Rb–Sr isotopic systematics of alkali-rich fragments in the Yamato-74442 LL-chondritic breccia
Rb–Sr isotopic systematics of alkali-rich fragments in the Yamato-74442 LL-chondritic breccia
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Yamato-74442 LL 球粒状角砾岩中富碱碎片的 Rb-Sr 同位素系统学
DOI：
10.1016/j.epsl.2013.01.037
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
S. Yoneda
中科院分区：
文献类型：
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作者：
T. Yokoyama;K. Misawa;O. Okano;Chi;L. Nyquist;Justin I. Simon;M. Tappa;S. Yoneda
对Yamato (Y)-74442 ll -球粒质角砾岩中富碱火成岩碎屑进行了矿物学、岩石学和Rb-Sr同位素研究。碎片大小为几毫米，主要由斑状橄榄石和枝晶辉石组成，镶嵌在富碱的玻璃基质中。次要相包括铬铁矿、三硝石和金属镍铁。碎片的大部分化学成分几乎与宿主球粒陨石相同，除了钠的消耗和钾的富集。对Y-74442的9个碎片进行同位素分析，λ(87Rb)=0.01402Ga−1，Rb-Sr年龄为4429±54Ma (2σ)，初始比值为87sr /86Sr=0.7144±0.0094 （2σ）。假设这些碎片的前体在太阳系形成时形成于4568Ma， 87sr /86Sr=0.69889，则这些碎片源物质的时间平均Rb/Sr（重量）比为2.58+0.91/−0.93。这个来源的极高Rb/Sr值很难用任何火成岩分馏或液体不混溶来解释，但可以用球粒陨石成分与早期太阳星云中形成的富碱成分的混合来解释。在我们首选的模型中，Rb/Sr⪢30的碱组分在除去难熔锶后会从残余的星云气体中冷凝出来，并且必须在温度足够低的区域隔离很长时间，以防止与其他硅酸盐/氧化物发生反应。碱组分（早期星云凝聚物）和镁铁组分的混合物可以反映至少4429Ma前对l -球粒质母体的撞击引起的闪热，并且在此事件中可能发生了铷和钾相对于锶的进一步富集。由此产生的撞击熔体岩石可能在后来的撞击事件中破碎，并最终合并到Y-74442母体中。因此，在早期太阳星云和后来的l -球粒陨石母体中，碱富集的显著特征作为一些球粒质角砾岩（如Y-74442）的一小部分被保存下来。
We have undertaken mineralogical, petrographical and Rb–Sr isotopic studies on alkali-rich igneous rock fragments in the Yamato (Y)-74442 LL-chondritic breccia. The fragments are a few mm in size and are composed mainly of porphyritic olivine and dendritic pyroxene set in alkali-rich groundmass glass. Minor phases include chromite, troilite and metallic nickel–iron. Bulk chemical compositions of the fragments are almost identical to the host chondrite except for a depletion of sodium and an enrichment of potassium. Isotopic analyses of nine fragments from Y-74442 yield a Rb–Sr age of 4429±54Ma (2σ) for λ(87Rb)=0.01402Ga−1with an initial ratio of87Sr/86Sr=0.7144±0.0094 (2σ). Assuming precursors of the fragments formed 4568Ma with87Sr/86Sr=0.69889 when the Solar System formed, a time-averaged Rb/Sr (weight) ratio of the source material for the fragments is calculated to be 2.58+0.91/−0.93. The extremely high Rb/Sr value of this source is difficult to interpret by any igneous fractionation or liquid immiscibility, but can be explained by mixing of a chondritic component with an alkali-rich component formed in the early solar nebula. In our preferred model, the alkali component with Rb/Sr⪢30 would have condensed from the residual nebular gas after removal of refractory strontium and must have been isolated for a long time in a region where the temperature was sufficiently low to prevent reaction with other silicates/oxides. A mixture of the alkali component (early nebular condensates) and the ferromagnesian component could reflect flash heating induced by impact on an LL-chondritic parent body at least 4429Ma ago, and further enrichments of rubidium and potassium relative to strontium could have occurred during this event. The resulting impact-melt rocks could have been fragmented by later impact event(s) and finally incorporated into the Y-74442 parent body. Thus, a remarkable signature of alkali enrichments both in the early solar nebula and later on the LL-chondrite parent body is preserved as a minor component of some chondritic breccias such as Y-74442.