Trace element geochemistry of CR chondrite metal

Abstract

We report trace element analyses by laser ablation inductively coupled plasma mass spectrometry of metal grains from 9 different CR chondrites, distinguishing grains from chondrule interior ("interior grains"), chondrule surficial shells ("margin grains") and the matrix ("isolated grains"). Save for a few anomalous grains, Ni-normalized trace element patterns are similar for all three petrographical settings, with largely unfractionated refractory siderophile elements and depleted volatile Au, Cu, Ag, S. All types of grains are interpreted to derive from a common precursor approximated by the least melted, fine-grained objects in CR chondrites. This also excludes recondensation of metal vapor as the origin of the bulk of margin grains. The metal precursors presumably formed by incomplete condensation, with evidence for high-temperature isolation of refractory platinum-group-element (PGE)-rich condensates before mixing with lower temperature PGE-depleted condensates. The rounded shape of the Ni-rich, interior grains indicates melting and equilibration with silicates upon slow cooling (1-100 K/h), largely by oxidation/evaporation of Fe. We propose that Ni-poorer, amoeboid margin grains, often included in the pyroxene-rich periphery common to type I chondrules, result from less intense processing of a rim accreted onto the chondrule subsequent to the melting event recorded by the interior. This means either that there were two separate heating events, which formed olivine/interior grains and pyroxene/margin grains, respectively, between which dust was accreted around the chondrule, or there was a single high-temperature event, of which the chondrule margin record a late "quenching phase", in which case dust accreted onto chondrules while they were molten. In the latter case, high dust concentrations in the chondrule-forming region are indicated.

arXiv comment35 pages, 8 figures

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• The formation conditions of enstatite chondrites: Insights from trace element geochemistry of olivine-bearing chondrules in Sahara 97096 (EH3)
• Collisions and compositional variability in chondrule-forming events
• Chondrule destruction in nebular shocks
• Depletion of Moderately Volatile Elements by Open-System loss in the Early Solar Nebula
• Trace Elemental Behavior in the Solar Nebula: Synchrotron X-ray Fluorescence Analyses of CM and CR Chondritic Iron Sulfides and Associated Metal
• Abundance, Sizes, and Major Element Compositions of Components in CR and LL Chondrites: Formation from Single Reservoirs
• Igneous Rim Accretion on Chondrules in Low-Velocity Shock Waves
• Oxygen Isotope Exchange Between Molten Silicate Spherules and Ambient Water Vapor with Nonzero Relative Velocity: Implication for Chondrule Formation Environment
• Chondrites as thermal and mechanical archives of accretion processes in the Solar protoplanetary disk