Hoppe P, Strebel R, Eberhardt P, Amari S, Lewis R S
Physikalisches Institut, Unversitat Bern, Switzerland.
Geochim Cosmochim Acta. 1996 Mar;60(5):883-907. doi: 10.1016/0016-7037(95)00435-1.
We report the results of SIMS isotopic analyses of carbon, nitrogen, oxygen, and silicon made on 849 small (approximately 1 micrometer) individual silicon carbide grains from the Murchison meteorite. The isotopic compositions of the major elements carbon and silicon of most grains (mainstream) are similar to those observed in larger grain studies suggesting an AGB star origin of these grains. In contrast, the trace element nitrogen shows a clear dependency on grain size. 14N/15N ratios increase with decreasing grain size, suggesting different stellar sources for grains of different size. Typically observed 14N/15N ratios in the small grains of this study are approximately 2700, clearly larger than the values expected from model calculations of AGB stars. In addition to the three dredge-up episodes characteristic for the evolution of AGB stars, extra-mixing of CNO-processed matter in low mass AGB stars appears to be a promising possibility in order to explain the high 14N/15N ratios of the small circumstellar SiC grains. A small fraction of grains shows a silicon isotopic signature not observed in larger circumstellar SiC grains from Murchison. Their stellar origin is still uncertain. The minor type A, B, Y, and X grains were found to be present at a level of a percent, which is similar to their abundance in the larger-grain SiC separates from Murchison. Oxygen isotopic compositions are normal within the experimental uncertainties of several 10%, indicating that oxygen of stellar origin is rare or even absent in the SiC grains. We conclude that most of the oxygen is a contaminant which was introduced into the SiC grains after their formation, e.g., during sample processing in the laboratory. We identified a nitride grain, most likely Si3N4 with little carbon, with highly anomalous isotopic compositions (12C/13C = 157 +/- 33, 14N/15N = 18 +/- 1, delta 29 Si = -43 +/- 56%, delta 30 Si = -271 +/- 50%). The isotopic patterns of carbon, nitrogen, and silicon resemble those of the rare SiC X grains suggesting that these two rare constituents of circumstellar matter formed in the same type of stellar source, namely, Type II supernovae.
我们报告了对默奇森陨石中849颗小（约1微米）的单个碳化硅颗粒进行的碳、氮、氧和硅的二次离子质谱（SIMS）同位素分析结果。大多数颗粒（主流）的主要元素碳和硅的同位素组成与在较大颗粒研究中观察到的相似，这表明这些颗粒起源于渐近巨星分支（AGB）恒星。相比之下，微量元素氮显示出对颗粒大小的明显依赖性。14N/15N比值随着颗粒尺寸的减小而增加，这表明不同大小的颗粒有不同的恒星来源。在本研究的小颗粒中通常观察到的14N/15N比值约为2700，明显大于AGB恒星模型计算预期的值。除了AGB恒星演化特有的三次物质混合过程外，低质量AGB恒星中碳氮氧（CNO）处理物质的额外混合似乎是解释小的星周碳化硅颗粒高14N/15N比值的一个有希望的可能性。一小部分颗粒显示出在默奇森较大的星周碳化硅颗粒中未观察到的硅同位素特征。它们的恒星起源仍然不确定。发现少量的A型、B型、Y型和X型颗粒的含量为百分之一左右，这与它们在默奇森较大颗粒碳化硅分离物中的丰度相似。在百分之几十的实验不确定度范围内，氧同位素组成正常，这表明恒星来源的氧在碳化硅颗粒中很少见甚至不存在。我们得出结论，大部分氧是一种污染物，是在碳化硅颗粒形成后引入的，例如在实验室样品处理过程中。我们鉴定出一个氮化物颗粒，最有可能是含碳量很少的Si3N4，其同位素组成高度异常（12C/13C = 157 +/- 33，14N/15N = 18 +/- 1，δ29Si = -43 +/- 56%，δ30Si = -271 +/- 50%）。碳、氮和硅的同位素模式与罕见的碳化硅X型颗粒相似，这表明这两种罕见的星周物质成分是在同一类型的恒星源中形成的，即II型超新星。
