Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.565r&link_type=abstract
Meteoritics, vol. 30, no. 5, page 565
Other
1
Cores, Fractionation, Metal-Silicate, Partition Coefficients, Siderophile Elements, Silicate Melts
Scientific paper
Quantification of the effect of pressure on siderophile element metal-silicate partition coefficients (D) is essential in modelling the accretion histories of the the Earth and terrestrial planets [1], as metal-silicate equilibria may have been set over a range of pressures [2]. We report siderophile element partition coefficients from metal-silicate equilibrium experiments done at 10 and 15 kb, and 1300 degrees C. These new results show that metal-silicate partition coefficients for Ni (Fig. 1), Co, and P decrease with increasing pressure (at constant T and relative fO(sub)2), while those for Mo and W increase. Experiments were done in a 1/2" piston cylinder apparatus, with T, P and fO(sub)2 controlled and monitored as described in a previous study [3]. Synthetic basalt [see 3] powder, doped with 5 wt% levels of either MoO3, WO3 or apatite, was loaded into Fe54Ni29Co17 or Fe64Ni36 tubing, which was closed either by welding or plugging the open ends with small, tapered caps of the same alloy composition. The samples were quenched after 4 to 6 hrs. The metal and glass in the run products are then analyzed by electron microprobe to obtain a solid metal/liquid silicate (SM/LS) partition coefficient for a given element (D = wt% element in metal/ wt% element in glass). For several experiments, NiS was added as a sulfur source, and thus stabilizing a sulfur-bearing metallic liquid. For these experiments, both solid metal/ liquid silicate and liquid metal/ liquid silicate (LM/LS) partition coefficients are reported (Table 1). In order to isolate the effect of pressure on siderophile element partition coefficients, we have compared our results at high pressures to calculated 1 bar values at the same T and fO(sub)2 as our experiments (based on experiments of [4 - 11]; see results for Ni in Fig. 1; data from [3] and this study). The effect of pressure and other intensive variables on metal-silicate D's can be quantified using the thermodynamically-based relation: lnD (metal/silicate) = a/T + b + clnfO(sub)2 + dln(1-2X(sub)S) + e(P-1)/T. (1) Values for a, b, c, d and e were determined by multiple linear regression of the 1 bar experimental data cited above, together with data from this study and available high pressure experimental data [3, 12 - 16]. Equation 1 can be used to predict the abundances of the siderophile elements in a planetary mantle that has undergone a metal separation event, at a specific T, P, fO(sub)2 and metal sulfur content. Such calculations for Mars indicate that metal segregation in the Martian mantle (based on SNC meteorite analyses) may have occurred at low pressures, in agreement with the conclusions of several other studies [17, 18, 19]. Similar calculations for Earth indicate that the upper mantle abundances of the siderophile elements are unlikely to have been set by simple metal-silicate equilibrium at pressures less than 100 kb. References: [1] Drake M. J. (1989) Z. Naturforsch., 44a, 883-890. [2] Newsom H. (1992] LPI Tech. Rpt. 92-03, 42-43. [3] Righter K. et al. (1995) LPS XXVI, 1169-1170. [4] Hillgren V. J. (1993) Ph.D. Thesis, Univ. of Arizona, Tucson. [5] Capobianco C. J. and Amelin A. (1994) GCA, 58, 125-140. [6] Schmitt et al. (1989) GCA, 53, 173-186. [7] Newsom H. and Drake M. J. (1982) GCA, 46, 2483-2489. [8] Newsom H. and Drake M. J. (1983) GCA, 47, 93-100. [9] Lodders K. and Palme H. (1991) EPSL, 113, 311-324. [10] Jones J. H. and Drake M. J. (1986) Nature, 322, 221-228. [11] Holzheid A. et al. (1994) GCA, 58, 1975-1981. [12] Thibault Y. and Walter M. J. (1994) GCA, 59, 991-1002. [13] Hillgren V. J. et al. (1994) Science, 264, 1442-1445. [14] Walker D. et al. (1993) Science, 262, 1858-1861. [15] Peach C. L. and Mathez E. A. (1993) GCA, 57, 3013-3032. [16] Seifert et al. (1988) GCA, 52, 603-616. [17] Drake M. J. et al. (1995) LPS XXVI, 345-346. [18] Gaetani G. A. and Grove T. L. (1995) LPS XXVI, 437-438. [19] Treiman A. H. et al. (1986) GCA, 50, 1071-1091. Acknowledgment: NASA Grant NAGW 3348 Table 1 shows a summary of experimental results.
Drake Michael J.
Righter Kevin
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