Chondrules in undifferentiated meteorites are former silicate melt droplets of variable texture and composition. Although widely studied, the chondrule formation mechanisms and conditions that explain all properties of chondrules are yet to be identified. To further constrain the processes that affected chondrules in the solar nebula and on the meteorite parent body, we determined in situ Si isotope ratios and major and trace element compositions of minerals in chondrules of variable types and sizes from the Allende CV3 chondrite.

The δ³⁰Si in chondrule minerals ranges from −1.28 ± 0.19 to 0.55 ± 0.20‰ (2SE). The δ³⁰Si in chondrules shows no direct relationship with chondrule sizes or with distance between core and rim. Barred olivine-rich chondrules record the highest δ³⁰Si, likely because of faster cooling and less interaction with isotopically light nebular gas. Type I non-porphyritic and some porphyritic chondrules show overall higher δ³⁰Si compared to type II porphyritic chondrules. Furthermore, Mg-rich olivine and Mg-rich pyroxene have systematically higher δ³⁰Si compared to Fe-rich olivine and Fe-rich pyroxene.

The variable δ³⁰Si of type I chondrule silicates (Mg-rich) compared to type II chondrule silicates (Fe-rich) may be explained by variable interaction of chondrule silicates with the nebular gas in the solar nebula. We envision a combination of equilibrium and kinetic isotope fractionation of Si between nebular gas and Fe-poor silicates (such as forsterite, anorthite, enstatite and mesostasis) and Fe-rich olivine and orthopyroxene. Petrographic evidence suggests that the enrichment of Fe in some highly altered porphyritic chondrules and at chondrule rims was likely caused by hydrothermal alteration on the parent body. Therefore, the correlation of Fe and δ³⁰Si of the chondrule minerals might serve as an indicator for the extent of further secondary processing of some chondrule minerals. The sum of these observations suggests that the formation and alteration of type II chondrules occurred by oxidation of originally reduced, metal-rich type I chondrules, both in the solar nebula and later on the meteorite parent body. Remaining ³⁰Si depleted gas contributed to the isotopic composition of matrix silicates. The evidence favours the formation of chondrules and matrix of the Allende meteorite in nebular settings rather than by asteroid impacts.

未分化陨石中的软骨是质地和组成可变的前硅酸盐熔滴。尽管已被广泛研究，但解释软骨的所有特性的软骨形成机理和条件尚待确定。为了进一步限制影响太阳星云和陨石母体中球粒体的过程，我们确定了Allende CV3球粒体中各种类型和大小的球粒中原位Si同位素比以及矿物中主要和微量元素的组成。

的δ ³⁰在球粒矿物质范围从-1.28±0.19至0.55±0.20‰（2SE）的Si。的δ ³⁰中示出了球粒的Si与球粒尺寸或具有芯部和轮辋之间的距离没有直接关系。禁止富橄榄石球粒记录最高δ ³⁰的Si，可能是因为较快的冷却，并用同位素光星云气体少的交互。I型非斑状和一些斑状球粒整体表现出较高的δ ³⁰的Si相比，II型斑状球粒。此外，富含Mg的橄榄石和Mg-富辉石具有系统性更高δ ³⁰的Si相比，富铁橄榄石和富铁辉石。

变量δ ³⁰ I型球粒硅酸盐的Si（富含Mg的）相比，II型球粒硅酸盐（富Fe）可以通过球粒硅酸盐的变量相互作用与星云气体在太阳星云说明。我们设想在星云气和贫铁硅酸盐（例如镁橄榄石，钙长石，顽辉石和介晶）和富铁橄榄石和邻苯二茂铁之间平衡和动态同位素分馏硅的组合。岩石学证据表明，Fe富集在一些斑状的球状软骨和软骨边缘中，可能是由于母体的热液作用引起的。因此，Fe及相关δ ³⁰软骨矿物的Si可以作为某些软骨矿物进一步二次加工程度的指标。这些观察结果的总和表明，II型软骨的形成和改变是由最初还原的，富含金属的I型软骨在太阳星云中以及后来在陨石母体上的氧化而发生的。剩余的³⁰ Si贫化气体有助于基质硅酸盐的同位素组成。有证据支持在星云状环境中形成阿连德陨石的软骨和基质，而不是受到小行星的撞击。