HR: 08:15h
AN: V51E-02 INVITED     [Abstracts]
TI: Diffusive Fractionation of U-Series Nuclides During MORB Production
AU: * Van Orman, J A
EM: jav12@case.edu
AF: Case Western Reserve University Department of Geological Sciences, 10900 Euclid Avenue, Cleveland, OH 44106 United States
AU: Saal, A E
EM: Alberto_Saal@brown.edu
AF: Brown University Department of Geological Sciences, 324 Brook Street, Providence, RI 02912 United States
AU: Bourdon, B
EM: bourdon@ipgp.jussieu.fr
AF: Laboratoire de Geochimie et Cosmochimie IPGP-CNRS UMR7579, 4, Place Jussieu, 75252 Paris, cedex 05 France
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, 5241 Broad Branch Rd. NW, Washington, DC 20015 United States
AB: Experimental studies indicate that the distribution of U-series nuclides among minerals and liquids during mantle melting is unlikely to reflect equilibrium partitioning if solid-state diffusion is the rate-limiting process. We have developed a numerical model that considers the diffusion-controlled partitioning of parent and intermediate daughter nuclides between multiple minerals and liquid during near-fractional adiabatic decompression melting, and apply it here to the $^{238}$U-$^{230}$Th-$^{226}$Ra and $^{235}$U-$^{231}$Pa systems in mid-ocean ridge settings. Diffusion coefficients are derived either from experimental data or, where no data exist, from an elastic model (Van Orman et al., 2001, CMP 141:687-703). Before melting begins, the solid mantle is assumed to be in bulk secular equilibrium. Parent nuclides are distributed in chemical equilibrium among the minerals, and intermediate daughters have a steady-state distribution, calculated numerically, that reflects a balance between production, decay and diffusive exchange. During melting, a small fraction of melt remains and interacts with the upwelling solid, while the rest is instantaneously removed from the system and pooled with other extracted melts. A primary control on the composition of the melt is the solid upwelling rate, which sets the time available for diffusive exchange and ingrowth of daughters. With increasing upwelling rate, the [$^{230}$Th]/[$^{238}$U] activity ratio decreases because less time is available for ingrowth and because diffusion of U is slightly faster than Th in high-Ca pyroxene at mantle solidus temperatures. The [$^{226}$Ra]/[$^{230}$Th] ratio increases with upwelling rate as the effective partition coefficients for U and Th increase while the effective partition coefficient for Ra remains small due to its significantly higher diffusivity. The [$^{231}$Pa]/[$^{235}$U] ratio increases with melting rate when melting begins in the garnet facies and has the opposite trend when melting begins in the spinel facies. The resulting correlations between [$^{230}$Th]/[$^{238}$U] and [$^{226}$Ra]/[$^{230}$Th], and between [$^{230}$Th]/[$^{238}$U] and [$^{231}$Pa]/[$^{235}$U], resemble the data for MORB. However, disequilibrium dynamic melting of a single homogeneous source cannot account for the observed correlations of $^{230}$Th excess and $^{226}$Ra excess with the degree of enrichment of the basalt. We will discuss other possibilities for these correlations.
DE: 8434 Magma migration
DE: 3640 Igneous petrology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting