SPINEL FROM ARCHEAN IMPACT SPHERULES
SPINEL FROM ARCHEAN IMPACT SPHERULES
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DOI：
10.1016/0016-7037(94)90099-x
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发表时间：
1994-08-01
影响因子：
5
通讯作者：
LOWE, DR
中科院分区：
文献类型：
--
作者：
BYERLY, GR;LOWE, DR
大型陨石撞击记录在两个早期太古代绿岩带序列（3.2-3.5 Ga）保存的远坠沉积物。制图、地层学和地球化学研究已被用于确定南部非洲巴伯顿绿岩带中至少四个撞击层和西澳大利亚皮尔巴拉绿岩带中的一个撞击层。代表冲击熔滴的球体将这些冲击层与相关的凝灰岩区分开来。具有不寻常成分特征的尖晶石出现在其中两个撞击层的球粒中。尖晶石是唯一保存下来的原生相，在保存最好的层中发现的球粒中有10%。一些球粒含有高达40%的尖晶石，这通常是球粒内结晶的唯一主相。尖晶石形态包括枝晶和骨架八面体，典型的高温熔体中发现的那些。尖晶石成分与新生代撞击的尖晶石成分相似，包括非常高的Ni和三价铁含量，但太古代尖晶石的独特之处在于主要由亲铜元素Fe、Ni、Cr和V组成。这种尖晶石不是变质的或交代的起源。下伏翁韦尔瓦赫特群中紧密共生的科马提岩，无论是新鲜的还是高度蚀变的，都具有未蚀变的尖晶石，其火成岩成分和形态与冲击尖晶石截然不同。初始高温还原产生熔融液滴，其中一些是不混溶的硅酸盐金属/硫化物。密度的差异允许分离的一些这些不混溶的熔体，随后独立发展，并形成具有高度可变的亲铜元素含量的球粒。大气再入过程中的氧化增加了亲铜-亲铁熔体中的Ni/Fe和Fe+3/Fe+2。太古代冲击尖晶石的Ni/Fe比高于新生代冲击尖晶石，但Fe+3/Fe+2明显较低，这可能反映了太古代期间氧化气氛明显较低。太古代大气氧化实际上可能是一个短暂的现象，与一个大的影响到陆地海洋导致非常高的大气H2O/H-2。根据含尖晶石球粒的平均直径为0.85毫米，估计火流星的直径为24公里。这一估计与在含有尖晶石的撞击层中观察到的约5.3 mg/cm 2的Ir通量一致， 这将是由一个直径30公里的南极火流星产生的。
Large meteorite impacts are recorded in distal fall deposits preserved in two early Archean greenstone belt sequences (3.2-3.5 Ga). Mapping, stratigraphic, and geochemistry studies have been used to identify at least four impact layers in the Barber-ton greenstone belt of southern Africa and one in the Pilbara greenstone belt of Western Australia. Spherules that represent impact melt droplets distinguish these impact layers from associated tuffs. Spinel with unusual compositional traits occurs within spherules in two of these impact layers. Spinel is the only preserved primary phase, and is found within up to 10% of the spherules in the best preserved layers. Some spherules contain up to 40% spinel, which is often the only primary phase that crystallized within the spherules. Spinel morphologies include dendrites and skeletal octahedra, typical of those found in high temperature melts. Spinel compositions are similar to those of Cenozoic impacts, including very high Ni and ferric iron contents, but the Archean spinel is distinctive in being composed dominantly of the chalcophile elements Fe, Ni, Cr, and V. This spinel is not metamorphic or metasomatic in origin. Closely associated komatiites in the underlying Onverwacht Group, both fresh and highly altered, have unaltered spinel with igneous compositions and morphologies quite distinct from the impact spinel.Characteristics of spinel preserved in these Archean impact spherules suggest a rather complex series of steps following a large bolide impact. Initial high temperature reduction produced melt droplets, some of which were immiscible silicate-metal/sulfide. Density differences allowed separation of some of these immiscible melts, which subsequently evolved independently and formed spherules with highly variable chalcophile element contents. Oxidation during atmospheric reentry increased Ni/Fe and Fe+3/Fe+2 in the chalcophile-siderophile melts. Archean impact spinel has higher Ni/Fe than Cenozoic impact spinel, though distinctly lower Fe+3/Fe+2, which probably reflects a substantially less oxidizing atmosphere during the Archean. The Archean atmospheric oxidation may in fact be a transient phenomenon associated with a large impact into the terrestrial ocean resulting in very high atmospheric H2O/H-2. Bolide size is estimated to be 24 km in diameter based on the average diameter of 0.85 mm for the spinel-bearing spherules. This estimate is consistent with the observed Ir fluence in spinel-bearing impact layers of approximately 5.3 mg/cm2, which would be produced by a 30 km diameter chondritic bolide.