Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.233h&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 233
Other
Scientific paper
Introduction. Heterogeneous accretion models have been invoked to account for the siderophile element abundance pattern in the Earth's mantle. In these models, the first material accreting to form the Earth is extremely reduced, and all Fe and other siderophile elements are metallic and, thus, separate to form the core. The last 10-20% of material is more oxidized allowing moderately siderophile elements to remain in the mantle. The more oxidized material implies that the metal segregating to the core is Ni-rich. Experiments and Results. To test this hypothesis, we have measured the partition coefficients of the moderately siderophile elements Ni, Co, Mo, W, P, Ga, and Sn at 1260 degrees C and 1 bar in systems containing basaltic liquid, S-bearing Ni-rich metallic liquid, and Ni-rich solid metal. When the measured partition coefficients are plotted against oxygen fugacity, lines of the form: logD^i(sub)j/k = -AlogfO(sub)2 + B can be fit to the data. D^i(sub)j/k is the distribution coefficient of element i between phases j and k; fO(sub)2 is the oxygen fugacity, and A and B are constants. The valence of the cation of interest can be extracted from the slope of this line. The calculated valences are shown in Table 1 along with the constants A and B for each line. Modeling. Preliminary calculations using simple mass balance and the measured partition coefficients have been used to test if the abundances of Ni, Co, Mo, W, and P in the Earth's mantle are consistent with heterogeneous accretion. We have also used literature values for solid silicate/liquid silicate partition coefficients for Ni and Co and values of 0.01 for Mo, W, and P. As metal is added to the surface of the planet, low pressure partition coefficients may be appropriate. In the calculations core formation is treated for convenience as occurring in two discrete events. The first event occurred after the initial 80% of the Earth accreted. This material is so reduced that all siderophile elements go into the core. The second core forming event occurs after the last 20% of material has accreted, and all siderophile elements are present in the mantle in 0.2 x CI abundances. Calculated abundances of Ni, Co, Mo, W, and P show a good match to the Earth mantle abundances when solid Ni-rich metal is segregated. Segregation of a small amount of Ni-rich sulfide liquid can also produce good matches to the observed pattern of these elements in the Earth's mantle. Thus, the heterogeneous accretion hypothesis appears to be consistent with observed elemental abundances and measured partition coefficients. Table 1: A, B, and the valences for each set of partition coefficients. SM is solid metal; LM is liquid metal, and LS is liquid silicate. Element Phases A B Valence Ni SM/LS 0.53 -3.09 2.1 Ni LM/LS 0.53 -3.09 2.1 Co SM/LS 0.68 -5.89 2.7 Co LM/LS 0.70 -6.24 2.8 Mo SM/LS 1.28 -12.47 5.1 Mo LM/LS 1.39 -14.01 5.6 W SM/LS 1.41 -16.76 5.6 W LM/LS 1.74 -21.57 6.9 P SM/LS 0.94 -12.37 3.7 P LM/LS 0.52 -6.52 2.1 Sn SM/LS 0.83 -0.16 0.6 Sn LM/LS -0.14 -0.14 0.6 Ga SM/LS 4.07 -0.39 1.6
Dearo J. A.
Drake Michael J.
Hillgren Valerie J.
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