53Mn-53 Cr chronology of Ca-Fe silicates in CV3 chondrites

High precision secondary ion mass-spectrometry (SIMS) analyses of kirschsteinite (CaFeSiO4) in the reduced CV3 chondrites Vigarano and Efremovka yield well resolved Cr-53 excesses that correlate with Mn-55/Cr-52, demonstrating in situ decay of the extinct short-lived radionuclide Mn-53. To ensure proper correction for relative sensitivities between Mn-55(+) and Cr-52(+) ions, we synthesized kirschsteinite doped with Mn and Cr to measure the relative sensitivity factor. The inferred initial ratio (Mn-53/Mn-55)(o) in chondritic kirschsteinite is (3.71 +/- 0.50) X 10(-6). When anchored to Mn-53-Cr-53 relative and U-corrected Pb-207-Pb-206 absolute ages of the D'Orbigny angrite, this ratio corresponds to kirschsteinite formation 3: 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. Published by Elsevier Ltd.