Plate subduction plays a critical role in plate tectonics, yet the interaction between the subducted plate and the mantle wedge in nascent subduction zones remains poorly understood. The southwestern Mariana arc located in the western Pacific near Micronesia, representing a modern analog of subduction infancy, provides an ideal setting to investigate subduction infancy and the recycling of subducted materials. Here we present petrographic, mineralogical, and whole-rock geochemical data for serpentinized forearc peridotites recovered from the southwest Mariana arc. These forearc peridotites can be divided into two groups based on their chromite Cr# values. The high-Cr# group (Cr#: 0.78−0.86) records interaction with high-Si boninitic melts, as indicated by the presence of chromite-hosted amphibole inclusions whose trace-element compositions are consistent with a high-Si boninitic affinity, whereas the low-Cr# group (Cr#: 0.49−0.74) is interpreted to have experienced metasomatism by melts of adakitic affinity, as suggested by the occurrence of clinopyroxene with adakite-like geochemical characteristics. Compared to the mature Izu-Bonin and central Mariana arcs, the southwestern Mariana forearc peridotites are geochemically more enriched, showing higher SiO2 and Fe2O3T, lower MgO, and up to an order of magnitude higher heavy rare earth element contents. These features indicate that the forearc peridotites in immature subduction zones preserve more pristine records of early mantle wedge processes, owing to limited subsequent modification after the initial stages of subduction. Our findings challenge the prevailing view that forearc peridotites are highly depleted and refractory and highlight the importance of early melt−rock interaction in shaping sub-arc mantle wedge compositions during subduction initiation.
Yao et al. (Thu,) studied this question.