Tracing ancient hydrogeological fracture network age and compartmentalisation using noble gases
Tracing ancient hydrogeological fracture network age and compartmentalisation using noble gases
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使用稀有气体追踪古代水文地质裂缝网络的年龄和分区
DOI：
10.1016/j.gca.2017.10.022
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发表时间：
2018
影响因子：
5
通讯作者：
Warr O
中科院分区：
文献类型：
--
作者：
Warr O
我们发现，前寒武纪结晶基底内的流体体积约占地球地下水总量（> 3000 万平方公里）的 30%。这种地下水的停留时间和科学重要性现在才受到人们的关注，在加拿大和南非发现的古代压裂液表明：（1）微生物生命已经孤立存在了数百万年； (2) 通过水-岩反应产生大量氢气和碳氢化合物； (3)保存早期大气中的稀有气体成分。稀有气体（He、Ne、Ar、Kr、Xe）丰度和同位素组成为流体平均停留时间 (MRT) 提供了主要证据。在这里，我们扩展了来自加拿大安大略省蒂明斯的 Kidd Creek 矿的稀有气体数据，这是一个形成于 2.7 Ga 的火山成因块状硫化物 (VMS) 矿床，其中在 2.4 公里深度处发现了 MRT 为 1.1-1.7 Ga 的压裂液（Holland 等人，2013 年）； 2.9公里深度的压裂液。我们在此将基德溪矿研究与在萨德伯里盆地地表以下 1.7 和 1.4 公里深度的两个矿井（矿井 1 和矿井 2）中提取的压裂液中测定的稀有气体成分进行比较，这些裂隙液是由 1.849 Ga 的陨石撞击形成的。基德溪矿的 2.9 公里样品显示了迄今为止在自由流体中观察到的最高放射性同位素比（例如 21Ne/22Ne = 0.6 和 40Ar/36Ar = 102,000），MRT 为 1.0–2.2 Ga。相比之下，重新采样的 2.4 km 流体表明，与之前的研究 (1.1–1.7 Ga) 相比，MRT (0.2–0.6 Ga) 不太古老。这与排水时注入裂缝的流体年龄分布的变化一致，最古老的端元流体的比例逐渐减少。这些流体的 129Xe/136Xe 比率证实，2.4 km 和 2.9 km 处的钻孔来自水文地质不同的系统。相比之下，萨德伯里矿场 1 号和 2 号矿场的 MRT 分别为 0.2–0.6 和 0.2–0.9 Ga。虽然与迄今为止文献中报道的几乎所有地下水相比仍然古老，但与基德溪矿相比，这些更年轻的停留时间与撞击事件造成的显着压裂一致，促进了盆地内更多的水文地质联系和流体混合。在基德溪矿和萨德伯里的所有样本中，均发现 124-128Xe 超出了现代空气值。这归因于先前在 Kidd Creek 矿发现的早期大气氙成分，但迄今为止尚未在停留时间最近为 0.2-0.6 Ga 的流体中观察到。Kidd Creek 矿的时间和空间采样也用于验证我们提出的概念模型，该模型提供了关于较小区域尺度上压裂液的分布、体积和停留时间的关键约束。
We show that fluid volumes residing within the Precambrian crystalline basement account for ca 30% of the total groundwater inventory of the Earth (> 30 million km3). The residence times and scientific importance of this groundwater are only now receiving attention with ancient fracture fluids identified in Canada and South Africa showing: (1) microbial life which has existed in isolation for millions of years; (2) significant hydrogen and hydrocarbon production via water–rock reactions; and (3) preserving noble gas components from the early atmosphere. Noble gas (He, Ne, Ar, Kr, Xe) abundance and isotopic compositions provide the primary evidence for fluid mean residence time (MRT). Here we extend the noble gas data from the Kidd Creek Mine in Timmins Ontario Canada, a volcanogenic massive sulfide (VMS) deposit formed at 2.7 Ga, in which fracture fluids with MRTs of 1.1–1.7 Ga were identified at 2.4 km depth (Holland et al., 2013); to fracture fluids at 2.9 km depth. We compare here the Kidd Creek Mine study with noble gas compositions determined in fracture fluids taken from two mines (Mine 1 & Mine 2) at 1.7 and 1.4 km depth below surface in the Sudbury Basin formed by a meteorite impact at 1.849 Ga.The 2.9 km samples at Kidd Creek Mine show the highest radiogenic isotopic ratios observed to date in free fluids (e.g.21Ne/22Ne = 0.6 and40Ar/36Ar = 102,000) and have MRTs of 1.0–2.2 Ga. In contrast, resampled 2.4 km fluids indicated a less ancient MRT (0.2–0.6 Ga) compared with the previous study (1.1–1.7 Ga). This is consistent with a change in the age distribution of fluids feeding the fractures as they drain, with a decreasing proportion of the most ancient end-member fluids.129Xe/136Xe ratios for these fluids confirm that boreholes at 2.4 km versus 2.9 km are sourced from hydrogeologically distinct systems. In contrast, results for the Sudbury mines have MRTs of 0.2–0.6 and 0.2–0.9 Ga for Mines 1 and 2 respectively. While still old compared to almost all groundwaters reported in the literature to date, these younger residence times compared to Kidd Creek Mine are consistent with significant fracturing created by the impact event, facilitating more hydrogeologic connection and mixing of fluids in the basin. In all samples from both Kidd Creek Mine and Sudbury, a124-128Xe excess is identified over modern air values. This is attributed to an early atmospheric xenon component, previously identified at Kidd Creek Mine but which has to date not been observed in fluids with a residence time as recent as 0.2–0.6 Ga. The temporal and spatial sampling at Kidd Creek Mine is also used to verify our proposed conceptual model which provides key constraints regarding distribution, volumes and residence times of fracture fluids on the smaller, regional, scale.
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