Potassium isotope evidence for slab-derived fluids in the sub-arc mantle

Abstract

Subduction recycles water, other volatiles and incompatible elements into Earth's mantle and plays a major role in exchanges between its surface and interior. Potassium (K), a highly fluid-mobile and incompatible alkali element, is strongly affected by these processes. The emerging K stable isotope system has been proposed as a new tracer of subduction events, e.g. to distinguish the inputs to arc magmas from sediments, altered basalts and other near-surface materials. Yet, it remains unknown if and how the K isotopic signatures of these components are affected by extraction of K-bearing media from dehydrating slabs and their transfer to the mantle wedge. Fragments (xenoliths) of sub-arc mantle carried to the surface by arc magmas may retain these slab-derived components. We report high-precision K isotope data on mantle harzburgite xenoliths from Avacha, an active arc volcano in Kamchatka (Russia) to further investigate the nature of slab-derived fluids and their impact on arc mantle. We find a narrow δ41K range for amphibole-rich cumulates from host andesite magmas (−0.53 ± 0.09‰ to −0.34 ± 0.09‰) that overlaps those of the bulk silicate Earth (BSE) and the upper continental crust (UCC). In contrast, the Avacha peridotites derived from the uppermost lithospheric mantle (refractory melting residues metasomatized by subduction zone fluids) show an extremely broad K isotopic range (δ41K ∼ 3‰) and are commonly enriched in heavy K isotopes (up to +1.16 ± 0.08‰) relative to the BSE and UCC; some of the peridotites show the highest δ41K values for any reported mantle rocks. The new data on the Avacha xenoliths, along with available K isotope data on arc lavas and eclogites, suggest that slab-derived fluids may develop heavy K isotopic signatures, distinct from those in the original subducting materials (e.g., marine sediments and altered oceanic crust). These results provide direct evidence that (a) subduction dehydration and fluid percolation in the mantle lithosphere can significantly fractionate K isotopes, and (b) slab-derived fluids infiltrated the sub-arc mantle wedge. Due to the large K concentration difference between the depleted mantle and slab-derived fluids, K isotopes may be a sensitive tracer of fluid activity in sub-arc mantle.