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Title: Paleotemperatures at the lunar surfaces from open system behavior of cosmogenic 38Ar and radiogenic 40Ar

The simultaneous diffusion of both cosmogenic 38Ar and radiogenic 40Ar from solid phases is controlled by the thermal conditions of rocks while residing near planetary surfaces. Combined observations of 38Ar/37Ar and 40Ar/39Ar ratios during stepwise degassing analyses of neutron-irradiated Apollo samples can distinguish between diffusive loss of Ar due to solar heating of the rocks and that associated with elevated temperatures during or following impact events; the data provide quantitative constraints on the durations and temperatures of each process. From sequentially degassed 38Ar/37Ar ratios can be calculated a spectrum of apparent 38Ar exposure ages versus the cumulative release fraction of 37Ar, which is particularly sensitive to conditions at the lunar surface typically over ~106–108 year timescales. Due to variable proportions of K- and Ca-bearing glass, plagioclase and pyroxene, with variability in the grain sizes of these phases, each sample will have distinct sensitivity to, and therefore different resolving power on, past near-surface thermal conditions. Furthermore, we present the underlying assumptions, and the analytical and numerical methods used to quantify the Ar diffusion kinetics in multi-phase whole-rock analyses that provide these constraints.
 [1] ;  [2]
  1. Univ. of California, Berkeley, CA (United States). Dept. of Earth and Planetary Science; Berkeley Geochronology Center, Berkeley, CA (United States)
  2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States). Chemical Sciences Div.
Publication Date:
OSTI Identifier:
Report Number(s):
Journal ID: ISSN 0016-7037
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Geochimica et Cosmochimica Acta
Additional Journal Information:
Journal Volume: 155; Journal Issue: C; Journal ID: ISSN 0016-7037
The Geochemical Society; The Meteoritical Society
Research Org:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Sponsoring Org:
Country of Publication:
United States
58 GEOSCIENCES; 79 ASTRONOMY AND ASTROPHYSICS moon; noble gas; cosmogenic 38Ar; 40Ar/39Ar; diffusion; Apollo; solar heating