Altered oceanic crust as repository for highly incompatible elements in the global geochemical cycle
Final Report 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. Being a mobile element, the budget of W in the oceanic crust, an important geochemical reservoir in the global subduction cycle, may be severely affected by hydrothermal alteration, but W data for altered oceanic crust (AOC) have not been available so far. We tackled this issue by measuring concentrations of W and related highly incompatible elements (Th, U, Nb, Ta) and other more compatible HFSE (Zr, Hf, Lu) by isotope dilution in different types of oceanic crust, namely upper oceanic crust (leg 1256D) and abyssal peridotites (leg 209). In the case of IODP-Borehole 1256D, W is systematically enriched over Th, U, and Ta over the whole depth of the drilled succession. Uranium is also hydrothermally enriched locally, whereas Th and Ta both behaved entirely immobile over the entire depth succession. Based on 230Th-238U data obtained for our samples, most alteration activity can be confined in age to >350 kyrs. Generally, the elemental ratios of W/Th, Ta/W and W/U show only minor overlap with pristine MORB ratios and often exceed the values reported for global arc lavas with respect to selective W enrichment. Ultramafic samples from leg 209 are extremely enriched in W, indicating that ultramafic portions of altered oceanic lithosphere are a major sink for W. 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 (enrichment factor of up to 10). Trace element modelling indicates that hydrothermal enrichment of W by redistribution is a more likely source for the W enrichment than seawater-derived W. Collectively, our data show that altered oceanic crust constitutes an important geochemical reservoir in the global geochemical cycle of W, contributing to the W enrichment found in arc lavas and to the recycling of W into the Earth’s mantle.
Publications
- (2021): Evidence for tungsten mobility during oceanic crust alteration. Chemical Geology 584: 120504
Reifenröther, R., Münker, C., Scheibner, B.
(See online at https://doi.org/10.1016/j.chemgeo.2021.120504) - (2022): Alteration of abyssal peridotites is a major sink in the W geochemical cycle. Geochimica et Cosmochimica Acta 321: 35-51
Reifenröther, R., Münker, C., Paulick, H., Scheibner, B.
(See online at https://doi.org/10.1016/j.gca.2021.12.030)
