Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmmr11a..01o&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #MR11A-01
Physics
1025 Composition Of The Mantle, 1037 Magma Genesis And Partial Melting (3619), 1038 Mantle Processes (3621), 8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105), 8125 Evolution Of The Earth (0325)
Scientific paper
Density of magma is important for chemical fractionation of the Earth interior. Since magma is extremely compressible compared to crystals, a density crossover between magma and crystals is expected to occur in the deep mantle. Several regions of the magma-crystal density crossover are expected to occur in the mantle. The density crossover between peridotite magma and equilibrium olivine was observed at a pressure around 13 GPa [1]. Thus, in the early earth neutral buoyancy of olivine occurs in the primordial magma ocean and effective separation of olivine did not occur at the depth. This could produce an olivine rich upper mantle in the magma ocean stage. Density crossover in the deep upper mantle also provides a maximum depth for generation of komatiites, i.e., although komatiites can be generated by melting at various depths in the upper mantle, they can not ascend at the depths greater than the base of the upper mantle, the region where melt-crystal density crossover occurs in the early Earth. The density of hydrous magma is also important in the present Earth. We determined the density of hydrous magma by using sink-float method, and estimated the partial molar volume of H2O in magmas at high pressures [2]. The result implies that a density crossover exists between the mantle and hydrous magmas containing H2O up to about 5.2 wt percent, and the magmas could be gravitationally stable at the base of the upper mantle. Low V and low Q regions are reported at the base of the upper mantle by seismic studies [3, 4], implying that the region is likely to be caused by accumulation of gravitationally stable hydrous magma. The base of the lower mantle is also a possible region where a dense magma can accumulate [5]. The ultralow velocity zone has been explained by existence of a dense magma [6, 7]. Ohtani [8] and Ohtani and Maeda [9] speculated a possible existence of dense magmas at the base of the lower mantle by extrapolation of the magma density into the CMB condition. They suggested that the magma density is greater than that of the lower mantle mineral assemblage at the base of the lower mantle. Some amount of metal component can be dissolved into the FeO melt, and closure of the liquid immiscibility gap in the FeO-Fe system is espected at high pressure [10]. Thus, we can expect dissolution of core derived metallic iron component into silicate melts at the core-mantle boundary, producing dense magmas and a gravitationally stable region with ultralow velocity. [1] Suzuki and Ohtani (2003), Phys. Chem. Minerals., 30: 449. [2] Sakamaki et al. (2005), Nature, in review. [3] Revanaugh and Shipkin (1994), Nature 369, 474. [4] Song et al. (2004), Nature 427, 530. [5] Rost et al. (2005), Nature 435, 666. [6] Williams and Garnero (1996) Science, 273, 1528. [7] Garnero et al.(1998), The core-mantle boundary region, AGU monograph, pp319-334. [8] Ohtani (1983), Phys. Earth Planet. Inter., 33, 12. [9] Ohtani and Maeda (2001), Earth Planet. Sci. Lett., 193, 69. [10] Ohtani and Ringwood (1984), Earth Planet. Sci. Lett., 71, 85.
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