Oxygen and Carbon Isotopic Ratios of Carbonates in the Nogoya CM Chondrite
Oxygen and Carbon Isotopic Ratios of Carbonates in the Nogoya CM Chondrite
复制标题
Nogoya CM 球粒陨石中碳酸盐的氧和碳同位素比率
DOI：
--
复制
发表时间：
2016
期刊：
影响因子：
--
通讯作者：
Y. Sano
中科院分区：
文献类型：
--
作者：
W. Fujiya;K. Fukuda;M. Koike;A. Ishida;Y. Sano
简介：CM化石显示出不同程度的水蚀变证据[1，2]。物理化学条件（例如，温度、水岩比和开/闭系统）下发生的含水蚀变一直是争论的主题。碳酸盐是水蚀变的次生矿物，记录了水和溶解的无机C物质的O和C同位素组成（例如，3.他所创造的，3，4]。水和溶解C的O和C同位素组成随着蚀变的增加而变化，其演化取决于水/岩比以及水蚀变是否发生在开放或封闭系统中[5-8]。碳酸盐的O和C同位素比值也由形成温度确定[9]。因此，碳酸盐可能提供有关CM球粒陨石母体的蚀变环境的信息。虽然碳酸盐的O和C同位素组成即使在单个陨石中也是高度可变的[例如，10]，关于O和C同位素组成如何变化以及主要控制它们的原因还不清楚。在这项研究中，我们进行了现场O和C同位素测量的方解石颗粒在Nogoya CM 2.2-2.3球粒陨石。详细的岩石学和矿物学观察沿着以及先前研究中的O-同位素测量表明，CM榴辉岩中的碳酸盐不是在单一事件中形成的，而是间歇性形成的[11，12]。多“代”碳酸盐岩的同位素测定可以揭示水蚀变过程中O和C同位素组成的演化。实验：我们准备了Nogoya CM球粒陨石的抛光薄切片，并用Au涂覆。我们用SEM-EDS观察了薄切片，并寻找足够大的碳酸钙颗粒用于随后的同位素测量。在AORI，UTokyo使用NanoSIMS 50进行氧和碳同位素测量。O离子（在O同位素测量），或C，O，CN，和Si离子（在C同位素测量）由20-30 pA的Cs离子束产生的检测与FC和两个EM，或与四个EM，分别。在碳同位素测量中，以组合的峰跳跃/多检测模式用相同的EM检测C离子。在C-同位素测量中监测O、CN和Si离子，以检查是否存在可能的污染物，如有机物或硅酸盐。δ O和δ C值的典型误差分别为5.3 ‰和6.4 ‰（2 ‰）。氧和碳同位素比值归一化通过使用陆地方解石与已知的O和C同位素比值通过常规方法测量。结果与讨论：通过扫描电镜观察，我们发现了大量的钙钛矿颗粒。虽然我们没有任何数据来区分碳酸钙的多晶型（即，方解石或文石），我们在下文中将碳酸钙描述为方解石，因为文石在更严重蚀变的CM方解石中不太常见[13]。
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].