Melting Relations in the MgO-SiO2 and CaO-MgO-SiO2 Systems at the Earth's Lower Mantle Conditions: New Methodological Approach and Preliminary Results
Abstract:Melting and crystallization of magma ocean(s) during the evolution of the early Earth may have led to extensive chemical differentiation. Some of the resulting mantle heterogeneities might have survived convective mixing and remained chemically distinctive, possibly residing today at the base of the lower mantle. To gain insights into the origin, composition and properties of these possible lower mantle heterogeneities, we are investigating melting phase relations on compositions in the binary MgO-SiO2 (MS) and ternary CaO-MgO-SiO2(CMS) systems at lower mantle pressures, using the laser-heated diamond anvil cell (LH-DAC) technique. An important objective of this study is to determine the effect of pressure on invariant melt compositions involving the following liquidus mineral assemblages, corresponding to peridotitic and basaltic model compositions:
1. Mg-perovskite (mgpv) + periclase (pc) and mgpv + silica in the MS-system.
2. Mgpv + pc + Ca-perovskite (capv) and mgpv + capv + silica in the CMS-system.
Two methodologies were developed in order to explore melting relations under high pressure conditions. Firstly, exploratory experiments were performed to investigate invariant melting temperatures (Tm) using postulated near-eutectic compositions. Secondly, in order to greatly reduce temperature gradients compared to conventional LH-DAC experiments, a novel technique for micro-fabrication of metal-encapsulated samples has been developed. The selected near-eutectic compositions show an expected positive P-Tm correlation, with lower Tm for the CMS-system. Our preliminary results indicate that the dTm/dP slope for the pv-silica eutectic is lower than for the pv-pc eutectic in the MS-system. Overall, there is good agreement between our results and the results of simulations based on density functional theory(1) and thermodynamic extrapolations of experimental data at lower pressures(2).
(1)de Koker et al. (2013), Earth Planet. Sci. Lett. 361, 58-63.
(2) Liebske and Frost (2012), Earth Planet. Sci. Lett. 345-348, 159-170.