53Mn-53Cr Chronology of Ca-Fe Silicates in CV3 Chondrites

1Glenn J. MacPherson, 2Kazuhide Nagashima, 2Alexander N. Krot, 3Patricia M. Doyle, 1Marina A. Ivanova
Geochmica et Cosmochmica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.09.032]
1US National Museum of Natural History, Smithsonian Institution, Washington, D.C., 20560, USA
2Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
3Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, RS91
Copyright Elsevier

High precision secondary ion mass-spectrometry (SIMS) analyses of kirschsteinite (CaFeSiO4) in the reduced CV3 chondrites Vigarano and Efremovka yield well resolved 53Cr excesses that correlate with 55Mn/52Cr, demonstrating in situ decay of the extinct short-lived radionuclide 53Mn. To ensure proper correction for relative sensitivities between 55Mn+ and 52Cr+ ions, we synthesized kirschsteinite doped with Mn and Cr to measure the relative sensitivity factor. The inferred initial ratio (53Mn/55Mn)0 in chondritic kirschsteinite is (3.71±0.50)×10–6. When anchored to 53Mn-53Cr relative and U-corrected 207Pb-206Pb absolute ages of the D’Orbigny angrite, this ratio corresponds to kirschsteinite formation View the MathML source3.2-0.7+08 Ma after CV Ca-, Al-rich inclusions. The kirschsteinite data are consistent within error with the data for aqueously-formed fayalite from the Asuka 881317 CV3 chondrite as reported by Doyle et al. (2015), supporting the idea that Ca-Fe silicates in CV3 chondrites are cogenetic with fayalite (and magnetite) and formed during metasomatic alteration on the CV3 parent body. Concentrically-zoned crystals of kirschsteinite and hedenbergite indicate that they initially formed as near end-member compositions that became more Mg-rich with time, possibly as a result of an increase in temperature.

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