Chemistry and mineralogy of Precambrian paleosols at the base of the Dominion and Pongola Groups (Transvaal, South Africa)
Chemistry and mineralogy of Precambrian paleosols at the base of the Dominion and Pongola Groups (Transvaal, South Africa)
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Dominion 群和 Pongola 群底部前寒武纪古土壤的化学和矿物学（南非德兰士瓦）
DOI：
10.1016/0301-9268(86)90003-3
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发表时间：
1986
影响因子：
3.8
通讯作者：
M. M. Kimberley
中科院分区：
文献类型：
--
作者：
D. Grandstaff;M. Edelman;R. Foster;E. Zbinden;M. M. Kimberley
我们研究了（最早元古代）自治领和（太古代）Pongola群的沉积岩与下面的花岗岩（南非德兰士瓦）之间的蚀变带。蚀变带具有过渡的下边界，原生火成岩矿物向上逐渐被破坏，与上覆沉积岩接触。Ca、Mg、Na、P和Mn在该带中向上逐渐损失。化学、矿物学变化和已有的地层学证据都与2.8- 3.0Ga前寒武纪风化作用形成的古土壤的解释相一致，古土壤形成后经历了交代和变质作用。古土壤中的绢云母可能是原始粘土低温钾交代的产物。Pongola前古土壤中的红柱石可能是由不渗透带中的原始粘土变质而形成的，这些不渗透带阻止了地下水流动和钾的交代作用。除铁的行为外，三个地点的风化影响相似。一个网站失去了约20%的铁，而其他网站失去了75%和80%的初始铁含量。铁损失的差异可能是由于土壤通风和大气气体扩散的变化作为粒度和地形的函数。铁损失的变化可以用来对前寒武纪大气的组成进行一些限制。考虑到自治领古土壤、上覆含铀砾岩、化学风化和温室效应的要求以及可能存在非扩散光化学氧化剂的限制，氧气和二氧化碳气体的分压可能在以下范围内：PO 1 - 0.02%至0.5%PAL（当前大气水平）和PCO 2 - 5至30 PAL。
We have studied alteration zones found between sedimentary rocks of the (lowest Proterozoic) Dominion and (Archean) Pongola Groups and underlying granitic rocks (Transvaal, South Africa). The alteration zones have a transitional lower boundary, primary igneous minerals are gradually destroyed upward toward the contact with overlying sedimentary rocks. Ca, Mg, Na, P, and Mn are progressively lost upwards in the zone. The chemical and mineralogical variations, and available stratigraphic evidence are all consistent with the interpretation that these zones are paleosols formed by Precambrian weathering 2.8-3.0 Ga ago.After formation the paleosols underwent metasomatism and metamorphism. Sericite now present in the paleosols was probably formed by low temperature potassium metasomatism of original clays. Andalusite in the pre-Pongola paleosol was probably formed by metamorphism of original clays in impermeable zones which prevented groundwater flow and potassium metasomatism.The Dominion paleosol was investigated at three localities. The effects of weathering were similar at the three sites except for the behavior of iron. One site lost about 20% of iron whereas the other sites lost 75% and 80% of initial iron content. The differences in iron loss may be due to variation in soil ventilation and diffusion of atmospheric gases as a function of grain size and topography. Variations in iron loss may be used to place some constraints on the composition of the Precambrian atmosphere. Given constraints imposed by Dominion paleosols, overlying uraniferous conglomerates, requirements for chemical weathering and the greenhouse effect, and possible presence of non-diffusing photochemical oxidants, the partial pressures of oxygen and carbon dioxide gases probably are in the ranges:PO1- 0.02% to 0.5% PAL (Present Atmospheric Level) andPCO2- 5 to 30 PAL.