Liquid–liquid immiscibility has crucial influences on geological processes, such as magma degassing and formation of ore deposits. Sulfate, as an important component, associates with many kinds of deposits. Two types of immiscibility, including (i) fluid–melt immiscibility between an aqueous solution and a sulfate melt, and (ii) fluid–fluid immiscibility between two aqueous fluids with different sulfate concentrations, have been identified for sulfate–water systems. In this study, we investigated the immiscibility behaviors of a sulfate- and quartz-saturated Na₂SO₄–SiO₂–H₂O system at elevated temperature, to explore the phase relationships involving both types of immiscibility. The fluid–melt immiscibility appeared first when the Na₂SO₄–SiO₂–H₂O sample was heated to ∼270°C, and then fluid–fluid immiscibility emerged while the sample was further heated to ∼450°C. At this stage, the coexistence of one water-saturated sulfate melt and two aqueous fluids with distinct sulfate concentrations was observed. The three immiscible phases remain stable over a wide pressure–temperature range, and the appearance temperature of the fluid–fluid immiscibility increases with the increased pressure. Considering that sulfate components occur extensively in carbonatite-related deposits, the fluid–fluid immiscibility can result in significant sulfate fractionation and provides implications for understanding the formation of carbonatite-related rare earth deposits.

中文翻译：

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Na2SO4-SiO2-H2O 系统中的熔体-流体和流体-流体不混溶性以及对稀土矿床形成的影响

液-液不混溶对地质过程具有重要影响，例如岩浆脱气和矿床的形成。硫酸盐作为重要的成分，与多种矿床伴生。两种类型的不混溶性，包括（i）水溶液和硫酸盐熔体之间的流体 - 熔体不混溶性，以及（ii）具有不同硫酸盐浓度的两种水性流体之间的流体 - 流体不混溶性，已被确定为硫酸盐 - 水系统。在本研究中，我们研究了硫酸盐和石英饱和的 Na ₂ SO ₄ -SiO ₂ -H ₂ O 系统在高温下的不混溶行为，以探索涉及两种不混溶类型的相关系。当 Na₂ SO ₄ -SiO ₂ -H ₂ O 样品被加热到~270°C，然后当样品被进一步加热到~450°C 时出现流体-流体不混溶。在这个阶段，观察到一种水饱和的硫酸盐熔体和两种具有不同硫酸盐浓度的水性流体共存。三个不混溶相在很宽的压力 - 温度范围内保持稳定，流体 - 流体不混溶的出现温度随着压力的增加而增加。考虑到硫酸盐组分广泛存在于与碳酸岩相关的矿床中，流体与流体的不混溶性会导致显着的硫酸盐分馏，并为理解与碳酸岩相关的稀土矿床的形成提供了启示。