Potassium isotope compositions of Mariana arc lavas and their sedimentary input.

We apply the stable potassium isotope system (41K/39K) to well-studied Mariana arc lavas, in which inter-island geochemical variability has been interpreted to reflect near constant addition of an aqueous fluid flux, that dominantly samples the subducting mafic oceanic crust and variable amounts of sediment melt addition to the sub-arc mantle wedge. The nature of the sediment component in the Mariana arc lavas remains enigmatic, with a mixture of melts from all subducted sediment lithologies and the altered-mafic oceanic crust seeming likely, but some interpretations point towards a component from melting of the volcaniclastic sediment alone. We present K isotopic data on a set of well-characterised Mariana arc lavas and sediment samples (from Ocean Drilling Program site 801). Our data show that the majority of Mariana arc lavas are isotopically heavy (ẟ 41K), by up to ∼0.2 ‰, relative to mid-ocean ridge basalt (MORB), but most of the subducting sediment samples are slightly isotopically lighter than, or indistinguishable from MORB. The volumetrically important volcaniclastic sedimentary unit however is significantly isotopically lighter than MORB, by ∼0.8 ‰, which reflects marine diagenetic processes. Thus, volcaniclastic material significantly influences the bulk sediment K isotope composition. We show that ẟ 41K compositions of the Mariana arc lavas can be reproduced by the addition of an aqueous fluid with isotopically heavy K (relative to MORB) and an additional fraction of an incompatible-element-enriched isotopically light melt component. Modelling of K isotopes together with K/La and radiogenic Nd indicate that the melt component is best explained by a mix of melts from bulk sediment and altered-mafic oceanic crust. Our results show that distinctive K isotopic variations in subduction zone inputs and K isotopic fractionation during dehydration reactions makes K a useful tracer of subduction zone processes. [ABSTRACT FROM AUTHOR]