HR: 0800h
AN: U41A-0718    [Abstracts]
TI: Origin of Tungsten Excess in Komatiites
AU: * Becker, H
EM: hbecker@geol.umd.edu
AF: Harry Becker, LPI,3600 Bay Area Boulevard, Houston, TX 77058 United States
AU: Brandon, A D
EM: alan.d.brandon1@jsc.nasa.gov
AF: Alan D. Brandon, NASA-JSC 2101 NASA Road 1 Mail Code SR, Houston, TX 77058 United States
AU: Walker, R J
EM: rjwalker@geol.umd.edu
AF: Richard J. Walker, Dept. of Geology, University of Maryland, College Park, MD 20742 United States
AB: The limited database available for W abundances in komatiites (n=7, Newsom et al., 1996) suggests that when melting and fractional crystallization effects are filtered out, these komatiites have about 10 times higher W, compared to other mantle-derived mafic-ultramafic magmas (MORB, OIB). The excess of W in the komatiites relative to lithophile highly incompatible elements becomes obvious when compared with the low concentrations of the light REE Ce and Nd (about 1-2 ug/g in many komatiites, compared to $>$ 10 ug/g in most MORB and OIB). In order to increase the komatiite W database, komatiite samples from Phanerozoic (Gorgona Island) and Archean terraines (Boston Creek/Canada, Belingwe/South Africa, 2.7 Ga) were dissolved and W was separated in order to obtain W concentrations by isotope dilution. Except for one sample from Gorgona Island with low W (23 ng/g), samples from all three locales show high W (516 to 2643 ng/g), with most samples containing near 700 ng/g W. Three Hawaiian picrites (H23, LO-02-04, MK-1-6) were also analyzed for comparative purposes and contain 75, 163 and 418 ng/g W, respectively. The W concentrations in the Hawaiian picrites are comparable or lower than W concentrations in Hawaiian tholeiites (Newsom et al., 1996). Mass balance considerations suggest that it is unlikely that the W excess in komatiites reflects W contributions to the mantle sources of komatiites from the outer core. The W enrichment could result from shallow-level alteration processes if primary W abundances of komatiites were low and W was added via fluids, containing W and other fluid-mobile elements derived from crustal rocks. Because most W in such samples would be of crustal origin, small contributions from the outer core may be difficult to detect using 182W systematics (Schersten et al., 2003).
DE: 1025 Composition of the mantle
DE: 1065 Trace elements (3670)
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Union [U]
MN: 2004 AGU Fall Meeting