I (Al–Ti-augite type mafic granulites): plagioclase + Ti augite + olivine + orthopyroxene + Ti pargasite (or kaersutite) + green spinel + titaniferous magnetite + ilmenite + sulfide;
- Type II (Cr-diopside type mafic granulite): plagioclase + Cr diopside + olivine + orthopyroxene + pargasite + spinel + magnetite + sulfide.
The mafic layers vary from a few centimeters to several meters in thickness, and are usually oriented parallel to the foliation plane of the peridotite. The primary mineral assemblages in the mafic layers (now transformed by subsolidus reaction) indicate multiple episodes of melt injection over a wide range of pressure from the garnet stability field (Type I) to the plagioclase stability field (Type II). Type I mafic layers have intralayer compositional heterogeneity, with the centers interpreted as garnet clinopyroxenite cumulates and the margins as mid-ocean ridge basalt (MORB)-like melt in equilibrium with these cumulates, whereas Type II layers are interpreted as plagioclase-rich cumulates (Takazawa et al., 1999).
The Bozu section
We chose the Bozu section for our study, as it is a representative 140 m section from the Lower Zone. This section is a continuously exposed lithologic sequence ranging from harzburgite through lherzolite to plagioclase lherzolite, and it has been well characterized, with petrologic and geochemical data obtained on scales ranging from micrometers to tens of meters [Takazawa et al. (2000) and references therein] (Fig. 2a). On the basis of study of the Bozu section, Takazawa and co-workers considered that the layered structure was produced by melting of an upwelling fertile mantle [the N (normal)-Type plagioclase lherzolite] at 0·9 Ga. Subsequently, within the lithosphere and the garnet stability field, the Horoman massif reacted with a light rare earth element (LREE)-rich melt or fluid, producing enrichment of incompatible elements in harzburgite, lherzolite and in a special type of plagioclase lherzolite that Takazawa et al. (2000) defined as E (enriched)-Type (Fig. 2b). Although the E-Type plagioclase lherzolite is enriched in incompatible elements, the abundances of major and moderately incompatible trace elements are similar to those of the lherzolite. During uplift of the massif, the peridotite was transformed from the garnet- to the spinel- to the plagioclase-facies mineralogy by subsolidus reaction (Takazawa et al., 1996).
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, N-Type plagioclase lherzolite; •, E-Type plagioclase lherzolite; 
, lherzolite; crossed square, harzburgite.We selected two harzburgite, two lherzolites, four E-Type plagioclase lherzolites and 10 plagioclase lherzolites for determination of Re–Os isotopic systematics. With the exception of the plagioclase lherzolites, where we analyzed all the samples available, the peridotite samples are from the center of their respective layers. In addition, we analyzed two Type I mafic layers; one each from the Lower and Upper Zones.
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ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The analytical techniques for Re–Os isotopes were described by Ravizza & Turekian (1989)
and Hauri & Hart (1993). Samples weighing 200–500 g were powdered in an agate shatterbox (Takazawa et al., 1999). Re and Os were determined on separate powder splits. For the analysis of Os, 1 g of powder was fused by the NiS fire-assay technique using a 4:1 flux-to-sample ratio. The Os concentrations and isotopic compositions were measured by negative thermal ionization mass spectrometry on NIMA-B [at Woods Hole Oceanographic Institution (WHOI)], with oxygen-enhanced emission and single-collector analog detection using an electron multiplier and a dynamic collection routine. For the Re analysis, 1 g of powder was dissolved in two steps using first a HF–HNO3 mixture and later HCl; the Re was extracted by a simple ion exchange technique. The Re measurements were performed on a Finnigan MAT Element ICP-MS (WHOI) by rapid peak hopping using electrostatic scanning.
