Silicon isotopic variation in enstatite meteorites: Clues to their origin and Earth-forming material

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• 作者：Paul S. Savage ; ^{savage@levee.wustl.edu} ; Fré ; dé ; ric Moynier
• 关键词：enstatite chondrites ; silicon isotopes ; refractory lithophile elements ; metal&ndash ; silicate fractionation ; core formation
• 刊名：Earth and Planetary Science Letters
• 出版年：2013
• 出版时间：1 January, 2013
• 年：2013
• 卷：361
• 期：Complete
• 页码：487-496
• 全文大小：466 K

文摘

Of the primitive meteorite groups, the enstatite chondrites are among the most chemically dissimilar to terrestrial, which should preclude them as major components of the proto-Earth. However, for many isotope systems (most notably oxygen), enstatite chondrites show very little variation away from terrestrial, hinting at a common origin. One isotope system which appears to differ from this trend is silicon, but no satisfactory explanation has been proposed as to why these meteorites should be significantly lighter than both the silicate Earth and other primitive meteorite groups.

This study presents a comprehensive investigation into the Si isotope composition of enstatite chondrites and their differentiated counterparts, the aubrites, and confirms that these meteorites are, with respect to Si isotopes, the lightest macroscale solar system objects so far analysed. Crucially, the results show that EH chondrites are significantly lighter (¦Ä³⁰Si=?0.77¡À0.08¡ë) than EL chondrites (¦Ä³⁰Si=?0.59¡À0.09¡ë) and aubrites (¦Ä³⁰Si=?0.60¡À0.11¡ë). Silicon isotope analyses of the metal-free components of EH and EL reveal that these are identical, within error, to each other and with carbonaceous/ordinary chondrite bulk measurements (viz., ¦Ä³⁰Si ¡«?0.47), which is taken as evidence that Si isotope variation in the nebular gas is not the cause of the light Si isotope enrichment in enstatite chondrites. From this, one can infer that silicates condensing from the solar nebula over a wide range of compositions and, presumably, heliocentric distances have very similar Si isotope compositions. A statistically significant negative correlation between bulk ¦Ä³⁰Si and Si content in enstatite chondrite kamacite indicates that the presence of isotopically light Si in the metal phase (as the result of formation under reducing conditions) is the principal cause of the bulk enrichment in lighter Si isotopes.

Based on the Si isotope offset between enstatite meteorites and the silicate Earth, the extant enstatite meteorites cannot represent a significant proportion of the material that accreted to form the proto-Earth, as unfeasibly large amounts of Si would have to enter the core to satisfy mass balance equations. However, we posit that material enriched in refractory lithophile elements which condensed in the same region as E-chondrites, could still be an important component.