HR: 1340h
AN: V53A-0608    [Abstracts]
TI: Th, Nd, Sr and Pb Isotopes in Samoan Lavas: Implications for Mantle Heterogeneity.
AU: * Sims, K W
EM: ksims@whoi.edu
AF: Dept of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Hart, S R
EM: shart@whoi.edu
AF: Dept of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AB: We report new U-Th disequilibria data for a suite of 14 young basaltic samples from the Samoan Islands, whose Nd, Sr, and Pb isotopic compositions have been previously determined (Workman et al., 2004). ($^{230}$Th/$^{232}$Th) range from 0.610 to 0.778 and ($^{238}$U /$^{232}$Th) ranges from 0.579 to 0.767. All samples have ($^{230}$Th /$^{238}$U) greater than one, indicating that $^{230}$Th has been enriched relative to its parent $^{238}$U. These samples ($^{230}$Th /$^{238}$U) are essentially constant and independent of ($^{230}$Th/$^{232}$Th), ($^{238}$U /$^{232}$Th), and major and trace element variations (e.g. La/Sm, Sm/Nd, etc.). However, ($^{230}$Th/$^{232}$Th) and ($^{238}$U /$^{232}$Th) are well correlated with $^{87}$Sr/$^{86}$Sr, $^{143}$Nd/$^{144}$Nd and $^{208}$Pb/$^{206}$Pb. These samples ($^{230}$Th/$^{232}$Th) and ($^{238}$U /$^{232}$Th) are some of the lowest values yet measured in oceanic basalts and are consistent with the enriched nature of the Samoan mantle source (EM2). The low ($^{230}$Th/$^{232}$Th), ($^{238}$U /$^{232}$Th) and high $^{87}$Sr/$^{86}$Sr (up to 0.708) greatly extend the global correlations that have been observed between ($^{230}$Th/$^{232}$Th) vs. $^{87}$Sr/$^{86}$Sr and ($^{238}$U /$^{232}$Th) vs. $^{87}$Sr/$^{86}$Sr for oceanic basalts. A compilation of oceanic basalt data shows that the functional forms of these relationships are hyperbolic rather than linear, as has been previously suggested. Our data compilation also shows that the correlation between ($^{230}$Th/$^{232}$Th) and $^{87}$Sr/$^{86}$Sr is better defined than the correlation between ($^{238}$U /$^{232}$Th) and $^{87}$Sr/$^{86}$Sr, suggesting that for at least OIBs, ($^{230}$Th/$^{232}$Th) is a better estimate of the U/Th source ratio and that net elemental fractionation of U/Th plays an important role in establishing a basalts ($^{230}$Th /$^{238}$U) disequilibria.
DE: 8434 Magma migration
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting