Altered oceanic crust as repository for highly incompatible elements in the global geochemical cycle

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Contrary to what is expected, recent studies revealed that some highly incompatible element ratios, for instance those of W relative to Th-U-Ta, exhibit large variations between different geochemical reservoirs. These findings have fundamental implications for the understanding of processes in the silicate Earth, as (1) highly incompatible elements serve as important tracers for processes active during crust-mantle differentiation, and (2) the mass balance of these elements is essential for employing isotope systems like Hf-W or UTh-Pb to address problems of Earth evolution. Several studies on processes triggering fractionation of W-U-Th-Ta between different silicate reservoirs on Earth could not entirely explain the variations observed. Notably, these studies overlooked an important silicate reservoir, namely altered oceanic crust, where submarine hydrothermal processes are capable of efficiently fractionating ratios of these incompatible elements.In the proposed project, we therefore intend to test the hypothesis that altered oceanic crust may be an important reservoir triggering significant fractionation of W-U-Th-Ta ratios in the global geochemical cycle. We argue that altered oceanic crust is a likely candidate, because even at low element concentrations it can efficiently control the budget of these elements via submarine hydrothermal processes, since altered oceanic crust makes up a significant portion of the Earths surface. Moreover, a selective mobility of W and U in aqueous fluids is predicted from ore deposit studies or aqueous geochemistry. So far, verification of our working hypothesis was hampered by (1) by limited access to complete sample sets and (2) by the lack of suitable analytical techniques for such low concentrations. The analytical limitation may now be overcome by the availability of isotope dilution techniques that permit sufficient precision and accuracy, in particular for elements like W-U-Th-Ta with low-ppb abundances in the oceanic crust. Our group is one of the few groups worldwide that can perform high precision isotope dilution measurements of W and Ta, and it is proposed here to expand these protocols to U and Th.We propose for the first time to perform a comprehensive high-precision W-U-Th-Ta survey on different sections of upper and lower oceanic crust. These sections cover (1) the only available complete section of altered upper oceanic crust (Hole 1256D) ranging down to the sheeted dike-gabbro transition, and (2) variably altered lower oceanic crust comprising abyssal peridotites and gabbros (Leg 209). Measurements will be performed on variably altered and petrologically well characterized bulk samples as well as on mineral separates covering potential carriers of W-Th-U-Ta. First high precision Ta-W-data for 1256D samples obtained in a pilot study revealed marked enrichments of W and U relative to Th-Ta that cannot be explained by igneous processes and close the Ta-W gap between MORBs and arc lavas.