HR: 10:20h
AN: V22B-01 INVITED    [Abstracts]
TI: PtFe Nano and Micro-Nuggets in Experimental Silicate Glasses
AU: * Cottrell, E
EM: cottrellE@si.edu
AF: Smithsonian Institution, National Museum of Natural History, Department of Mineral Sciences, P.O. Box 37012, Washington, DC 20013, United States
AU: Walker, D
EM: dwalker@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, PO Box 1000, Palisades, NY 10964, United States
AB: Noble metal nuggets (<20nm - 2μm) are ubiquitous in the quenched silicate glass of experiments that equilibrate solid or liquid noble metals (or Fe-metal alloys) with liquid silicate under reducing conditions. Inclusion or exclusion of nuggets from the analysis of 1-atm experiments alters the solubility estimates by up to 12 orders of magnitude at low fO₂. Most studies have attributed the presence of nuggets to mechanical contamination from fragments of the equilibrating metal. In contrast, Cottrell and Walker (2006) presented evidence that nuggets of Pt emerge from solution in the silicate upon thermal quench of the experiment. The former scenario requires judicious removal of the nugget signature from the solubility estimate; the latter does not. Here we show that the inclusion/exclusion of nuggets from analysis is largely irrelevant to the solubility at high T and P using data from two very similar sets of high pressure experiments: one which included nuggets (Cottrell and Walker, 2006) and one which excluded nuggets (Ertel et al., 2006). The pivotal issue of "to count or not to count" nuggets appears to be moot at very high T. Both studies document the positive T dependence of Pt²⁺ solubility in excess of that extrapolated from 1-atm data. If Pt is neutrally speciated in solution at high T, and if this is the nugget-forming species, then elimination of nuggets from the analysis only eliminates the contribution from Pt⁰. Therefore, in a 60 second LA-ICPMS trace, inclusion of only the 2-3 seconds yielding the lowest Pt concentration, only attempts to return the Pt²⁺ solubility, which demonstrably covaries systematically with T and fO₂, and does not document the full solubility including Pt⁰. Nevertheless, above 1550 °C, the correlation between Pt concentration and T is far more systematic for the nugget-bearing analyses than the nugget-excluded ones in Ertel et al.'s experiments. Moreover, in our very high T experiments, in which the silicate melt quench rate varies with respect to charge margins, areas of silicate glass in which no nuggets are present return the same Pt concentration as areas in which Pt is present in the melt as Pt²⁺ and as exsolved Pt⁰ nuggets. Evidently, rapidly quenched nugget-free areas have both species still dissolved at the atomic level. Our conclusion is that both sets of experiments suggest Pt partition coefficients relevant to magma ocean settings that are orders of magnitude lower than values extrapolated from high fO₂ experiments at 1 bar. This conclusion significantly reduces the need for a late veneer contribution to the mantle's Pt budget.
UR: http://mineralsciences.si.edu/staff/pages/cottrell.htm
DE: 1015 Composition of the core
DE: 1025 Composition of the mantle
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 3630 Experimental mineralogy and petrology
DE: 5455 Origin and evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting