Journal of Mineralogical and Petrological Sciences, Volume 109, page 79–84, 2014 LETTER Formation process of olivine–clinopyroxene cumulates inferred from Takashima xenoliths, Southwest Japan arc Ritsuko MUROI and Shoji ARAI Department of Earth Sciences, Kanazawa University, Kanazawa 920–1192, Japan Rocks of the dunite–clinopyroxenite series (dunite, wehrlite, olivine clinopyroxenite, and clinopyroxenite) are common as cumulates formed around Moho in arc–related environments, but their formation processes remain unclear. They are common as xenoliths of Group I from Takashima in the Southwest Japan arc, and a descrip- tion of their formation process is provided here. The rocks vary from dunite to clinopyroxenite via wehrlite and olivine clinopyroxenite, and all showing mosaic equigranular to weakly porphyroclastic textures. The rocks are completely free from plagioclase, and they contain <3 vol% chromian spinel. Some of them contain up to 3 vol% orthopyroxene; these are approximate mixtures of olivine and clinopyroxene. As the dunites change to clinopyroxenites, the Mg# [= Mg/(Mg + total Fe) atomic ratio] varies from 0.93 to 0.84 in olivine, and from 0.92 to 0.87 in clinopyroxene. The Cr# [= Cr/(Cr + Al) atomic ratio] of the chromian spinel varies from 0.8 to 0.2 with decreases in the Mg# of olivine and clinopyroxene. The Mg–Fe distribution relation between olivine and clinopyroxene suggests their subsolidus equilibration is around 800–900 °C. Initial Mg#s expected at a magmatic temperature indicate that their formation proceeded from magma in the order of Mg–rich dunites followed by clinopyroxenites and then less Mg–rich dunite–wehrlite–olivine clinopyroxenite. This suggests a zigzag liquid path, starting from a mantle–derived olivine–oversaturated magma, around the olivine–clinopy- roxene cotectic boundary. Continuous crystallization of olivine or clinopyroxene due to supersaturation could have enabled the magma to straddle the cotectic boundary to form alternately clinopyroxene– and olivine– oversaturated magmas. Keywords: Dunites, Wehrlites, Clinopyroxenites, Xenoliths, Cumulates, Takashima, Supersaturation INTRODUCTION enite series rocks (cf., Wandji et al., 2009). Igneous proc- esses for the formation of dunite–clinopyroxenite series Plutonic rocks of the dunite–clinopyroxenite series, i.e., rocks remain unclear despite their common occurrences. dunites, wehrlites, olivine clinopyroxenites, and clinopy- Crystal accumulation during the perfect fractional crys- roxenites, are commonly found in ophiolites (e.g., Ishi- tallization of a mantle–derived basaltic melt, which is oli- watari, 1985; Benn et al., 1988) and as xenoliths (e.g., vine–oversaturated at low pressures, may produce dunite Arai et al., 2000). They are especially important as the followed by olivine clinopyroxenite based on the appro- main constituents of the MTZ (Moho transition zone) of priate phase diagrams (cf., Kushiro, 1969). Wehrlite and arc–derived ophiolites (e.g., Parlak et al., 2002) as well as monomineralic clinopyroxenite cannot be, however, form- of Alaskan–type zoned complexes (e.g., Krause et al., ed by this process. 2007). Dunites and wehrlites distributed around the Moho We describe xenoliths of dunite–wehrlite–clinopy- transition zone are interpreted to be cumulates and/or re- roxenite of Group I (Frey and Prinz, 1978) from Takashima action products between the melt and harzburgite (Koga in the Southwest Japan arc (Arai et al., 2000), and discuss et al., 2001; Negishi et al., 2013). There have been many their formation process as cumulates in the sub–arc con- papers dealing with xenoliths of dunites (e.g., Sen and dition. The analyzed samples are representative of the up- Presnall, 1986) and clinopyroxenites (e.g., Neumann et permost part of the sub–arc mantle (e.g., Takahashi, 1978). al., 1988), but only a few studies on dunite–clinopyrox- doi:10.2465/jmps.131003 GEOLOGICAL BACKGROUND R. Muroi, [email protected] Corresponding author S. Arai, [email protected]–u.ac.jp Large amounts of ultramafic xenoliths from Takashima 80 R. Muroi and S. Arai are found in beach boulders of alkali olivine basalt of 3.0 a microprobe and analyzed the clinopyroxene for REE Ma (Nakamura et al., 1986) on Takashima Island, Kara- (rare earth elements) using La–ICP–MS. See the Appen- tsu City, Saga Prefecture, northern Kyushu, Japan (e.g., dix (available online from http://japanlinkcenter.org/DN/ Arai et al., 2000, 2001). Most of ultramafic xenoliths are JST.JSTAGE/jmps/131003) for analytical details. All Fe less than 30 cm across, and dominated by dunite–wehr- was assumed to be Fe2+ in silicates, and Fe2+ and Fe3+ lite–clinopyroxenite series rocks of both Group I (meta- were calculated assuming a spinel stoichiometry. Mg# morphic, green Cr–rich clinopyroxene bearing) and and Cr# represent Mg/(Mg + Fe2+) and Cr/(Cr + Al) atom- Group II (igneous, black Al–rich clinopyroxene bearing) ic ratios, respectively. Fe# is (1–Mg#). Fo is 100 × Mg# in the sense