In-run precision is 2
< 0·35% for 187Os/188Os, 2 < 1% for Os concentration and 2 < 2% for Re concentration. Analyses were corrected for total procedural blank of 4·8–5·6 pg for Os concentration and 2–3 pg for Re concentration with an Os isotopic ratio of 0·2–0·26. The correction is insignificant for Re and Os contents or Os isotopic composition, with the exception of samples Bz-116 and Bz-125, which have an estimated 2 error of 25% for Re concentration as a result of the blank correction. Re and Os replicates on separate powder splits of Bz-134 differ by <10% for Re, <2% for Os concentration, and <0·5% for 187Os/188Os ratios.
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RESULTS |
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Results for 20 ultramafic and mafic samples analyzed from the Horoman peridotite are listed in Table 1; whole-rock major and trace element compositions have been reported by Takazawa et al. (1999
, 2000). In the Bozu section, the 187Os/188Os ratios of the ultramafic rocks range from 0·1158 to 0·1283; a similar range was reported by Liu & Tanaka (1999), for another suite of samples from the Horoman massif. The Re and Os concentrations vary from 0·007 to 0·350 ppb and from 2·9 to 5·2 ppb, respectively. Thus, whereas the Re content varies by a factor of 50, the Os concentration varies by about a factor of two; results that are typical of peridotite massifs (Luck & Allègre, 1991; Reisberg et al., 1991; Reisberg & Lorand, 1995; Roy-Barman et al., 1996).
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Two major observations are: (1) the range of 187Os/188Os ratios in this 140 m section of peridotite is as large as those reported for peridotite massifs, such as Beni Boussera, Baldisero, Lanzo, Lherz and Ronda (Reisberg et al., 1991; Reisberg & Lorand, 1995; Roy-Barman et al., 1996) (Fig. 3a); (2) among the samples analyzed, the lowest and highest Os isotopic ratios measured correspond to E-Type and a N-Type plagioclase lherzolites separated by a distance of only 5 m (Fig. 3b).
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The 187Os/188Os ratios and the Re contents define a negative correlation with the MgO concentrations (Fig. 4). If the varying MgO content of these peridotites reflects varying extents of melt extraction, the negative correlation between Re and MgO contents suggests that Re behaves as a moderately incompatible element during melting. This interpretation is supported by the positive correlation between Re and Al2O3, Sc and Yb contents, and between Re and the modal proportion of clinopyroxene (see Fig. 5, below). On the other hand, the small variation in Os concentration (a factor of about two; Fig. 4) indicates that Os behaves as a highly compatible element during mantle melting (Morgan, 1986
; Hart & Ravizza, 1996). However, the Os contents do not correlate with the 187Os/188Os ratios, or the abundances of Re or MgO (Fig. 4). Possibly the expected positive correlation between Os and MgO abundance is masked by the nugget effect; that is, the difficulty in obtaining a representative abundance of an element that is highly concentrated in rare mineral phases.
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The 187Os/188Os ratios for Horoman peridotites define a strong positive correlation with the Re and Al2O3 contents (Figs 4 and 5) and a more scattered positive correlation with 187Re/188Os ratios (Fig. 6a). Thus, there is a gradual decrease in the Os isotopic composition from plagioclase lherzolites through lherzolites to harzburgites. The 187Os/188Os ratios for the ultramafic samples are lower than the Os isotopic composition estimated for the Primitive Upper Mantle (PUM, 0·1290 ± 0·0009; Meisel et al., 1996). Moreover, most of the samples have 187Re/188Os ratios below that of PUM (0·4–0·428 for PUM; McDonough & Sun, 1995; Meisel et al., 1996). However, five N-Type plagioclase lherzolites plot to the right of the geochron and three of the five samples have higher 187Re/188Os ratios than the estimates for PUM (Fig. 6a). It should be noted that the variation in 187Re/188Os ratios in the N-Type plagioclase lherzolites is controlled by both Re and Os contents, which vary by a factor of about two, and range to higher and lower values than estimates for PUM (Fig. 4).
