Correlated nucleosynthetic isotopic variability in Cr, Sr, Ba, Sm, Nd and Hf in Murchison and QUE 97008
Correlated nucleosynthetic isotopic variability in Cr, Sr, Ba, Sm, Nd and Hf in Murchison and QUE 97008
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DOI：
10.1016/j.gca.2011.10.009
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发表时间：
2011-09
影响因子：
5
通讯作者：
L. Qin;R. Carlson;C. Alexander
中科院分区：
文献类型：
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作者：
L. Qin;R. Carlson;C. Alexander
原始 C 球粒陨石 Murchison 和 O 球粒陨石 QUE 97008 的酸浸揭示了 Cr、Sr、Ba、Nd、Sm 和 Hf 的核合成异常。除 Cr 和 Sm 之外的所有异常现象最好的解释是，与普通太阳系物质相比，背景星云成分中纯 s 过程核素的不同添加量略微富含 r 过程同位素。浸出在默奇森留下的残渣富含 s 过程核素，其中 135Ba 的消耗量超过 0.1%，84Sr 的消耗量为 10,000 份中的七份。如果这两个元素中存在 p 过程异常，则它们会在由于 r、s 过程对标准化同位素的不同贡献而引起的变异中丢失。浓度和同位素系统学与默奇森残渣中的 Ba 和 Sr 同位素组成一致，受到富含 s 过程的前太阳 SiC 的强烈影响。一般来说，QUE 97008 中的核合成同位素异常比 Murchison 中小 2 到 5 倍。同位素异常的不同程度与 CM 和 O 球粒陨石之间的基体丰度差异相似，这与核合成异常材料的载体优先存在于基体中以及由于热变质作用，其中一些材料已分布在 O 球粒陨石矿物中的建议一致。与 Ba 和 Sr 一样，Nd、Sm 和 Hf 显示出可变的 s 过程核素丰度，但异常要小得多（例如，Murchison 中的 ε148Nd、ε148Sm=−5.7、2.1，QUE 97008 残基中的ε148Sm 分别为−0.43、0.16）。在针对 s、r 过程变异性校正 Nd 和 Sm 后，整个岩石球粒陨石中的 Sm 显示 p 过程同位素 144 Sm 的相对丰度可变，与 142 Nd 相关性较弱，表明 p 过程对 142 Nd 的直接贡献很小（~7-9%）。 Nd 的核合成变异性解释了 C 和 O、E 球粒陨石之间的 142Nd/144Nd 范围，但不能解释球粒陨石和所有现代地球岩石之间的差异，从而使 146Sm 的衰变和超球粒陨石 Sm/Nd 比率成为地球高 142Nd/144Nd 的可能解释。
Acid leaching of the primitive C-chondrite Murchison and O-chondrite QUE 97008 reveal nucleosynthetic anomalies in Cr, Sr, Ba, Nd, Sm and Hf. The anomalies in all but Cr and Sm are best explained by variable additions of pure s-process nuclides to a background nebular composition slightly enriched in r-process isotopes compared to average Solar System material. Leaching leaves a residue in Murchison that is strongly enriched in s-process nuclides with depletions of over 0.1% in135Ba and seven parts in 10,000 in84Sr. If there are p-process anomalies in these two elements, they are lost in the variability caused by different r-, s-process contributions to the normalizing isotopes. The concentration and isotope systematics are consistent with the Ba and Sr isotopic composition in the Murchison residue being strongly influenced by s-process-rich presolar SiC. In general, the nucleosynthetic isotope anomalies are 2- to 5-fold smaller in QUE 97008 than in Murchison. The different magnitudes of isotope anomalies are similar to the difference in matrix abundance between CM and O chondrites consistent with the suggestion that the carriers of nucleosynthetically anomalous material preferentially reside in the matrix and that some of this material has been distributed throughout the O-chondrite minerals as a result of thermal metamorphism. Neodymium, Sm and Hf display variable s-, r-process nuclide abundances as in Ba and Sr, but the anomalies are much smaller (e.g. ε148Nd, ε148Sm=−5.7, 2.1, respectively, in Murchison and −0.43, 0.16, respectively in QUE 97008 residues). After correcting Nd and Sm for s-, r-process variability, Sm in whole rock chondrites shows variable relative abundances of the p-process isotope144Sm that correlate weakly with142Nd suggesting that the direct p-process contribution to142Nd is small (∼7–9%). Nucleosynthetic variability in Nd explains the range in142Nd/144Nd seen between C and O, E-chondrites, but not the difference between chondrites and all modern Earth rocks, leaving decay of146Sm and a superchondritic Sm/Nd ratio as the likely explanation for Earth’s high142Nd/144Nd.