Nb/Ta Fractionations as Tracer of Subduction

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Intellectual merit. Using current estimates for the composition of the Earth's principal reservoirs, mass balance for niobium (Nb) and hafnium (Hf) does not add up and some Nb and Hf is unaccounted for. This proposal is aimed at testing the hypothesis of the existence of a mantle reservoir that contains the balance of the Nb and Hf. This "hidden" reservoir would be high in Nb/Ta and/or low in Lu/Hf. The recognition of such a reservoir would have significant implications for understanding mantle evolution and the recycling of crustal components back into the mantle. Subducted oceanic crust may be the key to solving the apparent mass balance problem. However, for oceanic crust to be the postulated hidden reservoir, it needs to lose Ta compared to Nb in the subduction process. Nb/Ta is an especially useful petrogenetic tracer because very few processes can fractionate these elements (given their similar chemical properties), and thus it can become a very reliable indicator of recycled material. However, because the total variation is limited, high precision analyses are required to recognize the variations. Low-precision analyses of island arc volcanics indicate that there is a significant fractionation in Nb/Ta during subduction. However, the "when" and "where" of the Nb/Ta fractionation is unknown. We propose to determine the behavior of these and other elements in subducted material from various stages of subduction to determine the reaction(s) that result in the fractionation of Nb from Ta and to characterize the material that is subducted. We will work on natural materials as the experimental evidence on elemental partitioning is inconclusive. Once the fractionation process is identified the Nb/Ta can be used as a fingerprint of that process. We complement the analyses of the subducted material with analyses of island arc samples - with the aim to understand where in the subduction process Nb and Ta fractionate. Broader impacts. One outcome will be development of "new" high precision trace element analytical methods that will have wide applicability. This project will support a young scientist and a graduate student and it will provide core support for the operation of the laboratory, which the PI tries to operate as an open use facility, especially for students. In combination with existing programs at the National High Magnetic Field Laboratory (NHMFL) the proposed research will train and educate undergraduate students as well as teachers. The presence of an active research program is an essential component in being able to expose students and teachers to scientific discovery.