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The positive correlation defined by the Horoman peridotites in the isochron diagram (187Os/188Os vs 187Re/188Os) yields an apparent ‘age’ of 0·91 ± 0·35 Ga (Fig. 6a, excluding sample Bz-257). If the two mafic layers are included in the regression, the apparent ‘age’ is 1·12 ± 0·24 Ga (Fig. 6b). Although these trends are not isochrons, these apparent ‘ages’ are within error of the 833 ± 78 Ma inferred from whole-rock Sm–Nd isotopic data of peridotites by Yoshikawa & Nakamura (2000)
. The age of 850 Ma has been interpreted as the time of melt extraction (Yoshikawa & Nakamura, 2000), but the Re depletion model age (TRD) for the E-type plagioclase lherzolite (Bz-257) with the lowest 187Os/188Os ratio (0·1158) yields a minimum age for the depletion event of 1·86 Ga (Table 1). Moreover, the trend in Fig. 6b suggests that the Type I mafic layers are genetically related to the peridotites, consistent with their Re–Os model ages of 1·1–1·3 Ga (Table 1). Thus, the age of the Type I mafic layers is much older than the 80 Ma inferred from Nd isotopic data for core–margin pairs by Takazawa et al. (2000). If the Type I mafic layers are 80 Ma, it would require an unrealistically high 187Re/188Os ratio of 1400 to produce the measured 187Os/188Os of about two, and a subsequent major modification of the 187Re/188Os ratios, lowering it to the observed value of 86 (Table 1). Therefore we consider the age of the Type I mafic layers to be approximately the same as that of the peridotites. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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What process controlled the Re–Os systematics of the Horoman massif?
The correlation between the 187Os/188Os ratios and the Re contents (Fig. 5) and the lack of correlation between the Os isotopes and the Os concentrations (not shown) suggest that the isotopic composition of the samples is mainly controlled by variable Re content. The negative correlation between Re and MgO and the positive correlation between Re and Al2O3, Sc and Yb contents, and the modal proportion of clinopyroxene indicates that these elements reflect a varying proportion of a ‘basaltic component’ in the Horoman peridotites. This observation is consistent with the results presented by Burton et al. (1999)
. They show that the Os budget in peridotites is mainly controlled by sulfides, but a significant proportion of the Re budget is located in the silicates. Thus, the Re/Os ratio is low in sulfides but high in silicates; for example, garnet has high Re/Os ratios (Roy-Barman et al., 1996; Burton et al., 1998; Righter & Hauri, 1998).
Two alternatives have been proposed to explain the variation in the proportion of a ‘basaltic component’ in the Horoman peridotites: (1) variable extents of melt extraction from a fertile mantle represented by the N-Type plagioclase lherzolite (Takahashi, 1992; Takazawa et al., 2000; Yoshikawa & Nakamura, 2000); (2) melting of lherzolite to create residual harzburgite and complementary plagioclase lherzolite, which represents a region of melt accumulation; i.e. refertilization (Obata & Nagahara, 1987).
The first model, variable extraction of melt, can explain the correlations between 187Os/188Os ratios, amount of modal clinopyroxene and abundances of MgO, Al2O3, Re, Sc and Yb (Figs 2 and 4). However, a major difficulty with the interpretation that the lithologic variation, from harzburgite to fertile lherzolite, formed as a result of partial melting is the repetitive layering of rock types on the scales of meters to hundreds of meters. These small-scale and spatially systematic heterogeneities are not expected from a melting process. Another example of local heterogeneity is that plagioclase lherzolites with the highest and the lowest Os isotopic ratios are located only 5 m apart (Fig. 3b). Perhaps severe deformation has juxtaposed rock types that were not adjacent when they formed (Ozawa & Takahashi, 1995; Yoshikawa & Nakamura, 2000; Takazawa et al., 2000). However, a melting process provides no explanation for either the fertile plagioclase lherzolites with high 187Re/188Os and low 187Os/188Os ratios relative to those estimated for PUM and chondrites, or the sample with the anomaluosly low 187Os/188Os ratios of 0·1156 (Bz-257, Fig. 6a). Also, it is surprising that fertile plagioclase lherzolite, 60% of the total outcrop (Takazawa et al., 1999), is so abundant in lithospheric mantle represented by the Horoman peridotite.
