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Title: Pd-Ag chronometry of IVA iron meteorites and the crystallization and cooling of a protoplanetary core

To constrain the timescales and processes involved in the crystallization and cooling of protoplanetary cores, we examined the Pd-Ag isotope systematics of the IVA iron meteorites Muonionalusta and Gibeon. A Pd-Ag isochron for Muonionalusta provides an initial 107Pd/ 108Pd = (2.57 ± 0.07) × 10 -5. The three metal samples analyzed from Gibeon plot below the Muonionalusta isochron, but these samples also show significant effects of cosmic ray-induced neutron capture reactions, as is evident from 196Pt excesses in the Gibeon samples. After correction for neutron capture effects on Ag isotopes, the Gibeon samples plot on the Muonionalusta isochron, indicating that these two IVA irons have indistinguishable initial 107Pd/ 108Pd. Collectively, the Pd-Ag data indicate cooling of the IVA core below Pd-Ag closure between 2.9 ± 0.4 Ma and 8.9 ± 0.6 Ma after CAI formation, where this age range reflects uncertainties in the initial 107Pd/ 108Pd ratios of the solar system, which in turn result from uncertainties in the Pb-Pb age of Muonionalusta. The Ag isotopic data indicate that the IVA core initially evolved with a modestly elevated Pd/Ag, but the low Ag concentrations measured for some metal samples indicate derivation from a source with much lower Ag contents and,more » hence, higher Pd/Ag. These contrasting observations can be reconciled if the IVA irons crystallized from an initially more Ag-rich core, followed by extraction of Fe-S melts during compaction of the nearly solidified core. Owing to its strong tendency to partition into Fe-S melts, Ag was removed from the IVA core during compaction, leading to the very low Ag concentration observed in metal samples of IVA irons. Alternatively, Ag was lost by evaporation from a still molten metallic body just prior to the onset of crystallization. The Pd-Ag isotopic data indicate that Muonionalusta cooled at >500 K/Ma through the Pd-Ag closure temperature of ~900 K, consistent with the rapid cooling inferred from metallographic cooling rates for IVA irons. Finally, combined, these observations are consistent with cooling of IVA irons in a metallic body with little or no silicate mantle.« less
Authors:
 [1] ;  [1] ;  [1] ;  [1]
  1. Univ. of Munster, Munster (Germany). Inst. fur Planetologie
Publication Date:
Report Number(s):
LLNL-JRNL-734230
Journal ID: ISSN 0016-7037
Grant/Contract Number:
AC52-07NA27344
Type:
Accepted Manuscript
Journal Name:
Geochimica et Cosmochimica Acta
Additional Journal Information:
Journal Volume: 220; Journal Issue: C; Journal ID: ISSN 0016-7037
Publisher:
The Geochemical Society; The Meteoritical Society
Research Org:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org:
USDOE; German Research Foundation (DFG)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; 58 GEOSCIENCES; Pd/Ag; IVA iron meteorites; Early solar system; Core formation; Volatile depletion; Chronology
OSTI Identifier:
1430987