South African crustal fracture fluids preserve paleometeoric water signatures for up to tens of millions of years
South African crustal fracture fluids preserve paleometeoric water signatures for up to tens of millions of years
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DOI：
10.1016/j.chemgeo.2018.06.011
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发表时间：
2018-08-20
期刊：
影响因子：
3.9
通讯作者：
Ballentine, Chris J.
中科院分区：
文献类型：
--
作者：
Heard, Andrew W.;Warr, Oliver;Ballentine, Chris J.
地壳中的裂缝流体可能保持数百万至数十亿年的隔离，并包含有关古水文地质学，地下微生物生命和保守成分的信息，有助于阐明早期地球的大气演化。例子包括南非卡普瓦尔克拉通（Kaapvaal craton）的流体，其中含有独立于光球生存的化学-岩石自养微生物群落，以及加拿大地盾（Canadian Shield）的数十亿年前的流体，其中保留了演化中的早期大气的同位素特征。水相的稳定同位素分析与溶解的稀有气体的同位素分析相结合，提供了对含水压裂液的时间变化历史的无与伦比的洞察。在这里，我们报告的稳定同位素和稀有气体数据的断裂流体在威特沃特斯兰德盆地和布什维尔德火成岩省系统，南非。我们确定封闭系统的放射性惰性气体的驻留时间为0.77-97万年（百万年）。开放系统的停留时间在600万年到1080万年之间。Masimong矿的一个样品的平均封闭系统停留时间为8500万年，使其成为有史以来记录的最古老的古沃茨之一。该样品中的水的三角洲（2 H）和三角洲O-18，以及先前报道的来自同一矿区的具有相似年龄的样品，需要同位素耗尽的地下水补给源。这可能反映了补给制度在一个较高的古纬度，海拔，或具有较高的降雨量，建立了几十万年前，也许类似的补给制度在现代莱索托高地。这些数据表明，地下水同位素可以提供有用的古气候信息，许多万年。
Fracture fluids in Earth's crust may remain isolated for millions to billions of years, and contain information on paleohydrogeology, subsurface microbial life, and conservative components that help elucidate the atmospheric evolution of the early Earth. Examples include fluids in the South African Kaapvaal craton which host chemo-lithoautotrophic microbial communities that survive independent of the photosphere, and billion-year-old fluids in the Canadian Shield, which preserve the Xe isotopic signature of an evolving early atmosphere. Stable isotope analyses of the aqueous phase combined with isotopic analyses of the dissolved noble gases provide unrivalled insight into the time-alteration history of aqueous fracture fluids. Here we report stable isotope and noble gas data for fracture fluids in the Witwatersrand Basin and Bushveld Igneous Province systems, South Africa. We determine closed-system radiogenic noble gas residence times of 0.77-97 million years (Myr). Open-system residence times range between 6.0 kyr and 10.8 Myr. One sample from Masimong Mine has a mean closed-system residence time of 85 Myr, making it one of oldest paleometeoric waters ever recorded. The delta(2H) and delta O-18 of water in this sample, and in previously reported samples from the same mining district that are shown to have similar ages, require an isotopically depleted source of groundwater recharge. This could reflect a recharge regime at a higher paleolatitude, elevation, or with higher rainfall, established up to tens of Myr ago, and perhaps similar to the recharge regime in the modern Lesotho Highlands. These data suggest that groundwater isotopes can provide useful paleoclimatic information for many Myr.