26Al–26Mg systematics of chondrules in a primitive CO chondrite
26Al–26Mg systematics of chondrules in a primitive CO chondrite
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DOI：
10.1016/j.gca.2008.05.038
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发表时间：
2008-08
影响因子：
5
通讯作者：
E. Kurahashi;E. Kurahashi;N. Kita;N. Kita;H. Nagahara;Y. Morishita
中科院分区：
文献类型：
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作者：
E. Kurahashi;E. Kurahashi;N. Kita;N. Kita;H. Nagahara;Y. Morishita
为了更好地了解最原始的碳质榴辉岩Yamato-81020（CO3.05）中铁镁球粒的成因，我们对其中的铁镁球粒的形成时代、化学成分和矿物学特征进行了系统的研究。用二次离子质谱仪测量了14个贫FeO（I型）、6个富FeO（II型）和2个富铝（Al）球粒的26 Al-26 Mg同位素体系。斜长石中26 Mg的过量（1.0-13.5‰）在除一个外的所有球粒中都能以足够的精度（在2σ水平上大多为0.4-6.6‰）分辨出来。为了评价26 Al-26 Mg计时器的适用性，详细研究了斜长石中Mg和Na的化学分带。我们认为Y-81020陨石球粒的Al-Mg同位素体系没有受到母体变质作用的干扰，26 Al分布均匀，可以作为计时器。假设太阳系早期的26 Al/27 Al初始比值为5×10− 5，则I型陨石在CAI形成后的26 Al-26 Mg年龄为1.7-2.5Ma，II型陨石为2.0-3.0Ma，富铝球粒为1.9和2.6Ma。Y-81020中铁镁球粒的形成年龄与文献中L和LL（3.0-3.1型）球粒陨石的形成年龄一致，表明CO球粒陨石中的共同球粒与L和LL球粒陨石中的共同球粒是同时形成的。CO和L/LL陨石球粒的同时形成表明，CO和L/LL陨石之间的化学差异可能是由原行星盘中球粒形成环境的空间分离造成的。
We have conducted systematic investigations of formation age, chemical compositions, and mineralogical characteristics of ferromagnesian chondrules in Yamato-81020 (CO3.05), one of the most primitive carbonaceous chondrites, to get better understanding of the origin of chemical groups of chondrites. The26Al–26Mg isotopic system were measured in fourteen FeO-poor (Type I), six FeO-rich (Type II) and two aluminum-rich (Al-rich) chondrules using a secondary ion mass spectrometer. Excesses of26Mg in plagioclase (1.0–13.5‰) are resolved with sufficient precision (mostly 0.4–6.6‰ at 2σ level) in all the chondrules studied except one. Chemical zoning of Mg and Na in plagioclase were investigated in detail in order to evaluate the applicability of26Al–26Mg chronometer. We conclude that the Al–Mg isotope system of the chondrules in Y-81020 have not been disturbed by parent-body metamorphism and can be used as chronometer assuming homogeneous distribution of26Al. Assuming an initial26Al/27Al ratio of 5×10−5in the early solar system,26Al–26Mg ages were found to be 1.7–2.5Ma after CAI formation for Type I, 2.0–3.0Ma for Type II and 1.9 and 2.6Ma for Al-rich chondrules. The formation ages of ferromagnesian chondrules in Y-81020 are in good agreement with those of L and LL (type 3.0–3.1) chondrites in the literature, which indicates that common chondrules in the CO chondrite were formed contemporaneously with those in L and LL chondrites. The concurrent formation of chondrules of CO and L/LL chondrites suggests that the chemical differences between CO and L/LL chondrites might be caused by spatial separation of chondrule formation environments in the protoplanetary disk.