The role of low-temperature 18O exchange in the isotopic evolution of deep subsurface fluids
The role of low-temperature 18O exchange in the isotopic evolution of deep subsurface fluids
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DOI：
10.1016/j.chemgeo.2020.120027
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发表时间：
2020-12
期刊：
影响因子：
3.9
通讯作者：
O. Warr;T. Giunta;T. Onstott;T. Kieft;R. Harris;D. Nisson;B. S. Lollar
中科院分区：
文献类型：
--
作者：
O. Warr;T. Giunta;T. Onstott;T. Kieft;R. Harris;D. Nisson;B. S. Lollar
世界各地的前寒武纪结晶岩石裂缝中含有高浓度溶解还原气体的Ca-Na-Cl流体，在南非、芬诺斯坎迪亚和加拿大地盾进行了最深入的研究。与地表沃茨、浅层地下沃茨、沉积盆地卤水和变质流体不同，这些Ca-Na-Cl流体的δ 18 O和δ 2 H值通常位于全球大气水线（GMWL）的左侧或上方。迄今为止，这些压裂液的大多数解释框架都集中在它们通过水-岩蚀变反应产生，该反应影响δ 18 O和δ 2 H值，导致水同位素值在GMWL以上的共变。这种蚀变过程包括硅酸盐水合作用与次生矿物的形成、辐解H2形成、与富H2气体的同位素交换或与含O和/或H矿物的同位素交换。这项研究提出了这些类型的流体的第一个编译的全球同位素数据集，将大量未发表的数据与以前发表的文献相结合，以便在全球范围内调查这些压裂液系统。重要的是，这种全球视角允许区分受后期与大气沃茨混合影响的流体，以及反映与地表水文循环水文地质隔离的宿主岩石中储存的最含盐端元的流体。大多数含盐流体占据的δ 18 O-δ 2 H空间范围比以前认识到的更有限，所有研究的前寒武纪岩石环境中的端元流体占据的δ 2 H δ 18 O同位素空间范围内，这些值没有共同变化。这些发现表明，一组共同的过程可能会定义这些最咸的端元流体在世界各地的前寒武纪设置的同位素签名-创建这些流体中可识别的共同签名，尽管在地质环境的差异。本研究确定了原始流体（与热液/变质活动相关）和寄主岩石之间的氧同位素交换的重要作用，发生在低温，低体积，水岩比超过长（Ma）的地质时间尺度。这一过程导致流体中的18 O随时间推移逐渐贫化，而δ 2 H值受影响较小。对于每个站点的具体同位素签名的断裂流体端员取决于初始热液/变质流体的组成，同位素交换率，水-岩石比，和原位停留时间。我们认为，在GMWL以上经常观察到的δ 18 O-δ 2 H的共同变化主要是由于后期压裂液端元与（古）大气降水的混合，导致同位素回归回到GMWL，端点由每个站点的当地气象气候条件定义。
Ca-Na-Cl fluids with high concentrations of dissolved reduced gases reside within fractures in crystalline Precambrian rocks around the world, and have been most intensively studied within South Africa, Fennoscandia and the Canadian Shield. In contrast to surface waters, shallow groundwaters, sedimentary basin brines and metamorphic fluids, the δ18O and δ2H values for these Ca-Na-Cl fluids typically plot to the left/above of the Global Meteoric Water Line (GMWL). To date, most interpretive frameworks for these fracture fluids have focused on their production via water-rock alteration reactions that affect both δ18O and δ2H values, resulting in co-variation of water isotope values above the GMWL. Such alteration processes include silicate hydration coupled with formation of secondary minerals, radiolytic H2formation, isotopic exchange with a H2-rich gas, or isotope exchange with O and/or H-bearing minerals. This study presents the first compiled global isotopic dataset for these types of fluids, integrating a large amount of unpublished data with the previously published literature in order to investigate these fracture fluid systems on a global scale. Importantly this global perspective allows differentiation between fluids impacted by late-stage mixing with meteoric waters, from fluids that reflect the most saline end-members stored in the host rocks in hydrogeologic isolation from the surface hydrologic cycle. The most saline fluids are shown to occupy a more restricted range of δ18O-δ2H space than previously recognised, with end-member fluids from all of the Precambrian rock settings investigated occupying a range of δ2H δ18O isotope space within which there is no co-variation in these values. These findings suggest a set of common processes may define the isotopic signatures of these most saline end-member fluids in Precambrian settings around the world – creating common signatures identifiable in these fluids, despite differences in geologic setting. This study identifies the important role of oxygen isotopic exchange between primary fluids (associated with hydrothermal/metamorphic activity) and the host rocks, taking place under low temperature, low-volume, water-rock ratios over long (Ma) geologic timescales. This process results in progressive18O depletion in the fluids over time, while δ2H values remain less affected. For each site the specific isotopic signature of the fracture fluid end-member depends on initial hydrothermal/metamorphic fluid composition, rates of isotopic exchange, water-to-rock ratios, and in-situ residence times. We suggest the often-observed co-variation in both δ18O-δ2H above the GMWL primarily results from late stage mixing of the fracture fluid end-members with (paleo)-meteoric water, resulting in isotopic regression back towards the GMWL, with end-points defined by the local meteoric-climatic conditions for each site.