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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >High-precision Mg isotopic systematics of bulk chondrites
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High-precision Mg isotopic systematics of bulk chondrites

机译：块状陨石的高精度镁同位素系统

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摘要

Variations of the mass-independent abundance of ~(26)Mg (Δ~(26)Mg~*) and stable Mg (Δ~(25)Mg) isotope composition of chondrites are important because they constrain the homogeneity of ~(26)Al and Mg isotopes in the proto-planetary disc and the validity of the short-lived ~(26)Al-to-~(26)Mg 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. Δ~(26)Mg~* 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 Δ~(26)Mg~* (-0.002 to +0.118%). Very small variations in Δ~(26)Mg~* anomalies in bulk chondrites (-0.006 to +0.019%) correlate with increasing ~(27)Al/~(24)Mg ratios and Δ~(50)Ti, reflecting the variable presence of calcium-aluminium-rich inclusions (CAIs) in some types of carbonaceous chondrites. Similarly, release of radiogenic ~(26)Mg produced by ~(26)Al decay from CAI material in the step-leaching of Murchison best explains the high Δ~(26)Mg~* observed in the last, aggressive, leaching steps of this experiment. Overall, the observed variations in Δ~(26)Mg~* 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 of ~(26)Al (≥±30%) or measurable Mg nucleosynthetic heterogeneity (≥±0.005%) to have existed on a planetesimal scale in the proto-planetary disc. Combined with published Δ~(26)Mg~* data for CAIs, the bulk chondrite data yield a precise initial (~(26)Al/~(27)Al)_0=(5.21±0.06)×10~(-5) and Δ~(26)Mg~*=-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 of ~(26)Al, although planetesimals and planets appear to have accreted from material with a mean initial (~(26)Al/~(27)Al)_0 in 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 (Δ~(25)Mg_(DSM-3)=-0.032±0.035%), is Δ~(25)Mg_(DSM-3)=-0.152±0.079% (2 sd) and is indistinguishable from that of the Earth's mantle.
机译：〜（26）Mg（Δ〜（26）Mg〜*）的质量无关丰度和球粒陨石的稳定Mg（Δ〜（25）Mg）同位素组成的变化很重要，因为它们限制了〜（26）的均匀性原行星盘中的Al和Mg同位素以及适用于陨石的〜（26）Al-to-（26）Mg短命天文钟的有效性。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，包括在CM2球粒陨石Murchison上进行的阶梯浸出实验。代表酸溶性物质的默奇森渗滤液中的Δ〜（26）Mg〜*变化比报道的中子富Ti和Cr同位素小≤30倍，并且没有显示出可解决的Δ〜（26）Mg〜*不足（- 0.002至+ 0.118％）。块状球粒体中Δ〜（26）Mg〜*异常的很小变化（-0.006至+ 0.019％）与〜（27）Al /〜（24）Mg比和Δ〜（50）Ti的增加相关，反映了变量在某些类型的碳质球粒陨石中存在富含钙铝的夹杂物（CAI）。同样，在Murchison分步浸出过程中，从CAI材料中〜（26）Al衰变产生的放射源〜（26）Mg的释放，最好地解释了在最后的侵蚀性浸出步骤中观察到的高Δ〜（26）Mg〜*。这个实验。总体而言，观测到的Δ〜（26）Mg〜*变化很小，并且在本研究的分析不确定性范围内，无法检测到超过由CAI样物质导致的电势差（±0.004％）。结果不允许在原行星盘上以行星状尺度存在〜（26）Al（≥±30％）的自由基异质性或可测量的Mg核合成异质性（≥±0.005％）。结合已发布的CAI的Δ〜（26）Mg〜*数据，块状陨石数据产生精确的初始（〜（26）Al /〜（27）Al）_0 =（5.21±0.06）×10〜（-5）太阳系的Δ〜（26）Mg〜* =-0.0340±0.0016％。但是，利用目前可获得的数据无法确定CAIs和包括球粒母体在内的小行星从中增生的材料是否具有与初始（〜26）Al完全相同的水平，尽管小行星和行星似乎从材料的平均初始（〜（26）Al /〜（27）Al）_0在2.1至6.7×10〜（-5）范围内。除来自CBa球粒陨石Gujba的球粒（Δ〜（25）Mg_（DSM-3）=-0.032±0.035％）外的所有球粒陨石的平均稳定Mg同位素组成为Δ〜（25）Mg_（ DSM-3）= -0.152±0.079％（2 sd），与地球地幔的区别不明显。

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