GSA Connects 2021 in Portland, Oregon

Paper No. 232-12
Presentation Time: 4:40 PM


MAZZA, Sarah, Department of Geosciences, Smith College, Northampton, MA 01063, STRACKE, Andreas, Institut für Mineralogie, University of Münster, Münster, 48149, Germany, GILL, James B., Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064 and KLEINE, Thorsten, Institut für Planetologie, University of Münster, Münster, 48149, Germany

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.