Nucleosynthetic strontium isotope anomalies in carbonaceous chondrite
Nucleosynthetic strontium isotope anomalies in carbonaceous chondrite
复制标题
碳质球粒陨石中的核合成锶同位素异常
DOI：
10.1016/j.epsl.2015.01.040
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发表时间：
2015
影响因子：
5.3
通讯作者：
Hiroshi Yamazaki
中科院分区：
文献类型：
--
作者：
Tetsuya Yokoyama;Yusuke Fukami;Wataru Okui;Nobuaki Ito;Hiroshi Yamazaki
精确的Sr同位素组成的样品从连续的酸浸实验已经确定了三个碳质陨石，阿连德，默奇森，和塔吉什湖，连同那些在这些陨石的散装等分试样。陨石中的硫酸浸出物和残留物具有Sr同位素异常的特征，其μ 84 Sr值（与标准物质的相对偏差为10 6）从+120到− 4700 ppm不等，证明了单个陨石中有多个核合成来源。此外，不同类型的花岗岩样品的μ 84 Sr模式也不同。对于阿连德和Murchison，在浸提步骤3（HCl+ H2 O，75° C）中观察到最高μ 84 Sr值，这可能是因为掺入了富钙和富铝夹杂物（CAIs）。相比之下，在Murchison和Tagish Lake的后期部分（步骤6和7）中观察到极低的μ 84 Sr值，表明存在来自AGB星的s-过程富集的前太阳SiC颗粒。一个μ 84 Sr-β 54 Cr图是用分别绘制的碳质陨石和其他陨石（非碳质）的CAI和大块等分试样制作的;然而，它们仍然形成了整体的正相关。CAIs的μ 84 Sr和μ 54 Cr值最高，而碳质陨石和非碳质陨石的μ 84 Sr和μ 54 Cr值分别介于两者之间和最低。这种积极的趋势被解释为全球热处理的结果，其中高μ 84 Sr和β 54 Cr载体的升华在CAIs中产生了过量的μ 84 Sr和β 54 Cr特征，而非碳质星子则来自经历了显著热处理的材料，因此具有相对较低的μ 84 Sr和β 54 Cr值。除了全球趋势外，碳质陨石和非碳质陨石都表现出内在的变化，突出了类似于其他同位素组合（例如，54 Cr-50 Ti，54 Cr-Δ 17 O）的同位素二分法。一个合理的情况下，创造的内在变化涉及局部热处理（例如，闪热形成球粒）所造成的额外的选择性破坏的前太阳晶粒不同于所造成的全球热处理。这样一个全球性的积极趋势和局部变化的两个陨石群的存在表明一个复杂的动力学历史的尘埃颗粒的热处理，材料运输，并混合在原行星盘星子形成之前。
Precise Sr isotopic compositions in samples from sequential acid leaching experiments have been determined for three carbonaceous chondrites, Allende, Murchison, and Tagish Lake, together with those in the bulk aliquots of these meteorites. The chondritic acid leachates and residues were characterized by Sr isotope anomalies with variable μ 84 Sr values (10 6 relative deviation from a standard material) ranging from+ 120 to− 4700 ppm, documenting multiple nucleosynthetic sources within a single meteorite. In addition, the μ 84 Sr patterns across leaching samples for individual chondrites differed from one another. The highest μ 84 Sr values were observed for leaching Step 3 (HCl+ H 2 O, 75° C) for Allende and Murchison likely because of the incorporation of calcium and aluminum-rich inclusions (CAIs). In contrast, extremely low μ 84 Sr values were observed in the later fractions (Steps 6 and 7) for Murchison and Tagish Lake, suggesting the existence of s-process-enriched presolar SiC grains derived from AGB stars. A μ 84 Sr–ϵ 54 Cr diagram was prepared with the CAIs and bulk aliquots of carbonaceous chondrites and other meteorites (noncarbonaceous) that were plotted separately; however, they still formed a global positive correlation. CAIs presented the highest μ 84 Sr and ϵ 54 Cr values, whereas carbonaceous chondrites and noncarbonaceous meteorites had intermediate and the lowest μ 84 Sr and ϵ 54 Cr values, respectively. The positive trend was interpreted as resulting from global thermal processing in which sublimation of high μ 84 Sr and ϵ 54 Cr carriers generated the excess μ 84 Sr and ϵ 54 Cr signatures in CAIs, while noncarbonaceous planetesimals accreted from materials that underwent significant thermal processing and thus had relatively low μ 84 Sr and ϵ 54 Cr values. Apart from the global trend, the carbonaceous chondrites and noncarbonaceous meteorites both exhibited intrinsic variations that highlight an isotopic dichotomy similar to that observed in other isotope combinations (eg, ϵ 54 Cr–ϵ 50 Ti, ϵ 54 Cr–Δ 17 O). A plausible scenario for creation of the intrinsic variations involves local thermal processing (eg, flash heating for chondrule formation) caused by additional selective destruction of presolar grains differently than that caused by global thermal processing. The existence of such a global positive trend and local variations for two meteorite groups suggests a complicated dynamic history for the dust grains with respect to thermal processing, material transportation, and mixing in the protoplanetary disk prior to planetesimal formation.