Alteration of abyssal peridotites is a major sink in the W geochemical cycle

Abstract

Arc lavas and the continental crust exhibit a selective enrichment of W relative to similarly incompatible elements, indicating that subduction zone environments are a tectonic setting where W is mobilized from the subducting slab. Here we present evidence that ultramafic portions of altered oceanic lithosphere are a major sink for W and evaluate how its W budget needs to be included in constraining the global geochemical cycle of W. We report high precision W, HFSE and U-Th data obtained by isotope dilution for altered basement formed at super-slow spreading rates along the Mid-Atlantic Ridge drilled during ODP Leg 209 (between 14°N and 16°N). The oceanic basement consists of abyssal peridotites and associated gabbroic rocks exhumed in a magma starved setting. Hydrothermal alteration styles covered in the sample set are serpentinization, talc alteration, and low-T seawater alteration. The drilled rock portions are among the most depleted in HFSE that have ever been studied.Tungsten concentrations range from 4 ppb in the least altered harzburgites to 500 ppb in strongly serpentinized dunites. Locally, W is significantly enriched relative to U, Th and Ta by factors of up to 100 compared to canonical mantle values, much higher than in mafic portions of altered oceanic crust such as hole 1256D (up to 10). The samples from ODP Leg 209 show strong W enrichment during progressive serpentinization (holes 1268A, 1270 and 1271) and during late-stage oxidative seawater alteration (hole 1270D and 1272A). In contrast, silica metasomatism associated with talc alteration (hole 1268A) is associated with selective W depletion relative to Th and Ta. Trace element modelling indicates that hydrothermal enrichment of W by redistribution is a more likely source than seawater-derived W. Collectively, our data show that altered ultramafic rocks likely constitute an important geochemical reservoir in the global geochemical cycle of W contributing to the enrichment of W found in arc lavas and to the recycling of W into the Earth’s mantle.