High-precision Mg isotopic systematics of bulk chondrites
High-precision Mg isotopic systematics of bulk chondrites
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DOI：
10.1016/j.epsl.2010.06.017
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发表时间：
2010-08
影响因子：
5.3
通讯作者：
M. Schiller;M. Handler;J. Baker
中科院分区：
文献类型：
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作者：
M. Schiller;M. Handler;J. Baker
球粒陨石的 26Mg (δ26Mg*) 丰度和稳定 Mg (δ25Mg) 同位素组成的与质量无关的变化非常重要，因为它们限制了原行星盘中 26Al 和 Mg 同位素的均匀性以及应用于陨石的短命 26Al-26Mg 计时器的有效性。我们提供了代表几乎所有主要球粒陨石类别的球粒陨石的高精度 Mg 同位素数据和 Al/Mg 比率，包括对 CM2 球粒陨石默奇森的逐步浸出实验。默奇森渗滤液中代表酸溶性物质的 δ26Mg* 变化比报告的 Ti 和 Cr 富中子同位素小 ≤30 倍，并且没有揭示 δ26Mg* 中可解决的缺陷（−0.002 至 +0.118‰）。大块球粒陨石中 δ26Mg* 异常的微小变化（−0.006 至 +0.019‰）与 27Al/24Mg 比率和 δ50Ti 的增加相关，反映了某些类型的碳质球粒陨石中富钙铝包裹体 (CAI) 的存在变化。类似地，在 Murchison 的逐步浸出过程中，CAI 材料中 26Al 衰变产生的放射性 26Mg 的释放最好地解释了在该实验的最后的侵蚀性浸出步骤中观察到的高 δ26Mg*。总体而言，观察到的 δ26Mg* 变化很小，并且在本研究的分析不确定度 (±0.004‰) 内无法检测到超出 CAI 类材料存在导致的潜在差异。结果不允许26Al的自由基异质性（≥±30%）或可测量的Mg核合成异质性（≥±0.005‰）在原行星盘的星子尺度上存在。结合已发布的 CAI δ26Mg* 数据，大块球粒陨石数据可得出太阳系精确的初始 (26Al/27Al)0=(5.21±0.06)×10−5 和 δ26Mg*=−0.0340±0.0016‰。然而，用目前可用的数据不可能确定地确定CAI和包括球粒陨石母体在内的星子吸积的材料是否具有完全相同的26Al初始水平，尽管星子和行星似乎是从平均初始(26Al/27Al)0在2.1到6.7×10−5范围内的材料中吸积的。除 CBa 球粒陨石 Gujba 的球粒 (δ25MgDSM-3=−0.032±0.035‰) 外，所有分析的球粒陨石的平均稳定镁同位素组成为 δ25MgDSM-3=−0.152±0.079‰ (2 sd)，与地球的球粒没有区别。 地幔。
Variations of the mass-independent abundance of26Mg (δ26Mg*) and stable Mg (δ25Mg) isotope composition of chondrites are important because they constrain the homogeneity of26Al and Mg isotopes in the proto-planetary disc and the validity of the short-lived26Al-to-26Mg chronometer applied to meteorites. We present high-precision Mg isotope data and Al/Mg ratios of chondrites representing nearly all major chondrite classes, including a step-leaching experiment on the CM2 chondrite Murchison. δ26Mg* variations in leachates of Murchison representing acid soluble material are ≤30 times smaller than reported for neutron-rich isotopes of Ti and Cr and do not reveal resolvable deficits in δ26Mg* (−0.002 to +0.118‰). Very small variations in δ26Mg* anomalies in bulk chondrites (−0.006 to +0.019‰) correlate with increasing27Al/24Mg ratios and δ50Ti, reflecting the variable presence of calcium–aluminium-rich inclusions (CAIs) in some types of carbonaceous chondrites. Similarly, release of radiogenic26Mg produced by26Al decay from CAI material in the step-leaching of Murchison best explains the high δ26Mg* observed in the last, aggressive, leaching steps of this experiment. Overall, the observed variations in δ26Mg* are small and potential differences beyond that which result from the presence of CAI-like material cannot be detected within the analytical uncertainties of this study (±0.004‰). The results do not allow radical heterogeneity of26Al (≥±30%) or measurable Mg nucleosynthetic heterogeneity (≥±0.005‰) to have existed on a planetesimal scale in the proto-planetary disc. Combined with published δ26Mg* data for CAIs, the bulk chondrite data yield a precise initial (26Al/27Al)0=(5.21±0.06)×10−5and δ26Mg*=−0.0340±0.0016‰ for the Solar System. However, it is not possible with the currently available data to determine with certainty whether CAIs and the material from which planetesimals accreted including chondrite parent bodies had precisely the same initial levels of26Al, although planetesimals and planets appear to have accreted from material with a mean initial (26Al/27Al)0in the range of 2.1 to 6.7×10−5. The average stable Mg isotope composition of all analysed chondrites, with the exception of a chondrule from the CBa chondrite Gujba (δ25MgDSM-3=−0.032±0.035‰), is δ25MgDSM-3=−0.152±0.079‰ (2 sd) and is indistinguishable from that of the Earth's mantle.