Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.u52a0014d&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #U52A-0014
Physics
1010 Chemical Evolution, 1015 Composition Of The Core, 1025 Composition Of The Mantle, 3630 Experimental Mineralogy And Petrology, 3672 Planetary Mineralogy And Petrology (5410)
Scientific paper
The earliest history of the accreting Earth involved the removal of metallic liquids to the core and segregation of silicates into a layered Earth. One hypothesis for core formation is that descending metallic liquids equilibrated with silicate liquids in the deep mantle. Of current interest is the possibility that a primordial magma ocean may have acted as a host for both silicate and metallic liquid segregation. The silicate liquid composition may have changed by processes such as crystal settling or flotation during accretion as the planet increased in size. If these were equilibrium processes, a P-T phase diagram of representative accretion material could be used to constrain the chemical evolution of the Earth by identifying silicate minerals and liquidus phases present at elevated pressures and temperatures (Agee, 1990; Agee et al., 1995). Experiments were carried out in a "Walker-type" 6-8 multi-anvil device in a 1100 ton press. Pressures from 5 to 11 GPa, at temperatures from 1050 to 2100 ° C, have been investigated using an 8 mm TEL assembly with a LaCrO3 furnace and either MgO or graphite capsules. Experiment durations were from 4 to 31 minutes. We chose the Richardton H-chondrite as starting material because it is a reasonable representation of the bulk Earth. An ongoing problem with these experiments is containment of the liquids within the MgO capsules. Additionally, the MgO capsule reacts with the silicate liquids, elevating the MgO content of the silicate melt and reducing the FeO content. Experiments conducted in graphite capsules do not have a containment problem. Phases present were tentatively identified using EDS spectroscopy. In the investigated P-T range, run products contain olivine of intermediate composition, low- and high-Ca pyroxene, and small amounts of garnet in subsolidus experiments. Runs conducted at 1700 ° C contain silicate liquid, olivine, and low Ca pyroxene at 6 GPa, but silicate liquid, olivine, low and high Ca pyroxene at 9 GPa. At 1800 ° C and 9 GPa runs contained silicate liquid, olivine and garnet. All runs contained metal-sulfide liquids. Our preliminary data indicates the liquidus near 9 GPa occurs at about 1975 ° C. This falls within the expected range between 1875 ° C for the Allende meteorite (Agee et al., 1995), and 2100 ° C for peridotite (Zhang and Herzberg, 1994). Future experiments will more fully characterize the slope of the liquidus, particularly in the pressure range of 20 to 27 GPa, the pressures most relevant to a deep magma ocean.
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