Modeling Major and Trace Element Chemistry of Zoned Metal Grains
نویسنده
چکیده
Introduction: The compositional variations [1 and references therein] in the zoned metal grains from the CH and CB chondrites are generally interpreted as unequivocal evidence for their condensation from a parcel of H 2-rich gas with solar or somewhat increased dust/gas (D/G) ratios [2,3]. Although different origins of such gas are discussed [e.g., 4-7], its high enrichment in H 2 [8] and low ambient pressures point to a nebular setting of condensation which was rapid enough to prevent the condensation-induced chemical zoning from equilibration by diffusion. Thermodynamic and kinetic modeling of condensation of either major [2,3,5] or trace elements [9] yielded some estimates of physicochemical conditions of the nebular source regions of zoned metal grains which are only broadly consistent. It was found [3] that such estimates are highly dependent upon the activity and diffusion coefficients of minor and trace elements in the Fe matrix, with the latter having been measured only recently [10,11]. Here I use the updated ZONMET code [3] to model compositional profiles of major and trace elements observed in zoned metal grains from the CH and CB chondrites in order to look for better constraints on their condensation histories. Code upgrade: The original ZONMET code was designed to model equilibrium and disequilibrium (CWPI-type) partitioning of 19 chemical elements Co, and Ni) among 203 gaseous and 488 solid species, including a number of non-ideal solid solutions. The code includes special subroutines for explicit modeling of sizes and chemical zoning profiles of zoned metal grains condensing from the gas. It is these subroutines that allow estimates of condensation timescales to be made. The updated code contains 14 more chemical ele-Pt, and Au). In addition to gaseous and solid species of these elements [12] the database also includes their gaseous monoxides [13] and, when available, gaseous and solid poly-oxides and sulfides [12,14,15]. The high-temperature PGE-enriched and Fe,Ni,Co-free metal is treated as an ideal solution; metal with X Fe +X Ni +X Co +X Cr > 0.9 is treated as non-ideal solution, with the activity coefficients being calculated either for ternary [16] or binary [17,18] systems. Because of the very low abundances of the trace elements, their contributions to the grain mass and radius are ignored. Like in the previous version of ZONMET [3], the concentrations of all elements at the metal-gas interface are
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