HR: 0800h
AN: V41D-1496    [Abstracts]
TI: Comparison of Th, Pb, Nd and Sr Isotopes in Oceanic Basalts: Implications for Mantle Heterogeneity and Magma Genesis
AU: Sims, M K
EM: ksims@whoi.edu
AF: Home, Capt Davis Lane, East Falmouth, MA 02536 United States
AU: * Sims, K W
EM: ksims@whoi.edu
AF: Dept of Geology and Geophysics Woods Hole Oceanographic Institution, Woods Hole Rd, 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 Rd, Woods Hole, MA 02543 United States
AB: In principle, comparison of 230Th/232Th, 238U/232Th and 230Th/238U with longer-lived radiogenic isotopes (e.g. 87Sr/86Sr) can provide first order insight into the nature of the mantle source, and the styles of melt generation and magma transport. Despite several studies comparing Th isotopes with longer lived radiogenic isotopes in oceanic basalts, only one of the four end member mantle components defining the mantle tetrahedron, DMM, is well represented in multi-dimensional isotope space. For the other three end member mantle components (HIMU, EM1 and EM2), there are no high precision measurements of Th isotopes for representative samples. Because of this lack of high quality Th isotopic data for the other three end-member mantle components (HIMU, EM1 and EM2), the form and relative significance of the relationships between U-Th systematics and the other longer isotopic systems like Sm-Nd, Rb-Sr and U-Pb are not yet well defined. Here we present new U-Th disequilibria data for a suite of young basaltic samples from the Samoan Islands representing the EM2 end member mantle component. These data are then compared with the MORB and OIB database for which U-series disequilibria and longer-lived radiogenic isotopes of Sr, Nd, and Pb have been measured by high precision mass spectrometric methods. This data compilation shows that: 1) 230Th/232Th and 238U/232Th are correlated with 87Sr/86Sr and 143Nd/144Nd; 2) these correlations can, to a first-order, be approximated by two-component mixing; and 3) the functional form of these relationships are hyperbolic rather than linear, as has been previously suggested. The relationships of 230Th/232Th and 238U/232Th with Pb ICs are more complex and require multiple source components. Using a maximum likelihood non-linear inversion method we show that the correlations of 230Th/232Th with 87Sr/86Sr and 143Nd/144Nd are better defined than the correlations of 238U/232Th with 87Sr/86Sr and 143Nd/144Nd, suggesting that 230Th/232Th is a better estimate of the U/Th source ratio and that net elemental fractionation of U from Th plays an important role in establishing a basalts (230Th/238U) disequilibria. In addition we show that the extent and variability in (230Th/238U) disequilibria decreases as a function of source enrichment, with MORB showing the largest extents and greatest variability in U-Th disequilibria. This latter observation suggests that melting processes beneath mid-ocean ridges are distinct from melting processes beneath ocean islands.
DE: 1000 GEOCHEMISTRY
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8125 Evolution of the Earth (0325)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005