¹W.S. Cassata, ¹L.E. Borg
¹Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA

Anomalously old 40Ar/39Ar ages are commonly obtained from Shergottites and are generally attributed to uncertainties regarding the isotopic composition of the trapped component and/or the presence of excess 40Ar. Old ages can also be obtained if inaccurate corrections for cosmogenic 36Ar are applied. Current methods for making the cosmogenic correction require simplifying assumptions regarding the spatial homogeneity of target elements for cosmogenic production and the distribution of cosmogenic nuclides relative to trapped and reactor-derived Ar isotopes. To mitigate uncertainties arising from these assumptions, a new cosmogenic correction approach utilizing the exposure age determined on an un-irradiated aliquot and step-wise production rate estimates that account for spatial variations in Ca and K is described. Data obtained from NWA 4468 and an unofficial pairing of NWA 2975, which yield anomalously old ages when corrected for cosmogenic 36Ar using conventional techniques, are used to illustrate the efficacy of this new approach. For these samples, anomalous age determinations are rectified solely by the improved cosmogenic correction technique described herein. Ages of 188 ± 17 and 184 ± 17 Ma are obtained for NWA 4468 and NWA 2975, respectively, both of which are indistinguishable from ages obtained by other radioisotopic systems. For other Shergottites that have multiple trapped components, have experienced diffusive loss of Ar, or contain excess Ar, more accurate cosmogenic corrections may aid in the interpretation of anomalous ages. The trapped 40Ar/36Ar ratios inferred from inverse isochron diagrams obtained from NWA 4468 and NWA 2975 are significantly lower than the Martian atmospheric value, and may represent upper mantle or crustal components.

Reference
Cassata WS, Borg LE (2016) A new approach to cosmogenic corrections in 40Ar/39Ar chronometry: Implications for the ages of Martian meteorites. Geochmica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.04.045]
Copyright Elsevier

¹Junko Isa, ²Chi Ma, ^1,3Alan E. Rubin
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California 90095, U.S.A.
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
³Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095, U.S.A.

Joegoldsteinite, a new sulfide mineral of end-member formula MnCr2S4, was discovered in the Social Circle IVA iron meteorite. It is a thiospinel, the Mn analog of daubréelite (Fe2+Cr2S4), and a new member of the linnaeite group. Tiny grains of joegoldsteinite were also identified in the Indarch EH4 enstatite chondrite. The chemical composition of the Social Circle sample determined by electron microprobe is (wt%) S 44.3, Cr 36.2, Mn 15.8, Fe 4.5, Ni 0.09, Cu 0.08, total 101.0, giving rise to an empirical formula of (Mn0.82Fe0.23)Cr1.99S3.95. The crystal structure, determined by electron backscattered diffraction, is a Fd3m spinel-type structure with a = 10.11 Å, V = 1033.4 Å3, and Z = 8.

Reference
Isa J, Ma C, Rubin AE (2016) Joegoldsteinite: A new sulfide mineral (MnCr2S4) from the Social Circle IVA iron meteorite. American Mineralogist 101,1217-1221
Link to Article [http://dx.doi.org/10.2138/am-2016-5594]
Copyright: The Mineralogical Society of America