These obstacles for the simple melting hypothesis can be overcome by a refertilization model, whereby a basaltic component is added to a depleted peridotite. The addition of a MORB-like component to a depleted peridotite can explain the depleted Os isotopic signature, and the suprachondritic 187Re/188Os ratios in the five plagioclase lherzolites (Fig. 6a), as well as the the correlations between Re, 187Os/188Os ratios, MgO, modal clinopyroxene, and Al2O3, Sc and Yb contents (Figs 2 and 4). As refertilization would be controlled by melt migration pathways, this process could also explain meter-scale geochemical variations and the lowest 187Os/188Os ratios of 0·1156 (Bz-257).
A variant of the refertilization model for the Horoman peridotite proposed by Obata & Nagahara (1987) has been advocated by Rehkämper et al. (1999), who measured platinum group elements (PGE) in six ultramafic rocks from the Horoman massif. They found that the three fertile Horoman lherzolites (>3% CaO) have suprachondritric Pt/Ir and Pd/Ir (as with Re and Os, the elements in the numerator are incompatible relative to the elements in the denominator). Although Pd–Ir data are not available for the Horoman samples with suprachondritic 187Re/188Os, the four samples with suprachondritic Pd/Ir ratios do not have anomalously high 187Re/188Os ratios, and the Os concentrations do not correlate with the Ir content (Table 1). Rehkämper et al. (1999) interpreted the high Pd/Ir ratios in the Horoman peridotite by a combination of two processes: (1) the refertilization of a depleted peridotite by sulfides derived from percolating magmas, with or without the addition of a basaltic component; (2) depletion of Ir abundance by secondary processes such as progressive dissolution of a trace carrier phase in percolating magma. The difficulty in obtaining representative Os and Ir abundances (the nugget effect), and the lack of correlation between Re/Os and Pd/Ir indicate that the complexity of Re and PGE abundance variations in mantle rocks is not well understood (e.g. Snow & Schmidt, 1998; Rehkämper et al., 1999). Nevertheless, suprachondritic Pd/Ir and Re/Os abundance ratios in the Horoman and other peridotite massifs (Reisberg et al., 1991; Roy-Barman et al., 1996; Burnham et al., 1998) stimulate us to evaluate a refertilization model using the major and trace element composition of the Horoman mafic and ultramafic rocks.
Can interaction between residual peridotite and mafic layers explain the major and trace element contents of the fertile plagioclase lherzolite?
Takazawa et al. (2000) showed that the full compositional range of the Horoman peridotites from fertile plagioclase lherzolite to harzburgite cannot be explained by two-component mixing. However, a subset of these trends are linear; namely, the compositional variation from the N-Type plagioclase lherzolite to lherzolite. Therefore, harzburgites are not considered in the following mixing calculations. As proposed by Obata & Nagahara (1987), harzburgites may have formed as residual rocks from melting of lherzolite, whereas the compositional variation from lherzolite to plagioclase lherzolite represents variable proportion of melt accumulation.
In the Horoman peridotite, there are two dominant types of mafic layers, Type I and Type II (Niida, 1984; Takazawa et al., 1999). To evaluate whether the mafic layers represent the basaltic component responsible for the formation of the N-Type plagioclase lherzolite, we used the trace element abundances of the ultramafic and mafic rocks (Takazawa et al., 1999, 2000) in a multiple component analysis. The results indicated that the Type II mafic layers are not a suitable basaltic component; however, the Type I mafic layers are an adequate mafic component for the mixing model.
