Geochemistry of the impact‐generated melt sheet at Manicouagan: Evidence for fractional crystallization
Geochemistry of the impact‐generated melt sheet at Manicouagan: Evidence for fractional crystallization
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马尼夸根撞击产生的熔体片的地球化学：分步结晶的证据
DOI：
10.1029/2010jb008084
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发表时间：
2011
影响因子：
--
通讯作者：
J. Spray
中科院分区：
文献类型：
--
作者：
C. O'Connell;J. Spray
[1]最近对Manicouagan撞击构造进行的勘探钻探显示，撞击熔融物的局部发展比以前接受的平均值1000米厚得多，熔融物片-基底界面更为复杂，显示出相当大的“地形”。最厚的部分（1045米的碎屑无碎屑穷人的影响熔体覆盖425米的碎屑负载的影响熔体）是与一个位于中心的，故障边界地堑，在熔体结晶之前的地方。本文报道了从9个钻孔和一组野外样品中获得的115个样品的地球化学调查结果。结果表明，大部分的岩心包括一个未分化的单位，显示出最小的地球化学变化。这通常是300米厚，相当于以前的研究从暴露的岩石样品。与此相反，更深的部分表现出分化，并分为三个层的化学，矿物学和纹理的变化的基础上：石英二长岩石英二长闪长岩上部带（276米），石英二长闪长岩中部带（244米），和一个主要二长闪长岩下部带（525米）。矿物学特征为斜长石>正长石>单斜辉石>斜方辉石，下部及以下普遍发育角闪石、黑云母和橄榄石。我们的研究结果表明，较厚的部分的冲击熔体片已经经历了分步结晶。部分Manicouagan的影响熔体的分化表明，月球的影响熔体样品具有不同的化学成分，以前被认为是来自不同的影响盆地，可能是通过分馏的共同的，小得多的直径的影响结构。
[1] Recent exploration drilling of the Manicouagan impact structure has revealed local developments of impact melt that are substantially thicker than the previously accepted average of ∼300 m and a more complex melt sheet–basement interface showing considerable “topography.” The thickest section (1045 m of clast-free to clast-poor impact melt overlying 425 m of clast-laden impact melt) is associated with a centrally located, fault-bounded graben that was in place prior to melt crystallization. Here we report the results of a geochemical investigation of 115 samples obtained from nine drill holes and a set of field samples. The results reveal that most of the drill core comprises an undifferentiated unit, showing minimal geochemical variation. This is typically ∼300 m thick and equates with previous studies performed on samples from exposed rock. In contrast, the deeper section exhibits differentiation and is divided into three layers based on chemical, mineralogical, and textural variations: a quartz monzonite to quartz monzodiorite upper zone (276 m), a quartz monzodioritic middle zone (244 m), and a mainly monzodioritic lower zone (525 m). The mineralogy is defined by plagioclase > orthoclase > clinopyroxene > orthopyroxene, with ubiquitous amphibole and biotite and olivine locally developed in the lower zone and below. Our results reveal that the thicker section of the impact melt sheet has undergone fractional crystallization. Differentiation of parts of the Manicouagan impact melt indicates that lunar impact melt samples possessing different chemistries, previously considered to be derived from distinct impact basins, may be related via the fractionation of common, much smaller diameter impact structures.