The Influence of Phase Separation on the Ca isotope Composition and Fluxes in Hydrothermal Systems

钙（Ca）是了解海洋化学史和长期地质时间尺度上全球气候的关键元素。这是因为Ca与碳循环相互作用，是大陆风化的主要成分。除了大陆径流外，洋中脊对海洋Ca浓度和同位素收支也有重要的定量意义。海水通过洋壳的热液循环变化对海洋Ca质量和同位素（δ44/40 Ca）平衡具有重要作用。热液活动导致循环海水在水-岩相互作用、含钙矿物形成和相分离过程中发生化学蚀变。在“特别优先项目SPP 1144”的子项目“CARLA”的框架内，详细研究了从Logatchev热液田（15°N/45°W）取样的热液流体中的Ca同位素比值（δ44/40 Ca）。可以证明，在高达300°C的高温下硬石膏沉淀期间，流体端元的Ca同位素组成经历分馏效应。Logatchev热液场提供了热液循环，而没有循环海水相分离的证据。然而，对于高温下检测到的Ca同位素分馏，相分离热液流体的Ca同位素组成仍有待考虑。这就是说，如果钙同位素在相分离过程中分馏，全球海洋钙同位素预算必须重新考虑。相分离导致形成低盐度蒸汽相和高盐度盐水相。后者富含金属阳离子，主要是氯络合物。实验研究和理论模型已经表明，在NaCl水溶液中，Ca主要在亚临界和超临界条件下作为稳定的氯络合物质出现。络合作用的后期变化可能会影响蒸气相和卤水相的络合敏感性Ca同位素组成。

Calcium (Ca) is a key element for the understanding of the chemical history of the ocean and for the global climate on long geological time scales. This is because Ca is interacting with the carbon cycle and is a major constituent of the continental weathering. Beside the continental runoff the mid-ocean ridges are quantitatively important concerning marine Ca concentration and isotope budget Variations of the hydrothermal circulation of seawater through oceanic crust have been recognized to play a significant role for the oceanic Calcium (Ca) mass and isotope (δ44/40Ca) balance. Hydrothermal activity leads to a chemical alteration of the circulating seawater during water-rock interaction, the formation of Ca-bearing minerals, and during phase separation. Within the framework of the subproject 'CARLA' in the 'Special Priority Program SPP1144’ Ca isotope ratios (δ44/40Ca) in hydrothermal fluids sampled from the Logatchev hydrothermal field (15°N/45°W) have been investigated in detail. It could be demonstrated that the Ca isotope composition of the fluid endmember undergoes fractionation effects during the precipitation of anhydrite at high temperatures of up to 300°C. The Logatchev hydrothermal field provides hydrothermal circulation without evidence for phase separation of the circulating seawater. However, with regard to the detected Ca isotope fractionation at high temperatures, the Ca isotopic compositions of phase separated hydrothermal fluids have still to be considered. This is if Ca isotopes become fractionated during phase separation, the global oceanic Ca isotope budget must be reconsidered. Phase separation leads to the formation of a low-salinity vapor and a high-salinity brine phase. The latter is enriched in metal cations that are mostly chloro-complexed. Experimental studies and theoretical models have shown that in aqueous NaCI solutions Ca occurs mainly as stable chloro-complexed species at sub- and supercritical conditions. Latter variation of complexation may influence the complexsensitive Ca isotope composition of the vapor and the brine phase.