Woods Hole Oceanographic Institution

Alison Shaw

»Long-term preservation of slab signatures in the mantle
»Lower crustal crystallization and melt evolution at mid-ocean ridges
»Heterogeneous hydrationEffects of heterogeneous hydration in the incoming plate, slab rehydration, and mantle wedge hydration on slab-derived H2O flux in subduction zones
»Deep pooling of low degree melts and volatile fluxes at the 85?E segment of the Gakkel Ridge
»Earth's deep water cycle
»The geochemistry of hydrothermal fluids from the SVZ of Chile
»Central American gas geochemistry
»H isotope matrix effects
»He isotopes in Central American phenocrysts
»C-He-Ar in Manus Basin glasses
»CO2 fluxes in Central American volcanoes
»Alarcon Basin glasses
»N recycling in Central America
»He-Ne decoupling in Manus Basin glasses

A. M. Shaw, E. H. Hauri, M. D. Behn, D. R. Hilton, C. G. Macpherson and J. M. Sinton, Long-term preservation of slab signatures in the mantle inferred from H isotopes, nature geoscience, March 2012

Seismic tomographic images indicate that subducted lithosphere is transported into the deep mantle1. Petrologic modelling shows that water contained in subducted slabs can be carried to depths of at least 200 km (ref. 2); however, whether the hydrated slab signature is preserved at greater depths depends on diffusion rates. Experimental studies give conflicting results on the question of hydrogen preservation. On a small scale, hydrogen equilibration with ambient mantle should be rapid3,4, implying that the slab hydrogen signature may not be preserved in the deep mantle5. However, on large scales the time required for diffusive equilibration is longer and hydrogen anomalies may persist6,7. Here we present hydrogen and boron data from submarine volcanic glasses erupted in the Manus back-arc basin, southwestern Pacific Ocean. We find that samples with low hydrogen-isotope
values also exhibit the geochemical signature of dehydrated, subducted lithosphere. Combined with additional geochemical and geophysical data, we interpret this as direct evidence for the preservation of hydrogen anomalies in an ancient slab in the
mantle. Our geochemical data are consistent with experimental estimates of diffusion for the upper mantle6 and transition zone7.We conclude that hydrogen anomalies can persist in the mantle without suffering complete diffusive equilibration over timescales of up to a billion years.

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