KAIDUN CARBONATES: RE-EXAMINING THE 53 Mn- 53 Cr SYSTEMATICS. M. Petitat 1 , M. Gounelle 1 , K. McKeegan 2 , S. Mostefaoui 1 , Y. Marrocchi 1 , A. Meibom 1 , M. E. Zolensky 3 . 1 Département d’Histoire de la Terre, Laboratoire d’Étude de la Matière Extraterrestre, Muséum National d’Histoire Naturelle, 57 rue Cuvier 75005 Paris, France; 2 Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA; 3 KT NASA Johnson Space Center Houston, TX 77058, USA. E-Mail: petitat@mnhn.fr. Introduction: The Kaidun meteorite is a complex polymict breccia containing lithic clasts spanning a wide range of chondrite groups, including enstatite, ordinary and carbonaceous chondrites [1]. The latter is represented by the CR, CI and CM lithologies. These lithologies all contain carbonates, derived from aqueous alteration. Given that many other clasts are anhydrous, aqueous alteration probably occurred before compaction. In the first and only 53 Mn- 53 Cr chronological study of Kaidun carbonates, Hutcheon et al. [2] used an ims 3f to analyze several carbonate grains from 3 different lithologies finding a good correlation of δ 53 Cr with 55 Mn/ 52 Cr implying an initial 53 Mn/ 55 Mn ratio of ~ 9×10 -6 . This value is within error of the bulk rock carbonaceous chondrite initial value ( 53 Mn/ 55 Mn = 8.5 ± 1.5) [3,4], a recent estimate of the initial ratio of the solar system. The result of [2] suggests very early, essentially simultaneous aqueous activity on several parent bodies prior to their disruption and the assembly of Kaidun. Such a chronology is surprising given that the onset of aqueous activity overlaps the period inferred for chondrule formation [5,6] and that carbonates in CI chondrites have significantly younger Mn-Cr closure ages. In this work, we used a NanoSims to characterize 53 Mn- 53 Cr internal isochrons on individual dolomite grains in Kaidun. Experimental methods: The mineral chemistry of individual Kaidun carbonates was determined at MNHN and Jussieu by using conventional SEM and EMPA techniques. The NanoSims at MNHN was used for 53 Mn– 53 Cr analysis by rastering a 1-3nA 16 O - primary beam over a 5×5 μm 2 area on the polished sample. The secondary ion intensities of 52 Cr + , 53 Cr + , and 55 Mn + were measured by coupling multi-collection to magnetic peak-switching at high mass resolution sufficient to resolve all molecular ion interferences, including hydrides. The relative sensitivity factor (RSF) for 55 Mn/ 52 Cr was determined from the mean of analyses of 4 standards: NBS611 (Mn/Cr = 1.2), San Carlos olivine (Mn/Cr = 7.8), T1-G (Mn/Cr = 50.5) and ATHO-G (Mn/Cr = 130.8). Unfortunately, as with previous ion probe investigations, no carbonate standards were available and thus our Mn/Cr ratios may suffer from a systematic error, but they are still comparable to previous results obtained on carbonates. Mass fractionation was corrected externally, i.e. 53 Cr/ 52 Cr ratios in the Orgueil carbonates were normalized relative to the average value of the 53 Cr/ 52 Cr ratios measured for the 4 standards and are reported as δ 53 Cr, expressed as the deviation, in parts per mil, from the reference 53 Cr/ 52 Cr value of 0.113457±0.000001 [7]. Given the magnitude of δ 53 Cr excesses, possible matrix effects on the mass fractionation correction are negligible. Samples: Three Kaidun dolomites, embedded in 2 different CI lithic clasts within the Kaidun_3.10.i and Kaidun_cavity mounts, were analysed. Both clasts are similar in mineral composition and size to the one described in [1]. The 3 carbonate grains are fine-grained, range from 20 to 70 μm in diameter, and occur as isolated matrix grains, without any association with other phases (picture 1). They are irregular in shape. The MgO content of these dolomites range from 16.0 to 20.9 wt% and their CaO content can reach 31.1 wt%. They contain significant amounts of iron (between 5.7 and 8.2 wt% FeO) and minor amounts of MnO (below 1.9 wt%). Picture 1. Kaidun_3.10.i-3 with its three measurement areas. Change of the Cr at the sub- micrometer range makes the 55 Mn/ 52 Cr ratios vary. Results: All 3 carbonates investigated show large enrichments in 53 Cr with δ 53 Cr up to ~ 600‰. Their respective 55 Mn/ 52 Cr ratios range up to 15000 and are linearly correlated with δ 53 Cr constituting strong evidence for in situ 53 Mn decay. A best-fit line forced through the origin yields a slope corresponding to initial 53 Mn/ 55 Mn ratios at the time of carbonate formation ranging from (4.27±0.43)×10 -6 to (5.96±1.05)×10 -6 . 1666.pdf 40th Lunar and Planetary Science Conference (2009)