Several observations indicate that Type I mafic layers may have been involved in a refertilization process:
- the present mineralogy of the mafic layers and of the seams in the plagioclase lherzolite do not represent primary igneous phases; the present mineralogy in both cases is the product of subsolidus reaction at low pressure (Takahashi & Arai, 1989; Ozawa & Takahashi, 1995; Takazawa et al., 1999). Using the major and trace element contents of the cores and margins of Type I layers, Takazawa et al. (1999) determined that they formed as garnet pyroxenites. The seams of fine-grained plagioclase, olivine, spinel, minor orthopyroxene and very rare clinopyroxene in the plagioclase lherzolite from the Lower Zone are interpreted to have formed by a decompression reaction between two pyroxenes and spinel, which in turn were derived from a reaction between garnet and olivine (e.g. Ozawa & Takahashi, 1995; Takahashi, 1997). The presence of garnet in the mafic layers and in the seams of the plagioclase lherzolite shows that both the seams and Type I layers formed within the garnet stability field. In contrast, the composition of the Type II mafic layers indicates that they are plagioclase-rich cumulates formed at low pressure (Takazawa et al., 1999).
- The collinearity of the Type I mafic layers and the ultramafic rocks in a Re–Os isochron diagram suggest that these rocks are genetically related. The Re–Os isotope systematics of these rocks defines an apparent age ranging from 0·91 ± 0·35 Ga (peridotites) to 1·12 ± 0·24 Ga (including the mafic rocks), similar to the age reported for the Horoman peridotites based on Nd isotopes (Takazawa, 1996; Yoshikawa & Nakamura, 2000). The similarity in the ages obtained from two different isotopic system supports the hypothesis that the mafic layers and the peridotites are genetically linked (Fig. 6a and b).
- The higher proportion of plagioclase lherzolite and mafic layers in the Upper Zone of the massif (Niida, 1984) is consistent with the hypothesis of refertilization by the addition of a basaltic component (mafic layer) to a depleted peridotite (lherzolite) as responsible for the formation of the plagioclase lherzolite.
We evaluate a model whereby the N-Type plagioclase lherzolite forms as a mixture of the depleted E-Type plagioclase lherzolite and the Type I mafic layers. We used the average composition of Type I layers reported by Takazawa et al. (1999); it is significant that this average is probably a composite of a melt composition and its associated garnet clinopyroxene cumulate. Hence this process, which we will model by two-component mixing, may be described more realistically as melt–peridotite reaction with the melt reacting and precipitating clinopyroxene and garnet within the peridotite. As garnet is an important phase that controls Re content of peridotites (Roy-Barman et al., 1996; Burton et al., 1998; Righter & Hauri, 1998), the addition of garnet pyroxenite to a depleted peridotite can lead to high Re concentration and Re/Os ratios in the N-Type plagioclase lherzolite. For example, addition of garnet pyroxenite with high Re/Os ratios, like the Type I mafic layers (Re/Os = 7–14), to a depleted peridotite can explain the N-Type plagioclase lherzolites plotting to the right of the geochron (Fig. 6a).
In the mixing model we do not use abundances of incompatible elements, which may have been affected by subsequent metasomatism. Specifically, we modeled the major elements (including K and P), and the trace elements from Gd to Lu, including Cr and Sc (Fig. 7). It should be noted that K and P were affected by metasomatism, and we include them only to illustrate that the abundances of such elements are not well explained by a two-component mixing model. First, the data are normalized following the scheme of Allègre et al. (1995) so that each element is given a comparable weight. Each element used is normalized to its standard deviation, multiplied by the concentration range and divided by the mean concentration and the analytical error. Clearly, the elements that have a larger concentration range will be better indicators of the processes responsible for the geochemical variation. To model the mixing process, we consider the simple model presented by Cantagrel et al. (1984); they expressed the mixing equation
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, 2000) and the errors in the linear regression.
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Figure 9 shows calculated abundances normalized to their measured values for four of the 10 N-Type plagioclase lherzolites. The mixing model reproduces the measured major and trace element composition for the fertile plagioclase lherzolite fairly well by using the same two end-members. As we previously mentioned, K and P are the two elements that are not well reproduced by this model. We believe that the K and P contents chosen for the depleted peridotite end-member are too high, as a result of late metasomatism (Fig. 7). We conclude that the refertilization process is a plausible process for creating the fertile plagioclase lherzolite as previously proposed by Obata & Nagahara (1987).
