TRACING DEHYDRATION OF THE SUBDUCTED SLAB WITH TUNGSTEN ISOTOPES IN ARC LAVAS

Subduction zones are fundamentally linked to the recycling of elements and the formation of continental crust. Tungsten, a moderately siderophilic element, is fluid mobile in the sub-arc mantle (König et al., 2011) but little is known about its geochemical cycle. Using a novel W double spike technique (Krabbe et al., 2017), this study evaluates W stable isotopic fractionation in subduction zones. A characteristic suite of samples from three subduction zones were chosen to trace W fractionation. Samples from the volcanic front and rear arc of the Sangihe and Izu-Bonin arcs were chosen to evaluate W fractionation between fluid/melt slab components. Samples from SW Japan were chosen to evaluate processes related to subduction of a young, hot slab.

W isotope ratios are reported as the ‰ deviation from NIST SRM 3163, i.e., δ¹⁸⁴W. Assuming average chondritic values (0.027±0.007 δ¹⁸⁴W) for bulk silicate earth, our results show both heavy and light W isotopic fractionation, ranging from ~0.077 to ~0.001 δ¹⁸⁴W. The heaviest δ¹⁸⁴W are observed in fluid rich samples from the volcanic fronts of the Sangihe and Izu-Bonin arcs. With increasing distance from the volcanic front, rear/back arc slab-melt rich samples are characterized by lighter δ¹⁸⁴W. Enriched alkali basalts from SW Japan have the lightest δ¹⁸⁴W. We suggest that the heavy δ¹⁸⁴W signature records seawater recycling in a subduction zone. As the heavy δ¹⁸⁴W is released during dehydration, any remaining W could preferentially fractionate into rutile, as W readily substitutes for Ti in rutile (Zach et al., 2002). This W incorporation into rutile could possibly account for the light δ¹⁸⁴W signature recorded in SW Japan were slab melts have OIB-like Na/La ratios. Our results indicate that stable W isotope systematics have the potential to improve our understanding of earth processes and trace slab dehydration in arc volcanoes.