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High Pressure and Temperature Metal-Silicate Partitioning Behavior of Moderately Siderophile Elements: Implications for the Early History of the Earth
| Content Provider | Semantic Scholar |
|---|---|
| Author | Hillgren, Valerie J. Drake, Michael J. Rubie, David C. |
| Copyright Year | 1994 |
| Abstract | Introduction. It has been known for some time that siderophile element abundances in the Earth's mantle are too high for metal to have been in equilibrium with silicate [e.g., 11, particularly if equilibrium was established at low pressures and temperatures near the surface of the accreting Earth. Although many hypotheses have been proposed to account for this apparent disequilibrium, none has been entirely satisfactory [2]. Murthy [3] proposed that the problem may be reconciled if metal-silicate equilibrium was established at high pressures and temperatures. We have performed experiments on the distribution of siderophile elements between liquid metal and liquid silicate at 100 kbars and 2000 OC. These experiments demonstrate that it is unlikely that siderophile element abundances were established by simple metal-silicate equilibrium at any combination of temperature and pressure, and that core formation in the Earth was probably a mixture of complex physical and chemical processes. Experimental Techniques. Experiments were conducted in the 1200 ton multianvil apparatus at the Bayerisches Geoinstitut. The sample was contained in either MgO or A1203 capsules which were surrounded by a MgO sleeve contained within a cylindrical LaCrOg heater with a geometry designed to minimize thermal gradients across the sample [4]. The sample assembly consisted of a MgO (+5 wt.% Cr203) octahedron with an edge length of 18 mm. Temperature was monitored with a 0.25 mm diameter W3%Re/W25%Re thermocouple in contact with the sample capsule. The starting material consisted of a synthetic basalt prepared by mixing together reagent grade oxides in the following proportions: 50 wt. % Si02, 19 wt. 96 FeO, 13 wt. % A1203, 11 wt. % CaO, and 7 wt. % MgO. This synthetic basalt was doped with 1.5 wt. % each of NiO and COO, or Mo@ and WO;?, or V2O3, Cr203, and MnO. These mixtures were melted and quenched to a glass and then ground back to a powder. This glass powder and Fe metal filings were mixed together in an approximately 50-50 ratio by mass. The samples were first brought to a pressure of 100 kbars, and then were heated to 1600 "C and held at that temperature for 45-60 minutes to sinter the capsule material in order to minimize its subsequent chemical interaction with the sample. The temperature was then raised to 2000 "C and held there for 3 minutes. At this temperature and pressure, both metal and silicate are liquids. The charges were quenched rapidly (to less than 300 "C in 1 second) by turning off the power to the LaCrOg furnace. Experimental results. The samples were analyzed with a Carneca SX50 electron microprobe. The initially homogeneously distributed metal filings had largely segregated into a single spheroid in the center of the charge. The silicate liquid did not quench to a glass but segregated into two phases with a dendritic texture. Thus to determine the composition of the silicate liquid prior to quenching the electron beam was rastered over an area approximately 20 ym on a side, and 30 to 40 analyses were taken and averaged together. For the metal a point beam was used. For major elements a beam current of 30 nanoamps and a counting time of 15 seconds were used. For the trace elements beam currents between 125 and 250 nanoamps and counting times of up to 10 minutes were used. The metal-silicate partition coefficients we determined for Ni, Co, Mo, W, Fe, V, Cr, and Mn are shown in Table 1 along with their one sigma uncertainties and an estimated oxygen fugacity relative to the iron-wiistite buffer. Molybdenum and W were both below analytic detection in the silicate glass, thus lower limits for their metausilicate partition coefficients are reported. Our results for Ni and Fe agree well with those of Walker et al. [5]. Discussion. In Figure 1, we compare our partition coefficients that were approximately 1.6 log units below the iron-wiistite buffer to 1260 "C, 1 bar data that were in a similar redox state. This Figure shows that the Ni and Co metal-silicate partition coefficients decrease with |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.lpi.usra.edu/meetings/lpsc1994/pdf/1276.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
