Determination of regional fluid flow and gas flux rates for a continental plateau using cosmogenic and radiogenic noble gas isotopes

利用宇宙成因和放射性成因的惰性气体同位素确定大陆高原的区域流体流量和气体通量托马斯基夫特，Tullis Onstt普林斯顿大学新墨西哥州采矿与技术研究所的Thomas Kieft直到最近，量化区域尺度地下流体流量的唯一可行方法是使用宇宙成因氯-36或放射性成因惰性气体分析测定古代地下水的年龄。然而，宇宙成因的氯-36很难在咸水地下水中定量，而放射性成因惰性气体测年依赖于太多不受约束的假设。基于氪-81的一种新的宇宙成因同位素方法由于输入率受到很好的限制而更加准确，这是由于激光原子陷阱轨迹分析的发展使之成为可能。该项目将比较南非威特沃特斯兰德盆地深部裂隙水中的宇宙成因氪-81测年数据和附近布什维尔德火成岩杂岩中的放射性惰性气体同位素数据，对于该盆地，作为正在进行的地球微生物学调查的一部分，已经存在大量的背景数据。调查人员将使用一种新型的脱气系统，从南非深层矿山的深层裂隙水中收集气体，同时防止空气污染。气体样本随后将被提纯，并被送往阿贡国家实验室进行氪-81分析。其他示踪剂，包括产生放射性的惰性气体，将用传统的质谱学方法进行量化。由地下水测年得出的地下停留时间将使：1)更好地了解与大气降水流经含有盐分流体的裂隙岩石有关的物理和地球化学过程，2)更好地校准来自大陆地壳的放射性惰性气体、非生物碳氢化合物和氢的通量，3)对储存在古气象水的氧和氢同位素中的潜在古气候记录进行时间描述，4)更清楚地了解这一隆起的大陆高原的区域尺度流体流动，其抬升速率和冷却已在已发表的报告中确定，5)更好地限制大陆深层生物圈有机和无机碳的循环速率，6)更好地限制微生物群落的地下停留时间，具有生物地理和演化意义。这项研究是首次将氪-81分析应用于具有大陆深部地壳特征的裂隙岩石流体系统。放射成因和宇宙成因惰性气体分析的结合将改善我们对这些系统中流体流动、气体扩散、元素生物地球化学循环和微生物演化速度的估计。这项研究还将更好地约束威特沃特斯兰德盆地旧矿山基础设施的水收支和水通量，这反过来又将为模拟这些深井的洪水提供科学依据，这导致今天酸性矿山条件的发展，这将成为南非未来一个日益严重的问题。

Determination of regional fluid flow and gas flux rates for a continental plateau using cosmogenic and radiogenic noble gas isotopesThomas Kieft, Tullis OnstottNew Mexico Institute of Mining and Technology, Princeton University Until recently, the only available approach for quantifying regional-scale subsurface fluid flow was by dating ancient groundwater using cosmogenic chlorine-36 or radiogenic noble gas analyses. However, cosmogenic chlorine-36 is difficult to quantify in saline groundwater, and radiogenic noble gas dating relies on too many poorly constrained assumptions. A new cosmogenic isotope method based on krypton-81 is more accurate due to well constrained input rates and is made possible by developments in laser-based atom trap trace analysis. This project will compare cosmogenic krypton-81 dating to radiogenic noble gas isotope data in deep fracture water in the Witwatersrand Basin of South Africa, for which considerable background data already exist as part of ongoing geomicrobiology investigations, and in the nearby Bushveld Igneous Complex. The investigators will collect gases from deep fracture waters sampled in deep South African mines using a novel degassing system, while preventing air contamination. Gas samples will then be purified and sent to Argonne National Laboratories for krypton-81 analysis. Other tracers, including radiogenic noble gases, will be quantified by traditional mass spectrometric methods. The subsurface residence times derived from groundwater dating will enable: 1) a better understanding of the physical and geochemical processes associated with meteoric water flow through fractured rock containing saline fluids, 2) a better calibration of the flux of radiogenic noble gases, abiogenic hydrocarbons and hydrogen from the continental crust, 3) the temporal delineation of a potential paleoclimate record stored in the oxygen and hydrogen isotopes of the paleometeoric water, 4) a clearer understanding of regional scale fluid flow for this elevated continental plateau for which the uplift rates and cooling have been determined in published reports, 5) better constraints on the rates of cycling of organic and inorganic carbon by the deep continental biosphere, and 6) better constraints on the subsurface residence times of microbial communities, with biogeographical and evolutionary implications.This study is the first time that krypton-81 analyses will be applied to fractured rock fluid systems that are a characteristic of deep continental crust. The combination of radiogenic and cosmogenic noble gas analyses will improve our estimates of the rates of fluid flow, gas diffusion, biogeochemical cycling of elements and microbial evolution in these systems. This study will also provide better constraints on the water budgets and fluxes in the old mine infrastructure of the Witwatersrand Basin, which in turn will provide the scientific basis for modeling the flooding of these deep mines, which is leading to the development of acid mine conditions today and which will become an increasing problem in the future for South Africa.