Mineralogy and geochemistry of ultramafic xenoliths from Wallis Island: insights into mantle processes in the Southwest Pacific litosphere

Wallis Island is located in the central South Pacific, between Fiji and Samoa. Its geodynamic context is dominated by intraplate volcanism; however, its relationship with the Samoa hotspot remains debated (Price et al., 2014). The island is composed of recent (< 0.5 Ma) lava flows with affinities ranging from alkaline to tholeiitic, and with associated pyroclastic rocks (Price et al., 1991). Here, we present a mineralogical and geochemical study on a suite of ultramafic xenoliths enclosed within pyroclastic tuffs. The xenoliths include dunites and websterites. The dunites are predominantly composed of olivine (Fo contents= 88-90 mol%), and Cr-rich spinel (Cr# = 73-80); trace amounts of Cpx were observed in a dunite sample. Mineral chemistry supports the hypothesis that the dunites formed as cumulates from near-primitive basaltic magmas. The websterites consist of approximately equal proportions of Opx (~50 vol%) and Cpx (~45 vol%), with minor Fe-rich olivine (Fo79-84) occurring interstitially or as inclusions within Opx. Al-rich, Cr-poor spinel occurs as tiny inclusions within clinopyroxene. Both pyroxenes exhibit extensive mutual exsolution textures. Cpx is characterized by elevated Al2O3 contents (7.69–8.52 wt%), moderate TiO2 (0.43–0.80 wt%), and Na2O (0.93–1.20 wt%). Using the model of Wood and Blundy (1997), equilibrium melts calculated from Cpx cores yield Mg# values ~ 50, suggesting crystallization from a moderately evolved melt. Cpx displays flat to slightly convex-upward REE patterns with fractionated heavy REEs. Calculated melts in equilibrium with Cpx are enriched in light REEs, with variable La/Yb fractionation (LaN/YbN = 3-12). Application of conventional pyroxene thermometry yield high equilibration temperatures (1070-1150 °C). The formation of Cpx + Opx (+ Ol + Spl), in the absence of plagioclase, suggests that the websterite crystallization occurred at relatively high pressures (> 9 kbar, Elthon, 1993). Our findings suggest that the dunites and websterite xenoliths represent cumulates from primitive to evolved melts within a relatively deep section of the Pacific oceanic lithosphere.