HR: 1340h
AN: V43C-1432    [Abstracts]
TI: New Results from High Pressure Phase Equilibria Experiments on the Richardton H-Chondrite
AU: * Danielson, L R
EM: albinosquid@sbcglobal.net
AF: Arizona State University, Department of Geolgical Sciences Arizona State University, Tempe, AZ 85287-1404 United States
AU: Sharp, T G
EM: tom.sharp@asu.edu
AF: Arizona State University, Department of Geolgical Sciences Arizona State University, Tempe, AZ 85287-1404 United States
AU: Leinenweber, K
EM: kurtl@asu.edu
AF: Arizona State University, Department of Chemistry and Biochemistry Arizona State University, Tempe, AZ 85287-1604 United States
AB: If the accretion of the Earth was accomplished by chondritic building blocks then melting and crystallization of chondritic material can be used to simulate the accreting and segregating bulk Earth. Because we may not have a representative "bulk Earth chondrite," silicate high P-T phase equilibria studies of a variety of chondrite compositions are needed to model liquids and crystallizing phases in a primordial segregating mantle. Crystallizing phases in a liquid magma ocean, whether persistent or intermittent, could significantly change the composition (and oxidation state) of that liquid by crystal flotation or settling. Also, crystals settling to the core-mantle boundary could provide a source for light elements in the core, or constraints on the abundance of some light elements. The Richardton H-chondrite was chosen as starting material for experiments because of its resemblance to the bulk Earth and higher FeO content than in a similar previous study of the Allende CV chondrite (Agee et al., 1995). Runs were conducted in a Walker-type multi-anvil apparatus and an end loaded piston cylinder. Pressures between 3 and 23 GPa and temperatures between 800 and $2300\deg$C were investigated, with a focus on higher temperatures bracketing the liquidus. Run durations were up to 50 minutes. Compositional identification of silicate run products was accomplished using either the Cameca SX-50 electron microprobe at the University of Arizona and the JEOL 8600 at Arizona State University. Mineral phases have been tentatively identified by stoichiometry, but Raman spectroscopy (ASU) is underway for more definitive mineral identification. The liquidus (olivine-out) at pressures less than 14 GPa occurs at temperatures less than about $1750\deg$C. Appropriate experiments for the full range of the liquidus at the garnet-out line have not yet been conducted. However, the probable garnet-olivine cotectic appears at about 13 GPa. Mg-Fe perovskite may be the liquidus phase above 21 GPa, with a perovskite-magnesiowustite cotectic occurring above 23 GPa. The lower liquidus below 14 GPa, relative to Allende and peridotite, is consistent with the higher FeO content. If Mg-Fe perovskite is the liquidus phase at higher pressures, an early crystallizing layer of perovskite could provide a source of Si and O for the core at the expense of other light elements, in addition to providing a potential sink for water in the lower mantle.
DE: 8125 Evolution of the Earth
DE: 8147 Planetary interiors (5430, 5724)
DE: 3630 Experimental mineralogy and petrology
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting