Contrasting melt percolation and melt-rock reactions in the Parece Vela back-arc oceanic lithosphere, Philippine Sea: A mineralogical perspective

Abstract

The accretion and evolution of oceanic lithosphere beneath back-arc spreading ridges remain less understood compared with mid-oceanic ridges (MOR). Here, we investigate mantle peridotites and gabbros recovered from the southernmost side of the Parece Vela back-arc basin, Philippine Sea. The peridotites contain multiple types of clinopyroxenes (Cpx) and amphiboles (Amp) which record contrasting melt percolation and melt-rock reaction processes. Type-1 Cpx shows primary texture and light rare-earth elements (LREE)-depleted pattern, which can be reproduced by ~8–10% fractional melting of depleted MORB mantle. Type-2 Cpx occurs in the deformed peridotites (SY224-1280-2a & -2b) and shows less depleted LREE at similar middle to heavy REE (M-HREE) contents compared with type-1 Cpx. Such features reflect re-equilibration with MORB-type percolating melts, which is further evidenced by elevated TiO 2 and Cr# in coexisting spinels and crystallization of associated MORB-type gabbros. Type-3 Cpx occurs in the undeformed peridotites (SY225-1255-1 & -2) and is texturally replaced by type-1 Amp. They show variably enriched large ion lithophile elements (LILE) and LREE at similar M-HREE contents compared with type-1 Cpx, which reflect an open-system (flux) melting and reaction process with a melt flux enriched in LILE and LREE. Type-1 Amp is almost magnesio-hornblende and shows trace-element pattern characteristic of supra-subduction amphiboles, which supports the hydrous melt that formed type-3 Cpx probably originated from subducted slab. In contrast, type-2 Amp occurs in a leucocratic vein in sample SY225-1255-1 and is characterized by nearly flat M-HREE pattern at 600–200 × CI, LREE depletion and marked negative Eu anomaly. These features are consistent with crystallization from an amphibole-saturated melt formed by extreme differentiation of MORB-type melts. Our data, together with previous observations, suggest co-occurrence of hydrous MORB-type and slab-derived melt percolations beneath the Parece Vela spreading ridge, which may affect the chemical evolution of back-arc oceanic lithosphere different from MOR. • Multiple types of clinopyroxenes and amphiboles occur in the PVB peridotites. • They reflect open-system melting and reaction with hydrous MORB and slab melts. • Hydrous open-system mantle processes are features of back-arc oceanic lithosphere.