346 M. Yoshikawa, S. Arai, Y. Ishida, A. Tamura and Y. ShimizuJournal of Mineralogical and Petrological Sciences, Volume 105, page 346─351, 2010

doi:10.2465/jmps.100622bM. Yoshikawa, masako@bep.vgs.kyoto-u.ac.jp Corresponding au-

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LETTER

Trace element and Sr-Nd isotopic features of ultramafic xenoliths from Kurose, Southwest Japan

Masako Yoshikawa*, Shoji Arai**, Yoshito Ishida**,***,Akihiro Tamura** and Yohei Shimizu**,****

*Institute for Geothermal Sciences, Graduate School of Sciences, Kyoto University, Beppu 874-0903, Japan**Department of Earth Sciences, Kanazawa University, Kanazawa 920-1192, Japan

***Graduate School of Bio-Applications and Systems Engineering Tokyo University of Agriculture and Technology, Tokyo, 183-0057, Japan

****Dia Consultants Company Limited, Tokyo, 101-0032, Japan

Ultramafic xenoliths from Kurose, Southwest Japan, are fragments of the lithospheric mantle beneath the back arc of the active Southwest Japan volcano-tectonic arc. Trace element and Sr-Nd isotopic compositions of the clinopyroxenes in the ultramafic xenoliths suggest that several enrichment processes are responsible for the geochemical variation. These xenoliths would have witnessed the following events: (1) open-system partial melting of the continental lithosphere with an influx of an asthenospheric melt with MORB-like isotopic com-position during the formation of the back-arc basin that resulted from the Japan Sea opening, (2) open-system melting under an influx of a fluid/melt derived from a subducted slab, and (3) metasomatism after these partial melting events by an island arc tholeiitic basalt melt in the Neogene period. Moreover, variations in the deple-tion levels of Zr and Ti in the clinopyroxenes suggest that source depletion preceded the melting and metaso-matic events.

Keywords: Back-arc peridotites, Open-system melting, Metasomatism, Subduction component

INTRODUCTION

Ultramafic xenoliths in arc magmas provide direct information on the petrological and geochemical charac-teristics of the mantle wedge. In comparison to other tectonic regimes such as continental rift zones and oceanic islands, these xenoliths are less frequently found in active arc settings (e.g., Abe et al., 1998). Cenozoic alkaline magmas that erupted within the back-arc region of the active Southwest Japan arc, where the Philippine Sea plate is currently subducting, commonly contain ultramaf-ic and mafic xenoliths (e.g., Takamura, 1973). The ultra-mafic xenoliths from Kurose in Hakata Bay, northern Kyu shu, Japan (Fig. 1; available online from http://www.jstage.jst.go.jp/browse/jmps), are one of these settings, and on the basis of petrographic observations and mineral che mistry, it has been suggested that they are free from Fe-rich metasomatism by the host alkali basalt (e.g., Arai et al., 2000). However, cryptic metasomatism by subducting slab components has been suggested on the

basis of the whole rock Sr, Nd, Os, and noble gas isotopic com positions and the trace element compositions of clino- pyroxenes (e.g., Abe et al., 1998; Ikeda et al., 2001; Senda et al., 2007). The Kurose ultramafic xenoliths are there-fore expected to provide information on the geo chemical characteristics and petrological processes in the upper mantle beneath the arc. Whole rock geochemical data are prone to contamination (e.g., Menzies and Murthy, 1978), and therefore, isotopic and trace element compositions of clinopyroxenes are more reliable for understanding mantle processes. In this paper, with the objective of characterizing the upper mantle beneath the Southwest Japan arc, we present major and trace element composi-tions and Sr-Nd isotopic compositions of clino pyroxenes obtained from selected xenolith samples from Kurose. We also present possible influx agents that could have led to the geochemical and isotopic variations in the xenoliths.

CHEMICAL AND ISOTOPIC FEATURES

The Kurose ultramafic xenoliths were free from hydrous minerals, garnet, and plagioclase (e.g., Arai et al., 2000). The analyzed rock types included four harzburgite

347Trace and isotopic features of the Kurose ultramafic xenoliths

xenoliths (KR6, KR120, KR363, and KR375) and one composite xenolith (KR202). There were no visible melt/fluid inclusions in the samples, except for KR202. The KR202 sample comprised harzburgite (KR202 H) inter-layered with a pyroxenite band with a thickness of around 2 cm (KR202 PX); the contact between harzburgite and pyroxenite was sharp and planar, and the sample con-tained melt inclusions within minerals.

