High Pressure and Temperature Metal-Silicate Equilibrium in the Early Earth: New Constraints from Moderately (Ga, Sn, and Cu) and Highly (Re, Au, Pd, and Pt) Siderophile Elements

K. Righter
1998 Mineralogical magazine  
Recent experimental work and calculations have shown that the primitive mantle abundances of six moderately siderophile elements (Fe, Ni, Co, Mo, W and P) and one highly siderophile element (Re) are consistent with metal-silicate equilibrium at the base of a 700-900 km deep magma ocean in the early Earth (i.e. [1]). Whereas heterogeneous accretion models can successfully explain the chondritic relative abundances of the highly siderophile elements (PGE's, Au), they cannot explain the depletion
more » ... lain the depletion patterns of the volatile moderately siderophile elements such as Ga, Sn and Cu (e.g. [2] ). This equilibrium magma ocean model must be tested for these highly and moderately siderophile elements. We have initiated systematic studies of three volatile, moderately siderophile elements whose mantle abundances are well characterized, and assess the effect of P, T, fo~ and metal and silicate liquid composition on the magnitude of four highly siderophile elements, Au, Re, Pd and Pt. Experimental Experiments have been carried out between 10 and 70 kbar, 1250 to 1750~ in piston cylinder and multianvil apparatuses. Basalt powder, doped with Ga203, SnO2, and CuO, was used in both types of experiments. Basalt in the piston cylinder experiments was enclosed by FeNiCo capsules, and basalt-FeNiCo mixtures in the multi anvil experiments were encapsulated with graphite. The FeNiCo alloy buffer oxygen fugacities near IW, while graphite buffers about 3 log fo~ units higher. Quenched metals and silicate liquids (glasses) in the run products were analysed for major elements and Ni, Co, Ga, Sn, and Cu by electron microprobe. Calculation of D(met/sil) as f(RT, fo2, X). It can be shown that increasing pressure causes D(Ga) to increase when all other variables are held constant; D(Cu) shows no change and D(Sn) decreases with pressure. Because pressure is an important variable to quantify, it is clear that it must be accounted for in any attempt to calculate metal-silicate partition coefficients for these three elements. Metal-silicate partition coefficients can be successfully predicted as a function of pressure, temperature, oxygen fugacity, and silicate and metallic melt composition with the expression (from [1]): lnD (metal/silicate) = alnfo: + b/T + c(P/T) + dln(1 -Xs) + e(nbo/t) + f (1) Constants a through f can be calculated by regression of all experimental data, including our new results and previous high temperature and pressure experimental work (see references cited in [3] ). For Ga and Sn, the metal-silicate D's are known to be sensitive to Fe/Ni ratio in the metal [4], and thus require attention to activity coefficients (7) in the metal phase. We have calculated activity coefficients for Ga and Sn using the approach of [4], and regressed against ln(yD) instead of InD. This type of approach (Equation 1) can also be applied to published data for Au, Re, Pd and Pt TABLE 1. Calculated partition coefficients and mantle abundances (Ga, Sn, Cu in ppm -HSE in ppb)
doi:10.1180/minmag.1998.62a.2.330 fatcat:7ihqaja5gzbsliokevzzljn33a