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The age of the melting and melt–rock mixing and reaction events
The ages of the melting, mixing and reaction events affecting the Horoman peridotite are not well constrained. The oldest inferred age is 1·86 Ga, the Re depletion model age for an E-Type plagioclase lherzolite (Bz-257, Table 1). Based on Nd isotopic data for lherzolites and plagioclase lherzolites, the age of a melting event is considered to be 833 ± 78 Ma (Yoshikawa & Nakamura, 2000). The ultramafic samples (omitting Bz-257, 187Os/188Os = 0·1156) define an apparent age of 0·91 ± 0·35 Ga in the Re–Os isochron diagram. If both mafic layers are included in the regression they define an apparent age of 1·12 ± 0·24 Ga (Fig. 6). Although the Re–Os isotope systematics does not define an isochron, the consistency of the 900 Ma age defined by both isotopic systems suggests that this age has a geologic meaning. If 900 Ma is the age of melting, it is difficult to explain the 1·86 Ga Re depletion model age. If local melting and refertilization of a lherzolite was responsible for the 900 Ma age, we can explain the relatively old model age of 1·86 Ga (Table 1) as representing an older melting event. In this case, sample Bz-257 was unaffected by the 900 Ma local melting and refertilization event.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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In the 140 m Bozu section of the Horoman peridotite massif the range in 187Os/188Os ratios is similar to that reported for other peridotitic massifs, thereby suggesting that the process responsible for the Re–Os isotope variation at the meter scale and the whole-massif scales are similar. The Re–Os isotopic systematics of Horoman peridotites indicates that the 187Os/188Os ratios are mainly controlled by variation in the Re content, with Re concentration governed by the amount of a ‘basaltic component’.
The systematic layering of rock types in the Horoman peridotite has been interpreted as resulting from varying extents of melt extraction (e.g. Takazawa et al., 2000; Yoshikawa & Nakamura, 2000). Stimulated by the suprachondritic 187Re/188Os ratios of some plagioclase lherzolites, we evaluated an alternative model; the refertilization process proposed by Obata & Nagahara (1987). The Re–Os isotopic systematics, major and trace elements contents, and field and petrographic information indicate that the N-Type plagioclase lherzolites can be explained by a refertilization process involving mixing melt-rock reaction of a MORB-like rock and depleted lithospheric mantle. This process can explain some of the large compositional variations occurring over the scale of meters. The consistency of the 900 Ma age defined by Sm–Nd (Yoshikawa & Nakamura, 2000) and Re–Os isotopic systems suggests that this age has a geologic meaning, and that this age may represent the time of local melting and refertilization. A Re depletion model age of 1·86 Ga may represent an older melting event that survived the local melting and melt–rock reaction event at 0·9 Ga. The apparent age of 0·9 Ga defined by the peridotites in the Re–Os isochron diagram and the 1·1–1·3 Ga Re–Os model ages for the Type I mafic layers suggest that these mafic and ultramafic rocks are genetically related. Although the refertilization model of Obata & Nagahara (1987) is a plausible process for explaining the meter-scale compositional variations from lherzolite to plagioclase lherzolite, the variable Re–Os model ages, and the suprachondritic 187Re/188Os ratios of some plagioclase lherzolites, we recognize that the strength of the constraints arising from Re and PGE abundances need further evaluation with more detailed studies of Re and PGE abundances in mantle rocks.
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ACKNOWLEDGEMENTS |
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We thank K. Koga for help in modelling, and G. Ravizza, B. Peucker-Ehrenbrink, J. Blusztajn and L. Ball for help during the analytical work. This work has benefited greatly from discussion with G. Ravizza and M. Rëhkamper, and the thoughtful reviews of J.-L. Bodinier, J. Snow, M. Obata, E. Rampone and G. Piccardo.
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FOOTNOTES |
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*Corresponding author. Present address: Lamont–Doherty Earth Observatory, Room 58, Geochemistry Building, Columbia University, PO Box 1000, Palisades, NY 10964-8000, USA. Telephone: (914) 365-8712. Fax: (914) 365-8155. E-mail: asaal{at}ldeo.columbia.edu
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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