Oxygen and carbon isotopic ratios of calcite in the Nogoya CM chondrite

Introduction: CM chondrites exhibit evidence for aqueous alteration to variable degrees [1,2]. Physicochemical conditions (e.g., temperature, water to rock ratio, and open/closed system) under which aqueous alteration occurred have been the subject of debate. Carbonate is a secondary mineral of aqueous alteration and recorded O and C isotopic compositions of water and dissolved inorganic C species (e.g., CO3) from which it formed [e.g., 3,4]. The O and C isotopic compositions of water and dissolved C changed with increasing alteration, and their evolution depends on water/rock ratios and whether aqueous alteration occurred in an open or closed system [5-8]. The O and C isotopic ratios of carbonates were also determined by formation temperatures [9]. Thus, carbonates potentially provide information about the alteration environment in the CM chondrite parent body. Although the O and C isotopic compositions of carbonates are highly variable even in a single meteorite [e.g., 10], much is unknown about how the O and C isotopic compositions changed and what predominantly controlled them. In this study, we conducted in-situ Oand Cisotope measurements on calcite grains in the Nogoya CM 2.2-2.3 chondrite. Detailed petrological and mineralogical observations along with O-isotope measurements in previous studies have suggested that carbonates in CM chondrites did not form in a single event but formed intermittently [11,12]. Isotope measurement of carbonates in multiple “generations” could shed light on the evolution of O and C isotopic compositions during aqueous alteration. Experimental: We prepared a polished thin section of the Nogoya CM chondrite and coated it with Au. We observed the thin section with an SEM-EDS and searched for Ca-carbonate grains large enough for subsequent isotope measurements. Oxygenand C-isotope measurements were performed with the NanoSIMS 50 at AORI, UTokyo. O ions (in O-isotope measurement), or C, O, CN, and Si ions (in Cisotope measurement) produced by a 20-30 pA Cs ion beam were detected with a FC and two EMs, or with four EMs, respectively. In C-isotope measurement, C ions were detected with the same EM in a combined peak-jumping/multi-detection mode. O, CN, and Si ions were monitored in the C-isotope measurement to check the presence of possible contamination such as organic matter or silicate. Typical errors on O and C values were 5.3 ‰ and 6.4 ‰ (2), respectively. Oxygen and C isotopic ratios were normalized by using terrestrial calcite with known O and C isotopic ratios measured by a conventional method. Results and discussion: We found many Cacarbonate grains by the SEM observation. Although we do not have any data to distinguish the polymorphism of the Ca-carbonate (i.e., calcite or aragonite), we hereafter describe the Ca-carbonate as calcite because aragonite is less common in more-heavily altered CM chondrites [13].