Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.u51a..06w&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #U51A-06
Physics
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 3630 Experimental Mineralogy And Petrology, 3672 Planetary Mineralogy And Petrology (5410), 5455 Origin And Evolution, 8125 Evolution Of The Earth
Scientific paper
Most of Earth's mass probably accreted as a consequence of numerous impacts between large bodies and proto-Earth, and a giant impact with a Mars-sized object is the most plausible explanation for a Moon forming event. 1 Physical models show that large impacts would have caused high-degrees of melting and a global magma ocean. 2 Crystallization differentiation in a deep magma ocean could impart stratification in the solidified mantle, forming large geochemical domains. To accurately model crystallization in a deep magma ocean the liquidus phase-relations of peridotite, as well as mineral/melt element partitioning, must be known at lower mantle conditions. Here, we report the results of liquidus experiments on fertile model peridotite compositions at 23 - 33 GPa. Experiments were performed in 6/8-type multi-anvil apparatus using carbide and sintered-diamond second-stage anvils with 4 and 2 mm truncations, respectively. Samples were encapsulated by either graphite or Re. High-temperatures were generated using LaCrO3 or Re furnaces, and temperatures were held from 2 to 50 minutes at 2300 - 2500 C. Run products were analyzed for major and trace elements using EPMA and SIMS. At 23 GPa the liquidus phase is majorite, followed closely down temperature by ferropericlase (Fp) and Mg-perovskite (Mg-Pv). At 24 GPa the liquidus phase has changed to Fp, followed closely by majorite and Mg-Pv. Ca-perovskite (Ca-Pv) is present only at much lower temperatures close to the solidus. At approximately 31 GPa Mg-Pv is the liquidus phase followed down-temperature by Fp then Ca-Pv. At ~ 33 GPa Ca-Pv crystallizes closer to the liquidus, within about 50 C, at a similar temperature to Fp. Thus, important phases crystallizing in a deep magma ocean are Mg-Pv, Ca-Pv and Fp. Crystallization models based on major element partitioning show that only very modest amounts of crystal separation of a Mg-Pv + Fp assemblage can be tolerated before Ca/Al, Al/Ti and Ca/Ti ratios become unrealistic for estimates of primitive upper mantle (PUM). 3 However, even small amounts of Ca-Pv in the crystal assemblage effectively buffer these ratios at values close to the starting composition (e.g. chondritic). Further, based on our new trace element partitioning data, models involving considerable Mg-Pv fractionation generally show poor matches with model PUM. For example, model PUM has sub-chondritic REE/Ti, whereas these ratios increase considerably during Mg-Pv crystallization. Notable exceptions are super-chondritic Zr/Ti, chondritic Sr/Ti, and sub-chondritic Zr/Nb and Sm/Yb ratios, all of which are well matched by considerable Mg-Pv crystallization. Although trace element D's for Ca-Pv are not yet measured quantitatively, the observed affinity of Ca-Pv for REE could conceivably account for the the sub-chondritic REE/Ti ratios in PUM. Ca-Pv also concentrates K, and could be an important source of heat from radioactive decay in the lower mantle. 1. Canup, R. and Agnor, C., Origin of the Earth and Moon, Righter and Canup, eds., U. Arizona Press, 113-144, 2000. 2. Melosh, H., Origin of the Earth, Newsom and Jones, eds., Oxford Press, 69-84, 1990. 3. McFarlane, E. et al., Geochimica et Cosmochimica Acta, 5161-5172, 1994.
Frost Daniel
Ito Emi
Nakamura Eita
Tronnes R.
Walter Michael J.
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