Tungsten geochemistry and implications for understanding the Earth's interior

Details Tungsten geochemistry and implications for understanding the Earth's interior Tungsten geochemistry and implications for understanding the Earth's interior

Aug 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008e%26psl.272..656a&link_type=abstract

Earth and Planetary Science Letters, Volume 272, Issue 3-4, p. 656-665.

Computer Science

10

Scientific paper

The concentration of tungsten (W) in basaltic melts provides a window into the behavior of this element during core mantle separation, crust formation, silicate differentiation, and potentially core mantle interaction. We have analyzed an extensive suite of modern basalts (n = 86) for their trace element chemistry via laser ablation ICP-MS, with barium (Ba), thorium (Th), uranium (U), and W concentrations typically determined to ≤ 5% (2σ) uncertainty. We find that the partitioning behavior of U mirrors that of W during basalt genesis, whereas Ba and Th both behave more incompatibly. The W/U ratio of our complete sample suite (0.65 ± 0.45, 2σ) is representative of the mean modern mantle, and is indistinguishable from that of mid-ocean ridge basalts (W/UMORB = 0.65 ± 0.41, n = 52), ocean island basalts (W/UOIB = 0.63 ± 0.07, n = 10), and back-arc basin basalts (W/UBABB = 0.62 ± 0.09, n = 12). This ratio is also consistent with the W/U ratio of the continental crust, and thus represents the W/U ratio of the entire silicate portion of the Earth. Assuming a concentration of 20 ± 8 (2σ) ng/g U in the bulk silicate Earth, the abundance of W in the silicate Earth is 13 ± 10 ng/g. Following mass balance, this implies a mean modern mantle and core composition of 8.3 ± 7.1 ng/g W and 500 ± 120 ng/g W, respectively. Additionally, the MORB source is modeled to contain approximately 3.0 ± 2.3 ng/g W, indicating a four-fold depletion of the highly incompatible elements in the MORB source relative to the silicate Earth. Although both the isotopic composition of W and the constancy of the silicate Earth W/U ratio allow for potential insight into core mantle exchange, both of these proxies are extremely dependent on the chemical composition of the source. A case study of three Hawaiian picrites with enrichments in 186Os 187Os but terrestrial ɛ182W can be explained by: i) a lack of a core component in the Hawaiian “plume,” ii) crustal contamination, or iii) a source composition enriched in incompatible trace elements relative to the bulk silicate Earth.

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