CHARACTERIZING THE EVOLUTION OF SLAB INPUTS IN THE EARLIEST STAGES OF SUBDUCTION: PRELIMINARY EVIDENCE FROM FLUID-MOBILE ELEMENTS FOR IZU-BONIN BONINITES RECOVERED DURING IODP EXPEDITION 352

IODP Expedition 352 drilled >800 m of boninitic lavas, hyaloclastites and shallow intrusives at two adjacent sites (U1439A-C and U1442A) along the outer margin of the overriding plate in the Izu-Bonin forearc. Given that boninites form as a result of fluid-addition melting in the early stages of subduction, these samples provide an opportunity to examine characteristics of slab-derived fluid components at the beginnings of the subduction process.

Expedition 352 boninitic lavas at both sites record a transition from low Si to high Si boninite upsection, with major and trace element and radiogenic isotope indications for differences in mantle composition between the low Si and high Si lavas (e.g., Reagan et al 2015; Prytulak et al., 2016 [Goldschmidt Conference]; Shervais et al. 2016, [this meeting]). Fresh boninitic glasses recovered from these cores show a similar dichotomy in their mobile element systematics, with the high-Si boninites showing much greater enrichments in B and other fluid-mobile elements (FMEs; As, Sb, Cs, Li) than low Si boninites. Both types of Exp. 352 boninites display two-component mixing relationships between an FME-poor mantle and a FME enriched slab-derived component, a pattern that is at odds with that of the Izu-Bonin or Mariana volcanic arcs, or that of other arcs globally, in which a second, FME-depleted slab-derived component is universally observed. While the fluid-mobile element enriched components in volcanic arcs are associated with elevated δ¹¹B signatures, and have thus been argued to reflect inputs from subduction-related serpentinite, preliminary boron isotope results from Exp. 352 boninites point to comparatively low values, suggesting that the fluxes and/or reservoirs for fluid-mobile species are different when subduction initiates, and evolve as it proceeds.