Compositional grading in an ~3.24 Ga impact-produced spherule bed, Barberton greenstone belt, South Africa: A key to impact plume evolution
Compositional grading in an ~3.24 Ga impact-produced spherule bed, Barberton greenstone belt, South Africa: A key to impact plume evolution
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南非巴伯顿绿岩带 ~3.24 Ga 撞击产生的球粒床的成分分级：撞击羽流演化的关键
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发表时间：
2006
期刊：
影响因子：
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通讯作者：
G. Byerly
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作者：
A. E. Krull;D. Lowe;G. Byerly
南非Barberton绿岩带~3.24 Ga球粒层S3中球粒的成分分级，为球粒形成的撞击产生的岩石蒸汽羽流演化提供了约束条件。S3的两个坠落沉积区相距40公里，厚度都在30厘米左右，几乎完全由小球粒组成。这些球体在大小、组成和质地上各不相同。虽然它们的化学成分发生了变化，但原始结构和一些矿物学特征被保留了下来。两个剖面显示出相似的成分分级，包括五个地层带，每个地层带由不同的球粒类型组合组成。岩石学和地球化学资料表明，这两个剖面的地层分层是相同的。这种区域性一致的分级表明，在撞击羽流中存在大规模的成分和热分层。稀土元素分析表明，这些剖面S3的基底部分含耐火物质最多，上部含玄武岩成分最多。总的来说，床层的时间演化与预测的球粒质和玄武岩物质分馏改良混合的凝结序列一致。
Compositional grading of spherules in the ~3.24 Ga spherule bed S3, Barberton greenstone belt, South Africa, provides constraints on the evolution of the impact-generated rock vapor plume within which the spherules formed. Two fall-deposited sections of S3, 40 km apart, are both about 30 cm thick and composed almost entirely of spherules. These spherules are diverse in size, composition and texture. Although they have been chemically altered, primary textures and some mineralogy is preserved. Both sections show a similar compositional grading, which includes five stratigraphic zones, each composed of a distinctive assemblage of spherule types. Petrographic and geochemical data suggest that this stratigraphic layering is the same in both sections. This regionally consistent grading suggests large-scale compositional and thermal layering in the impact plume. REE data indicate that the basal part of S3 in these sections contains the most refractory material and that the upper part contains the most basaltic components. Overall, the beds show a temporal evolution consistent with the condensation sequence predicted for a fractionation-modified mixing of chondritic and basaltic materials.