DURATION AND SEQUENCE OF CARBONATE CRYSTALLIZATION ON THE ORGUEIL PROTOLITH: 53 Mn- 53 Cr SYSTEMATICS OF THEIR EVOLUTION IN O AND C ISOTOPIC COMPOSITION. M. Petitat 1 , K. McKeegan 2 , M. Gounelle 1 , S. Mostefaoui 1 , Y. Marrocchi 1 , A. Meibom 1 , L.A. Leshin 3 . 1 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 90095-1567; 3 Office of the Director, NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States. E-mail: mpetitat@mnhn.fr. Introduction: Aqueous alteration of silicate primary minerals is an important process that occurred in the inner solar system on the parent bodies of meteorites, as evidenced by secondary mineralization such as the formation of carbonates in primitive meteorites (e.g. [1]). Zito et al. [2] measured the oxygen and carbon isotope compositions of individual carbonate grains from the Orgueil meteorite and discovered a clear trend of correlated increasing isotopic values in carbon and oxygen, from breunnerite to dolomite to calcite [2]. The formation of CI carbonates is known to be early from 53 Mn- 53 Cr studies [4,5,6] but the duration and sequence of precipitation is not well-constrained by available chronological data. The goals of the present work are to: (1) refine the timescales of carbonate formation on the Orgueil parent body, and (2) determine if a correlation exists between the measured initial 53 Mn/ 55 Mn ratios and O and C isotope compositions measured for the same grain thereby constraining the sequence of carbonate crystallisation during the progressive protolith alteration hypothesized by [2]. Experimental methods: Chemical compositions of Orgueil carbonates were determined at UCLA using conventional SEM and EMPA techniques. 53 Mn– 53 Cr data were gathered at MNHN using the NanoSims and at UCLA using the Cameca ims 1270. NanoSims: 53 Mn– 53 Cr analyses were made 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, San Carlos olivine, T1-G and ATHO-G. 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 aforementioned 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 [3]. Given the magnitude of δ 53 Cr excesses, possible matrix effects on the mass fractionation correction are negligible. Ims 1270: Elliptical beam spots of ~20×30μm. were sputtered by a 10 nA 16 O − beam and 52 Cr + , 53 Cr + and 55 Mn + were measured in multi-collection mode with 55 Mn + intensity analyzed by a Faraday cup or electron multiplier depending on the Mn concentration. Although sacrificing spatial resolution compared to the NanoSims, the ims 1270 configuration allowed measurement of high Mn/Cr phases with enough Cr intensity to attain good counting statistics. As previously, measured 53 Cr/ 52 Cr ratios in the Orgueil carbonates were normalized to those obtained for NBS610 glass and the San Carlos Olivine and the RSF was determined by measuring the same standards (and results agree with those found for the NanoSims). Isotope count rates were corrected for dynamic background and dead time. Uncertainties are expressed in 2σ. δ 53 Cr 55 Mn/ 52 Cr 0,0 3,0e+4 6,0e+4 9,0e+4 1,2e+5 1,5e+5 1,8e+5 0 500 1000 1500 2000 2500 3000 3500 ( 53 Mn/ 55 Mn) 0 = (2.21 ± 0.16) x 10 -6 ( 53 Mn/ 55 Mn) 0 = (3.20 ± 0.30) x 10 -6 Figure 1. Mn-Cr isochron diagram for breunnerite_33, Orgueil_2 (black squares) bearing the highest 53 Mn/ 55 Mn ratio and for dolomite_5, Orgueil_1 (red squares) grain bearing the lowest initial value from the Orgueil meteorite. Error bars are 2σ. Results: Measurements were made for 8 breunnerite grains (100 to 500 μm in diameter) and 3 dolomite grains (80 to 100 μm in diameter) that were previously studied for δ 18 O and δ 13 C [2] and 8 other breunnerite grains. Elemental and isotopic ratios vary widely within a carbonate grain but are linearly correlated constituting strong evidence for in situ 53 Mn decay (Fig. 1). The slope of this correlation line gives the initial 53 Mn/ 55 Mn ratio at the time of carbonate formation. These range from (0.58 ± 0.06) × 10 -6 to (2.21 ± 0.16) × 10 -6 for breunnerites and 1657.pdf 40th Lunar and Planetary Science Conference (2009)