Olivines in the harzburgites were Fo 90-91 (Table 1; available online from http://www.jstage.jst.go.jp/browse/jmps). The Cr# value [= Cr/(Cr + Al)] of spinels varied between 0.22 and 0.31, showing a refractory signature weaker than that reported in previous studies (Cr#: 0.19-

0.47; Arai and Hirai, 1983; Arai, 1994; Abe et al., 1998; Abe and Yamamoto, 1999; Senda et al., 2007). The wall harzburgite and pyroxenite band in KR202 showed small but clear differences in the TiO2, Al2O3, and Cr2O3 content of clinopyroxene and spinel (Table1).

Clinopyroxenes from KR6, KR120, and KR363 were depleted in highly incompatible elements such as light rare earth elements (LREEs) and Sr, and the KR375 clinopyroxene showed a V-shaped pattern with strong en-richment in elements more incompatible than Zr (Fig. 2A). Clinopyroxenes from KR202 H and KR202 PX showed a flat trace-element pattern with Zr depletion (Fig. 2A).

The Sr and Nd isotopic compositions of clino-pyroxenes, except for the sample KR375, plotted within the fields of the East Pacific Rise (Pacific MORB) and the Japan Sea basin (Fig. 3). Although the Nd isotopic composition of the KR375 clinopyroxene was relatively lower than that of the other clinopyroxenes, it was consistent with published Kurose dunite clinopyroxene data (KRS9806; Nishio et al., 2004; Fig. 3). The isotopic composition of clinopyroxene in KR202 PX (87Sr/86Sr = 0.7050, 143Nd/144Nd = 0.51274) was clearly enriched compared with that in KR202 H (87Sr/86Sr = 0.7048, 143Nd/144Nd = 0.51288) in the composite xenolith.

DISCUSSION AND CONCLUSIONS

In plots of Al2O3 content versus Na2O content, the Kurose harzburgites show positive correlations, indicating a par-tial melting trend (e.g., Arai, 1994; Fig. 4A; Fig. 4 is ava-ilable online from http://www.jstage.jst.go.jp/browse/jmps). Most of the Kurose ultramafic samples plot within the field of peridotites of the back-arc basin (BAB), and their composition is intermediate between abyssal peridotites and South Korean peridotite xenoliths (continental lithospheric man tle) (Fig. 4A). The Na2O content of clinopyroxene has pre v iously been considered as an indicator of the depth of the partial melting (e.g.,

Arai, 1994). If this consideration is valid, the Kurose harzburgites (Na2O: 0.2-0.6) should have undergone partial melting at a shallow depth, similar to abyssal peridotites.

The observed trends in a plot of Ti content versus Zr content for clinopyroxenes obtained from our harzburgites are similar to those in similar plots constructed for abyssal and BAB peridotites and are consistent with the fractional partial melting model curve (Fig. 4B). However, a dec-rease in the Ti content correlates with a decrease in Cr# of spinel in our harzburgites, except for the most fertile KR120 and composite KR202 xenoliths; this correlation indicates the occurrence of a process other than simple partial melting. On the other hand, previously published data on clinopyroxene (Abe et al., 1998) in refractory Ku-rose xenoliths (Cr# of spinel ≥ 0.31) display a different trend on the Ti-Zr diagram (Fig. 4B). They resemble the trend for clinopyroxene in peridotite xenoliths from the Japan arcs (Japan arc trend; Abe et al., 1998). These dis-tinct trends are possibly a result of different formation pro-cesses or they possibly originate from different sources.

