Exploration of hydrothermal carbonate magnesium isotope signatures as tracers for continental fluid aquifers, Schwarzwald mining district, SW Germany
Exploration of hydrothermal carbonate magnesium isotope signatures as tracers for continental fluid aquifers, Schwarzwald mining district, SW Germany
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DOI：
10.1016/j.chemgeo.2015.02.009
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发表时间：
2015-04
期刊：
影响因子：
3.9
通讯作者：
B. Walter;A. Immenhauser;A. Geske;G. Markl
中科院分区：
文献类型：
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作者：
B. Walter;A. Immenhauser;A. Geske;G. Markl
镁同位素（δ 26 Mg）分馏在大陆热液区的意义尚不清楚。在这里，记录并讨论了来自德国西南部施瓦茨瓦尔德矿区的各种含水层宿主岩石和相应热液碳酸盐沉淀物的详细镁同位素数据集。这项研究的动机是研究区现有大量热液矿化数据集和对采样点的良好区域控制。根据构造、矿物学和显微测温学参数，我们在此将矿脉细分为三组：（i）二叠纪，（ii）侏罗纪-白垩纪和（iii）后白垩纪（新生代）。重点是集群ii和iii及其相应的，纹理较老的热液流体包裹体丰富，粗粒，低镁方解石和方解石-铁白云石固溶体和年轻的，振荡分区的低镁方解石和方解石-铁白云石固溶体晶体从温度在50和350 °C之间的流体沉淀。根据其δ 26 Mg比值，可以区分出三组具有特征的热液碳酸盐岩：（i）黑森林中部的侏罗纪-白垩纪脉（δ 26 Mg = − 3.38至− 0.82‰）是从含有大量沉积盖层衍生沃茨的流体中沉淀出来的。它们产生了典型的沉积碳酸盐岩的26镁亏损镁同位素特征。(ii)黑森林南部的侏罗纪-白垩纪脉状碳酸盐岩主要由基底流体沉淀而成，具有26个富镁特征（δ 26 Mg = − 1.22至+ 0.05‰）。这些流体起源的支持证据来自Sr-C-Pb同位素系统学以及辉铝土矿和闪锌矿的微量元素组成变化。Mg同位素变化受流体来源特征差异的控制，其主导作用超过矿物学差异所产生的变化（其中低Mg方解石通常（但并非在所有情况下）比方解石-铁白云石固溶体更贫26 Mg）。(iii)后白垩纪（新生代），Rhinegraben相关的静脉代表了一个复杂的构造并列不同的含水层岩性。正如预期的那样，这个阶段涵盖了从硅酸盐到碳酸盐的镁同位素特征（+0.45至-3.4 ‰）的全部范围。镁同位素数据的支持纹理分析（光学和阴极发光显微镜），电子探针，显微测温和已发表的放射性87 Sr/86 Sr同位素系统。研究结果表明，碳酸盐镁同位素比值在热液流体来源和含水层岩性的示踪方面具有重要的应用价值。
The significance of magnesium isotope (δ26Mg) fractionation in the continental hydrothermal domain is poorly explored. Here, a detailed Mg isotope dataset from various aquifer host rocks and corresponding hydrothermal carbonate precipitates from the Schwarzwald mining area in SW Germany is documented and discussed. This study is motivated by the very considerable existing data set on hydrothermal mineralizations in the study area and the excellent regional control of sampling points. Based on structural, mineralogical and microthermometrical arguments, we here subdivide three clusters of veins: (i) Permian, (ii) Jurassic–Cretaceous and (iii) post-Cretaceous (Cenozoic). The focus is on clusters ii and iii and their corresponding, texturally older hydrothermal fluid inclusion-rich, coarse-grained, low-Mg calcite and dolomite–ankerite solid solutions and younger, oscillatory zoned low-Mg calcite and dolomite–ankerite solid solution crystals precipitated from fluids with temperatures between 50 and 350 °C. In terms of their δ26Mg ratios, three characteristic groups of hydrothermal carbonates can be distinguished: (i) Jurassic–Cretaceous veins (δ26Mg = − 3.38 to − 0.82‰) in the central Schwarzwald precipitated from fluids containing a significant proportion of sedimentary cover-derived waters. They yield26Mg-depleted Mg isotope signatures typical of sedimentary carbonate lithologies. (ii) Jurassic–Cretaceous vein carbonates in the southern Schwarzwald precipitated from mainly basement-derived fluids with26Mg-enriched signatures (δ26Mg = − 1.22 to + 0.05‰). Supporting evidence for the origin of these fluids comes from Sr–C–Pb isotope systematics and trace element compositional variations of fahlore and sphalerite. The Mg isotope variations are controlled by differences in fluid source characteristics dominate over variations exerted by mineralogical differences (where low-Mg calcites are generally, but not in all cases, more depleted in26Mg compared to dolomite–ankerite solid solutions). (iii) Post-Cretaceous (Cenozoic), Rhinegraben-related veins represent a complex tectonic juxtaposition of different aquifer lithologies. As expected, this phase spans the full range from silicate to carbonate Mg isotope signatures (+ 0.45 to − 3.4‰). Magnesium isotope data are supported by textural analyses (optical and cathodoluminescence microscopy), electron microprobe, microthermometry and published radiogenic87Sr/86Sr isotope systematics. Our data are not in agreement with a significant temperature-controlled Δ26Mgfluid-calciteand no obvious relation between fluid salinity and hydrothermal carbonate δ26Mg is found. The results of this study suggest that hydrothermal carbonate magnesium isotope ratios have a significant potential as tracer of hydrothermal fluid sources and corresponding aquifer lithologies.