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

钙是理解海洋化学史和长期地质时间尺度上的全球气候的关键元素。这是因为钙与碳循环相互作用，是大陆风化的主要组成部分。除大陆径流外，大洋中脊对海洋钙浓度和同位素收支具有重要的定量意义，海水通过洋壳的热液循环变化对海洋钙(Ca)质量和同位素(δ44/40Ca)平衡起着重要作用。在水-岩相互作用、含钙矿物的形成和分相过程中，热液活动导致循环海水发生化学变化。在“特别优先计划”中的“CALA”子项目框架内，对从洛加切夫热液地区(15°N/45°W)采集的热液流体中的钙同位素比值(δ44/40Ca)进行了详细研究。结果表明，在300℃高温下硬石膏沉淀过程中，流体端元的Ca同位素组成经历了分馏作用。Logatchev热场提供了水热循环，而没有证据表明循环海水的相分离。然而，对于高温下检测到的钙同位素分馏，仍需考虑相分离热液的钙同位素组成。这就是说，如果钙同位素在相分离过程中变得分馏，全球海洋钙同位素预算必须重新考虑。相分离导致低盐度蒸汽和高盐度卤水相的形成。后者富含主要以氯络合形式存在的金属阳离子。实验研究和理论模型表明，在亚临界和超临界条件下，在NaCI水溶液中，Ca主要以稳定的氯络合形式存在。后者的络合变化可能会影响蒸气和卤水相的络合敏感的钙同位素组成。

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.