Melting of Peridotite at Lower Mantle Conditions: Implications for Magma Ocean Differentiation and Melting Processes at the CMB

Details Melting of Peridotite at Lower Mantle Conditions: Implications for Magma Ocean Differentiation and Melting Processes at the CMB Melting of Peridotite at Lower Mantle Conditions: Implications for Magma Ocean Differentiation and Melting Processes at the CMB

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v13c2049f&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #V13C-2049

Physics

[1212] Geodesy And Gravity / Earth'S Interior: Composition And State, [1038] Geochemistry / Mantle Processes, [3630] Mineralogy And Petrology / Experimental Mineralogy And Petrology, [3924] Mineral Physics / High-Pressure Behavior

Scientific paper

The bulk silicate Earth (mantle before crust extraction) is thought to have a composition that matches the one of the most fertile or undepleted mantle peridotites (e.g. McDonough & Sun, Chem. Geol. 120, 223, 1995). A fully crystallized bulk silicate Earth as it is mostly the case today should contain at lower mantle pressures about 80 wt% Mg-perovskite, 15 wt% ferropericlase and 5 wt% Ca-perovskite (e.g. Irifune, Nature 370, 131, 1994; Wood, Earth Planet. Sci. Lett. 174, 341, 2000). However, early in Earth’s history, an episode of extensive melting probably affected the planet leading to the formation of a deep magma ocean. To investigate the geochemical consequences of the presence of such a magma ocean, we performed melting experiments on a fertile natural mantle peridotite (KLB-1) at 50, 85 and 110 GPa. To ensure chemical homogeneity and optimal Fe2+/Fe3+ ratio, the starting material (KLB-1) was melted and quenched into a glass by gas levitation and laser heating under slightly reducing conditions of oxygen fugacity. The experiments were conducted in diamond-anvil cells at the high-pressure beamline of the ESRF so as to use clear in situ melting criterion and to determine phase relationships from X-ray diffraction. FIB sections of the recovered diamond-anvil cell samples were further investigated at the nano-scale by scanning and analytical transmission electron microscopy to determine melting/crystallization sequences as well as variations of phase composition with temperature and pressure. Our results, which extend drastically the pressure range of results from previous multi-anvil studies (e.g. Ito et al., Phys. Earth Planet. Int. 143, 397, 2004), allow us to constrain the way the putative magma ocean would have crystallized and its implications for deep mantle differentiation. Our new results also yield strong constraints on the solidus curve of the lower mantle and provide a test for the only reference available (Zerr et al., Science 281, 243, 1998). Finally, our study provides experimental insights into the possible existence of a deep molten layer at the base of the present-day mantle.

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