The clinopyroxene of the most fertile sample KR120

Figure 2. (A) Chondrite-normalized (Sun and McDonough, 1989) trace element patterns of clinopyroxene in the Kurose ultramafic xenoliths considered in this study. KR202 H and KR202 PX show wall harzburgite and a pyroxenite band of the KR202 com-posite xenolith, respectively. (B) Comparison of our data with the clinopyroxene data of the Kurose dunite xenolith (KRS9806; Nishio Y. et al., unpublished data) and the composition range for published clinopyroxene data of the Kurose peridotites (Abe et al., 1998), LREE-depleted ((La/Yb)N < 1, N: chondrite-normal-ized value) clinopyroxene in the South Korean xenoliths (Choi et al., 2005 and references therein; Kil, 2006), back-arc-basin peri-dotites (Mariana trough and Parece Vela; Ohara et al., 2002, 2003), and Type I Takeshima seamount xenoliths (Ninomiya et al., 2007).

348 M. Yoshikawa, S. Arai, Y. Ishida, A. Tamura and Y. Shimizu

has a composition similar to LREE-depleted clinopyrox-enes from South Korean xenoliths (Fig. 2B). LREE-de-pleted trace element patterns of KR6 and KR363 clinopy-roxenes are similar to those of clinopyroxenes obtained from the BAB peridotites. The clinopyroxene of KR202 shows similar patterns to previously published Kurose clinopyroxene data (Abe et al., 1998), except their signifi-cant Ti depletion (Fig. 2B). The flat to V-shaped KR375, KR202, and published clinopyroxene patterns are similar to clinopyroxene patterns of Type I Takeshima seamount xenoliths (Fig. 2B; Ninomiya et al., 2007). The LREE en-richment cannot be explained by a simple partial melting event. Rather, enrichment processes should be considered.

On the basis of these observations and isotopic data of the clinopyroxenes, the Kurose clinopyroxenes can be divided into the following three types: (1) Type 1: The Sr-Nd isotopic compositions are similar

to those of BAB basalts and the clinopyroxenes have a typical BAB peridotite trace element pattern (KR6, KR363).

(2) Type 2: These clinopyroxenes have relatively low Nd isotopic compositions (KR375, KRS9806) with flat to V-shaped trace element patterns.

(3) Type 3: These have flat to enriched trace element patterns with refractory mineral compositions (Cr# of spinel ≥ 0.31; KR202 and published data on Kurose

samples). The trace element patterns suggest an influx of fluid/

melt and not a simple partial melting process. Open-

system melting (Ozawa and Shimizu, 1995) satisfactorily explains the peculiar clinopyroxene geochemical trend of Types 1 and 2, the decrease in the Ti content with an decrease in spinel Cr#, and the depleted isotopic com-positions that are found for high (Ce/Nd)N composition (subscript N indicates that the value is normalized to the chondrite value) (Table 1). On the other hand, we use a melt-solid interaction model (Kelemen et al., 1992) for Type 3 clinopyroxene. The injection of melt after partial melting explains the discrepancy in isotopic compositions and mineral chemistry, between clinopyroxens of KR202 H and KR202 PX and the absence of a relationship be-tween Cr# of spinel and the (Ce/Nd)N values obtained from published data.

For the models, we assume that the trace element composition of KR120 represents the clinopyroxene composition of the source mantle peridotite because KR120 harzburgite shows the most fertile mineral che-mistry (Cr# of spinel: 0.215; Al2O3 in clinopyroxene: 4.20 wt%). Additionally, the trace element composition is wi-thin the range observed in LREE-depleted clinopyroxenes of the peridotite xenoliths obtained from the South Korean continental lithospheric mantle (Fig. 2B), and it is close to the Ti/Zr value of the primitive mantle clinopy-roxene (Fig. 4B). The calculated Nd model age for the clinopyroxene in KR120 is 0.4 Ga, which is within the range of the Re depletion ages determined for Northeast China (2.7-0.13 Ga; Gao et al., 2002) and within the range of Nd model ages determined from clinopyroxenes of the peridotite xenoliths from South Korea (1.1-0.06 Ga; Choi et al., 2005). All these observations suggest that KR120 originates from the eastern Eurasian continental lithosphere.

By performing open-system melting model calcu-lations, trace element compositions of Type 1 clinopyrox-enes of KR6 and KR363 can be reproduced if we assume the host alkali basalt to be the influx agent; however, the depletion of Zr cannot be reproduced by this model calculation (Fig. 5A, Appendix 1; available online from http://www.jstage.jst.go.jp/browse/jmps). Significant de-pletion of Zr and Ti in clinopyroxene and discrepancies in numerical modeling results, such as the difference in the estimated degree of partial melting between REE-based and Ti-Zr-based calculations, have been reported by dif-ferent studies that have examined peridotites from differ-ent parts of the world. These observations have been at-tributed to the following factors: (1) variation of cli no py ro xene/melt partition coefficients of high field strength elements (HFSEs), including Zr and Ti, with

Figure 3. Nd-Sr isotopic diagram for clinopyroxene in the Kurose harzburgite xenoliths (open circle) and a pyroxenite band (open square), and comparison with published clinopyroxene data of the dunite xenolith (solid circle, KRS9806; Nishio et al., 2004) and host alkaline basalts (asterisk, Kagami et al., 1993; Hong and Uto, 2003; Senda et al., 2007). The error bars of clinopyroxenes except of the KR363 clinopyroxene are shown within the sym-boles. The fields of Pacific MORB (Hofmann, 1997), Japan-Sea basalts (Tatsumoto and Nakamura, 1991; Cousen et al., 1994), Northeast Japan tholeiites (Togashi et al., 1992; Shibata and Na-kamura, 1997), and clinopyroxene in South Korean peridotite xe-noliths (Choi et al., 2005 and references therein; Kil, 2006) are also shown. Dashed lines indicate the present day Nd-Sr isotopic compositions of the bulk earth.

349Trace and isotopic features of the Kurose ultramafic xenoliths

pressure, temperature, and mineral composition (e.g., Hill et al., 2010), (2) association of the source compositions with HFSE negative anomalies (e.g., Salters and Hart, 1991), and (3) interaction between HFSE-depleted car-bonatite melt and peridotite (e.g., Coltorti et al., 1999).

We exclude the interaction with carbonatite melt because the observed negative Zr anomaly cannot be reproduced even by considering a large decrease (by a factor of 1000) in the Zr concentration of the host basalt as carbonatite melt in the model calculation. We prefer a source with HFSE negative anomalies, which could be formed by variations in olivine-melt and orthopyroxene-melt par-tition coefficients at different temperatures, pressures, and mineral compositions, similar to the case of clinopy-roxene.

The Sr-Nd isotopic compositions of KR363 and KR6 clinopyroxenes are considerably depleted than that of KR120 clinopyroxene. KR363, which has more depleted Sr-Nd isotopic composition, requires a larger mass influx of the agent (β = 0.02) as compared to KR6 (β = 0.008), but with the same degree of melting (F = 7%). Additional isotopic constraints suggest that for KR6 and KR363, the influx agent should have a MORB-like isotopic composition if the source mantle has isotopic composition similar to KR120. Such influx might be related to the opening of the Sea of Japan. On the basis of observations of spreading center basalts of the Japan Sea basin, alkaline basalts of Ulreung and Dog islands, and tholeiitic basalts of the northern Japan rift zone (Nohda et al., 1988; Tatsumoto and Nakamura, 1991; Cousens et al., 1994), the back-arc spreading of the Sea of Japan has been interpreted as a response to the penetration of the asthenosphere into the cratonic lithospheric mantle with MORB-like Sr-Nd-Pb isotopic compositions. Our model calculations strongly indicate that the Type 1 ultramafic xenolith was formed by open-system melting with the in-volvement of an influx of alkali basalt asthenospheric melt with MORB-like isotopic composition. This is most likely to have occurred during the back-arc spreading of the Sea of Japan.

The trace element pattern and isotopic data of Type 2 KR375 clinopyroxene cannot be explained by the model considered for Type 1 clinopyroxenes. The pattern of KR375 may be explained by a 7% partial melt extraction from the same source as that of Type 1, with an influx agent having highly incompatible element concentrations (β = 0.008, Fig. 5B, Appendix 1). The estimated com-position of an influx agent indicates that the fluid/melt is likely to have been derived from a subducted slab (Kogiso et al., 1997). The estimated compositions of fluid from sub ducted sediment and oceanic crust (see the caption of Fig. 5B) indeed show high concentrations of highly in-compatible elements. However, middle to heavy REEs does not fit to the calculated composition of the influx agent (Fig. 5B). This difference can be adjusted by assu-ming different influx agent composition derived from different slab depths (Kimura et al., 2010).

Figure 5. Chondrite-normalized (Sun and McDonough, 1989) trace element compositions of modeled and measured clinopyroxenes. Parameters used for the modeling are given in Appendix 1. (A) Clinopyroxenes modeled by open-system melting (Ozawa and Shimizu, 1995) with host alkali basalt (Hong and Uto, 2003) as influxed melt and comparison with the KR6 and KR363 clinopy-roxenes. (B) Compositions of clinopyroxene, the influxing agent, and the extracted melt modeled using the open-system melting model. The composition of the KR375 clinopyroxene is also pre-sented. In the open-system melting model, the system is open to both input (fluid influx or metasomatism) and output (melt segre-gation). α, denotes the ratio of the trapped melt mass to the resid-ual solid mass; β, the amount of influxing material for a specific degree of melting or degree of the crystallization reaction; F, the degree of partial melting. For comparison, slab fluid calculated on the basis of the mobility of each element using the parameters of Tatsumi and Hanyu (2003), and the calculated melt equilibri-um with KRS9806 dunite using distribution coefficient between clinopyroxene-melt are also show. (C) Result of melt-mantle in-teraction modeling (Kelemen et al., 1992) between incoming melt equilibrated with KR202 PX and sur r oun ding residual peri-dotites of the HFSE depleted source. Tholeiite compositions of Northeast Japan basalts (Togashi et al., 1992; Shibata and Naka-mura, 1997) are also shown. r, denotes the ratio of the mass as-similated to the mass crystallized.

350 M. Yoshikawa, S. Arai, Y. Ishida, A. Tamura and Y. Shimizu

The composition of the melt in equilibrium with the dunite (KRS9806) clinopyroxene from Kurose (Fig. 5B) is consistent with the calculated composition of melt derived from KR375 (Fig. 5B). The isotopic composition of dunite clinopyroxene is similar to that of KR375 clinopyroxene (Fig. 3). These facts suggest that the dunite should have crystallized from a melt in equilibrium with the KR375 peridotite.

The Type 3 clinopyroxene composition of KR202 H can be reproduced by considering the interaction of 7% residue from the same source as that considered for Types 1 and 2, with the melt equilibrated with the KR202 PX clinopyroxene (Fig. 5C, Appendix 1). The Ti depletion in-dicated by published clinopyroxene data can be explained by the interaction of the same melt with the residue of a source that shows Zr and Ti depletions. The estimated composition of the melt is similar to the composition of the Northeast Japan arc tholeiitic basalt (Fig. 5C). The Sr-Nd isotopic compositions of the KR202 PX and KR202 PX clinopyroxenes are within the composition range re-ported for the Northeast Japan tholeiitic basalt (Fig. 3). Thus, we suppose that the Type 3 clinopyroxenes are products of a reaction between a tholeiitic melt with an arc signature and residual peridotite.

In conclusion, on the basis of mineral compositions and Sr-Nd isotopic and trace element compositions of clinopyroxene, we infer that the Kurose ultramafic xeno-liths recorded the various modes of partial melting of the continental lithosphere with influxes from BAB basalt, a slab component, and a tholeiitic melt with an arc signature in the late Cenozoic. HFSE depletions of the source should have occurred prior to the fluxing events. All these events may have occurred during the Japan Sea opening and the subsequent influx of subducting components.

ACKNOWLEDGMENTS

We are grateful to Dr. Y. Nishio for providing unpublished data, Dr. T. Shibata for discussions and useful technical advice, Drs. M. Python and H. Helmy and Prof. K. Take-mura for improving the text, and Dr. J. Kimura for edito-rial handling and useful comments. Critical comments by Prof. K. Ozawa and anonymous reviewers helped in sig-nificantly improving the manuscript. This work was sup-ported by KAKENHI(21540495) and (19GS0211) award - ed to M. Yoshikawa and S. Arai, respectively.

DEPOSITORY MATERIALS

Figures 1 and 4, Table 1, and Appendix 1 are available online from http://www.jstage.jst.go.jp/browse/jmps.

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Manuscript received June 22, 2010Manuscript accepted October 5, 2010

Manuscript handled by Jun-Ichi Kimura