53Mn-53Cr systematics of carbonates in CM chondrites: Implications for the timing and duration of aqueous alteration

CM chondrites contain carbonates and other secondary minerals such as phyllosilicates, sulfides, sulfates, oxides and hydroxides that are believed to have formed by aqueous alteration reactions on their parent asteroid. We report in situ Mn-Cr isotope measurements in the highly aqueously altered CM2.1 chondrites QUE 93005 and ALH 83100 using secondary ion mass spectrometry (Cameca ims-1270 ion microprobe). The Cr-53 excesses are correlated with the Mn-53/Mn-55 ratio and result from the in situ decay of Mn-53, a short-lived radioisotope with a half-life of 3.7 Ma. If we assume that carbonate grains in samples QUE 93005 and ALH 83100 are cogenetic, then the excesses define initial Mn-53/Mn-55 ratios ((Mn-53/Mn-55)(0)) of (4.1 +/- 1.2) x 10(-6) and (5.1 +/- 1.7) x 10(-6), respectively. These values are comparable to those in carbonates from other CM chondrites as reported in the literature. Initial Mn-53/Mn-55 ratios for calculated model isochrones for individual carbonate grains range from (3.8 +/- 1.4) x 10(-6) to (4.8 +/- 2.1) x 10(-6) for QUE 93005 and from (3.1 +/- 1.6) x 10(-6) to (1.3 +/- 0.5) x 10(-5) for ALH 83100. A possible interpretation for the ranges in (Mn-53/Mn-55)(0) could be that alteration in individual CM chondrites was episodic and occurred over an extended period of time. However, isochrones based on the entire set of carbonate grains in each of the CM chondrites imply that the degree of aqueous alteration is roughly correlated with the age of carbonate formation in CM chondrites of different subtypes and that alteration on the CM parent asteroid started contemporaneously with or shortly after CAI formation and lasted at least 4 Ma. (C) 2009 Elsevier Ltd. All rights reserved.