of Frey and Prinz (1978) (Arai et al., 2000). in olivine. Table 1 shows the results for the selected anal- Arai et al. (2000) presumed a thick ‘cumulus mantle’ lay- yses on the major to minor elements of olivine, clinopy- er (Takahashi, 1978) comprised of dunite–wehrlite–py- roxene, and chromian spinel, whereas results for the REE roxenites of Group I, which could be equivalent to the of clinopyroxene are shown in Table 2 (available online Moho transition zone of ophiolite (Arai and Abe, 1994), from http://japanlinkcenter.org/DN/JST.JSTAGE/jmps/ beneath Takashima. The ‘cumulus mantle’ layer has been 131003). intruded by pyroxenites of Group II (Arai et al., 2000, Olivines, clinopyroxenes, and chromian spinels are 2006). See Arai et al. (2001) for more details. homogeneous in chemistry except for rims in contact with different minerals, and homogeneous core composi- PETROGRAPHICAL DESCRIPTIONS tions were used in this study. Olivines were found to be highly Mg–rich (Fo >90) in clinopyroxene–free dunites, Rocks of the dunite–clinopyroxenite series have various and less Mg–rich (Fo >84) in clinopyroxene–rich rocks appearances on alkali olivine basalt boulders (Fig. 1). (wehrlites to clinopyroxenites) (Table 1 and Fig. 2). NiO Nearly homogeneous dunites (Fig. 1a) and olivine–bear- and MnO contents of olivine vary from 0.4 to 0.1 wt% ing clinopyroxenites (Fig. 1b) are frequent, while homo- and 0.1 to 0.2 wt%, respectively, in an approximate rela- geneous wehrlites are less frequent. Various dunite–clino- tion to decreases in Fo. The Mg# of clinopyroxene varies pyroxenite composite xenoliths are commonly found nearly with that of olivine from 0.92 (olivine–rich rocks) (Figs. 1c and 1d); it is noteworthy that the mixture of dun- to 0.86 (clinopyroxene–rich rocks) (Fig. 2). The Al2O3 ite and clinopyroxenite was more common than the homo- content of clinopyroxene varies from 1 to 4 wt% with a geneous wehrlite. For the nearly homogeneous samples, decrease in Mg# (Table 1). Clinopyroxenes are mostly dunites grade to clinopyroxenites with a gap at olivine rich in Cr2O3, containing 0.5 to 0.7 wt% irrespective of clinopyroxenites that could be due to incomplete sam- rock type (Table 1). The Cr# of chromian spinel changes pling. We could not observe fine–scale layering in these from 0.8 to 0.2 with the Fo of olivine. The TiO2 content rocks, which is in contrast to those in some ophiolites of chromian spinel varied from 0.3 wt% in dunites and up (e.g., Ishiwatari, 1985). to 0.8 wt% in clinopyroxene–rich rocks (Table 1). The The dunite–clinopyroxenite series rocks contain Fe3+/(Cr + Al + Fe3+) atomic ratio is around 0.1 (Table 1). small amounts (<3 vol%) of chromian spinel, and some The REE patterns of clinopyroxene, which was nor- of them contain orthopyroxene (<3 vol%), which is malized to the chondritic abundances (Sun and McDo- roughly in proportion to the amount of clinopyroxene. nough, 1989), are strikingly similar. The normalized val- They are completely free from plagioclase. Furthermore, ues exhibit a gentle convex upward trend from Lu to Sm, they commonly exhibit mosaic equigranular to weakly where their levels are also similar in individual samples, porphyroclastic textures. Coarse (<7 mm across) grains and roughly in sympathy with the initial igneous Mg# of olivine and clinopyroxene exhibit deformation fea- (see DISCUSSION) (Fig. 3). No zonation in terms of tures, i.e., kinking and/or undulatory extinction under REE contents was observed in the clinopyroxene grains. the microscope. Some coarse clinopyroxene grains exhib- The patterns were, however, highly variable from Sm to it exsolution lamellae, but the orthopyroxene is free from La, sometimes even in a single thin section (Fig. 3). The them. Chromian spinel is less than 0.5 mm across, and enrichment of light REEs in clinopyroxene is not corre- black to brown in color for the thin sections. Constituents lated with any petrographic or chemical features of the of composite xenoliths are similar in petrography to the host rock. equivalents occurring as discrete xenoliths. Variations of mineral chemistry are discussed below based on the initial recalculated high–temperature Mg# of MINERAL CHEMISTRY olivine and clinopyroxene. We analyzed minerals for major to minor elements using Formation process of olivine–clinopyroxene cumulates 81 2 (a) (b) 10 Wehrlite (TK-28-2, Mg#=0.898) Clinopyroxenite (TK-02, Mg#=0.883) Dunite (TK-20, Mg#=0.871) Wehrlite (TK-35, Mg#=0.856) 1 10 0 (c) (d) 10 Cpx / C1 chondrite -1 10 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 3. Chondrite–normalized REE patterns of clinopyroxene in dunite–clinopyroxenite series xenoliths from Takashima. The in- Figure 1. Xenoliths of dunite–clinopyroxenite series from Taka- itial Mg#s (discussed below) are shown in parentheses. Chon- shima, the Southwest Japan arc. (a) Dunite nearly free from drite values from Sun and McDonough (1989). Note the pat- clinopyroxene. (b) Homogeneous olivine clinopyroxenite.