HR: 17:30h
AN: V42H-07    [PDF]
TI: The Relationship Between Partial Melt Composition and Trace Element Partitioning Along the Peridotite Solidus
AU: * Gaetani, G A
EM: ggaetani@whoi.edu
AF: Dept Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AB: Peridotite partial melting occurs primarily within the upper 80 to 90 km of the Earth's mantle, where the fractionation of incompatible trace elements is controlled largely by their relative compatibilities in high-Ca clinopyroxene (Cpx). A significant number of Cpx-melt partition coefficients (D$_{i}$) have, therefore, been determined experimentally for geochemically important trace elements over a broad range in pressure, temperature, and bulk composition. These experiments have produced Cpx exceeding the compositional range produced in peridotite partial melting experiments. Collectively, the results from these studies demonstrate that composition-related variations in Cpx structure produce significant changes in D$_{i}$ by varying the misfit between the lattice site and the substituent cation, or by facilitating the charge balance required for heterovalent substitution. Gaetani and Watson [1,2] determined that the influence of melt structure on trace element partitioning increases with decreasing extent of partial melting at 1.5 GPa due to increasing polymerization of the melt. Systematic changes in the major element composition of peridotite partial melt generated near the peridotite solidus causes this effect to be very significant at 1.0 to 1.5 GPa, but relatively unimportant at higher pressures. This reconciles the large D$_{i}$ values determined by Blundy et al. [3] with the lower compatibility measured by Salters and Longhi [4]. The compositions of low-degree partial melts of mantle peridotite change dramatically over the pressure range of 1.0 to 2.5 GPa. With increasing pressure, the concentrations of SiO$_{2}$, Al$_{2}$O$_{3}$ and Na$_{2}$O in the melt decrease as FeO, MgO and CaO increase. This leads to a systematic decrease in melt polymerization as reflected in the ratio of non-bridging oxygens to tetrahedrally coordinated cations (NBO/T). As the value of NBO/T for low-degree melts increases from a value of $\sim$0.15 at 1.0 GPa to $\sim$0.95 at 2.5 GPa, the compatibility of the rare earth elements in Cpx monotonically decreases. For NBO/T values of $\sim$0.49 or less, melt structure plays a significant role in determining compatibility. In less polymerized melts, trace element compatibility is determined almost solely by the composition of the Cpx. Given these systematics and the range of pressures at which mantle peridotite is thought to cross its solidus beneath oceanic spreading centers, the large partition coefficients determined by Blundy et al. [3] are likely to be relatively unimportant for modeling trace element fractionations during partial melting. References: [1] Gaetani and Watson, 2000, EOS, 81:1383; [2] Gaetani and Watson, 2001, Eleventh Annual V.M. Goldschmidt Conf, Abst 3292. [3] Blundy, Robinson and Wood, 1998, EPSL 160:493-504; [4] Salters and Longhi, 1999, EPSL 166:15-30.
DE: 1065 Trace elements (3670)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting