During the Aptian (Cretaceous), in what is now the South Atlantic, the largest chemogenic (abiotic) carbonate factory so far identified in the Phanerozoic geological record developed as a vast hyper-alkaline lake system. This covered at least 330 000 km², producing carbonates, locally over 500 m thick, in what are now the offshore Santos and Campos basins (Brazil), and Kwanza Basin (Angola). Current evidence supports the view that almost all of this carbonate was chemogenic in origin, precipitated from hyper-alkaline, shallow lake waters, probably by evaporation. This unit, best documented from offshore Brazil and known as the Barra Velha Formation (Santos Basin) and the Macabu Formation (Campos Basin), consists of just two basic carbonate components, millimetre to centimetre sized crystal shrubs and spherulites. These are commonly in situ but can also be reworked into a range of detrital facies. Demonstrable microbialites are generally rare. These carbonates are associated with Mg silicates (as clays) which had a profound influence not only on the textural development of the in situ carbonates, but also on their diagenesis. The dissolution of the clays produced much of the porosity in these limestones, which are the hosts for multi-billion barrel oil fields. The source of the carbonate was most likely from metasomatic alteration of mafic rocks, such as continental flood basalts related to Atlantic opening, with some contribution from much older continental basement. Clear evidence that serpentinization of possible exhumed mantle is lacking but mantle CO₂ is likely to have been a critical factor in determining the composition of the fluids from which the carbonates formed and the high alkalinities of the lake waters.

中文翻译：

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南大西洋的地幔、二氧化碳和巨大的阿普蒂安化学成因湖相碳酸盐工厂：一些碳酸盐是制造出来的，而不是天生的

在阿普蒂安（白垩纪）期间，在现在的南大西洋，显生宙地质记录中迄今为止发现的最大的化学（非生物）碳酸盐工厂发展成为一个巨大的超碱性湖泊系统。这至少覆盖了 330 000 公里²，生产碳酸盐岩，当地厚度超过 500 m，在现在的离岸桑托斯和坎波斯盆地（巴西）以及宽扎盆地（安哥拉）。目前的证据支持这样一种观点，即几乎所有这些碳酸盐都是化学成因的，可能是通过蒸发从高碱性浅湖水中沉淀出来的。这个单元在巴西近海有最好的记录，被称为 Barra Velha 组（桑托斯盆地）和 Macabu 组（坎波斯盆地），仅由两种碱性碳酸盐组分组成，即毫米到厘米大小的晶体灌木和球晶。这些通常就地但也可以重新加工成一系列碎屑相。可证明的微生物岩通常很少见。这些碳酸盐与镁硅酸盐（作为粘土）有关，后者不仅对原位碳酸盐岩的结构发展产生深远的影响，而且对它们的成岩作用产生深远的影响。粘土的溶解在这些石灰岩中产生了大部分孔隙，这些石灰岩是数十亿桶油田的主体。碳酸盐的来源很可能来自基性岩的交代蚀变，例如与大西洋开放有关的大陆洪水玄武岩，还有一些来自更古老的大陆基底的贡献。清楚的证据表明可能挖掘出的地幔缺乏蛇纹石化，但地幔 CO ₂ 可能是确定形成碳酸盐的流体组成和湖水高碱度的关键因素。