Results From in Situ High P-T Melting and Phase Equilibria Experiments on the Allende Meteorite

Details Results From in Situ High P-T Melting and Phase Equilibria Experiments on the Allende Meteorite Results From in Situ High P-T Melting and Phase Equilibria Experiments on the Allende Meteorite

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmmr31a0144d&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #MR31A-0144

Physics

1025 Composition Of The Mantle, 1027 Composition Of The Planets, 3612 Reactions And Phase Equilibria (1012, 8412), 3630 Experimental Mineralogy And Petrology, 5455 Origin And Evolution

Scientific paper

Because chondritic materials are thought to be the building blocks of terrestrial planets and planetesimals, crystallization of chondritic and peridotitic material can be used to simulate accretion and differentiation of a bulk planet. The objective of this study is to measure the liquidus phases and temperatures for a number of planetary mantle analog materials at P>20 GPa. Experiments were conducted in the Large Volume Press at the Advanced Photon Source, Argonne National Laboratory. Phases were identified using energy-dispersive X-ray diffraction (EDXRD) with a fixed diffraction angle (2θ) of ~6° and data collection times of 60 sec. Heating runs up to 2200°C were performed at 400, 600, and 700 tons, sampling a pressure range from 18-32 GPa. A 3mm TEL beamline modified Fei-type assembly was used in experiments: a Re furnace with lanthanum chromite insulating sleeve, alumina or MgO end caps, graphite capsule packed with powdered starting materials, and X-ray windows of a slit in the Re furnace and alumina or graphite plugs in the lanthanum chromite. A pressed pellet of MgO powder doped with diamond powder was used as a pressure standard and packed between the capsule and thermocouple. The majorite liquidus temperature occurs near 2050°C, comparable to previous results (Agee et al. 1995; Asahara et al., 2004). The majorite-Mg-perovskite cotectic occurs at 22 GPa, as opposed to around 25 GPa. However, given the limited number of experiments and uncertainties introduced from previously not applying (1) a P-T relationship from relaxation of the assembly during heating, and (2) a thermal gradient from temperatures measured at the thermocouple across the capsule, significant differences in P and T are not surprising. It is possible to apply a P-T correction based on (1) and (2) above, and identify a majorite-ferropericlase cotectic for previous studies at around 22 GPa. This is particularly true for Agee et al. (1995), where calibrations were performed at 1200°C. Pressure at 2000°C can be 3 GPa lower than that at 1200°C (Leinenweber et al., 2006). Although Mg-perovskite was identified as the liquidus phase above 22 GPa, the ferropericlase-out line is very steep, and may become the liquidus phase above 25 GPa. This shallow liquidus is more consistent with a deeper, hotter magma ocean model.

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