Contemporaneous formation of chondrules in distinct oxygen isotope reservoirs
Contemporaneous formation of chondrules in distinct oxygen isotope reservoirs
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不同氧同位素库中同时形成球粒
DOI：
10.1016/j.gca.2013.01.045
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发表时间：
2013
影响因子：
5
通讯作者：
and Kita N. T
中科院分区：
文献类型：
--
作者：
Ushikubo T.;Nakashima D.;Kimura M.;Tenner T. J.;and Kita N. T
我们研究了来自最原始的碳质球粒陨石Acfer 094的I型（贫feo）球粒的26al - 26mg系统。推断的Acfer 094球粒的初始26al /27Al比值范围为（4.2±2.0）×10−6至（9.0±1.5）×10−6。这些球粒具有不同的氧同位素比率（Δ17O:−5‰，−2‰和0‰），但没有观察到推断的初始26al /27Al比率与氧同位素比率之间的相关性。这些结果表明，在球粒形成期间，太阳星云的区域氧同位素不均匀性。球粒中的一些低钙辉石颗粒具有少量但可溶解的26mg过量，其水平为~ 0.1‰，这可能是由于再加热事件导致球粒部分熔化的结果。如果是这样的话，这些特定球粒的真实初始26al /27Al比值可能比使用共存的斜长石和橄榄石的Mg同位素比值估计的值略低（<10%）。球粒中许多橄榄石颗粒的Mg稳定同位素比值（25Mg/24Mg）相对于同一球粒中其他相的Mg稳定同位素比值正分异~ 1‰。这表明橄榄石在球粒形成的冷却阶段显著凝结之前，保存了球粒形成熔体的正分馏Mg同位素比率。
We investigated the26Al–26Mg systematics of type I (FeO-poor) chondrules from one of the most primitive carbonaceous chondrites, Acfer 094. The inferred initial26Al/27Al ratios of chondrules from Acfer 094 ranged from (4.2±2.0)×10−6to (9.0±1.5)×10−6. These chondrules have distinct oxygen isotope ratios (Δ17O: −5‰, −2‰, and 0‰), though no correlation between inferred initial26Al/27Al ratios and oxygen isotope ratios were observed. These results indicate the regional oxygen isotope heterogeneity in the solar nebula over the duration of chondrule formation. A few low-Ca pyroxene grains in chondrules have small but resolvable26Mg excesses at the level of ∼0.1‰, which may be the result of a partial melting of chondrules by reheating events. If this is the case, the true inferred initial26Al/27Al ratios of these particular chondrule could be slightly lower (<10%) than estimates using Mg isotope ratios of coexisting plagioclase and olivine. Stable isotope ratios of Mg (25Mg/24Mg) of many olivine grains in chondrules are positively fractionated by ∼1‰ relative to those in other phases within the same chondrules. This suggests that olivine preserved a positively fractionated Mg isotope ratios of the chondrule-forming melt prior to significant condensation during the cooling stage of chondrule formation.