Melt–Fluid and Fluid–Fluid Immiscibility in a Na₂SO₄–SiO₂–H₂O System and Implications for the Formation of Rare Earth Deposits

Abstract

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 Na 2 SO 4 -SiO 2 -H 2 O system at elevated temperature, to explore the phase relationships involving both types of immiscibility. The fluid-melt immiscibility appeared first when the Na 2 SO 4 -SiO 2 -H 2 O 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.

Hunting the Datable Garnet using the LA‐ICP‐MS U‐Pb Method: Predicting Garnet U Concentration, based on Major and Minor Elements

“…1). Such a compositional variation can partly be accounted for by the varying growing conditions of minerals within the same rock unit, since metamorphic rocks typically have prolonged multistage thermal histories and a magmatic or hydrothermal system can show significant spatial heterogeneity in physicochemical conditions within a very limited area (e.g., Cui et al, 2021). For dating tasks relating to these rocks, HighUGarnet can be particularly useful for determining which garnets, or which parts of a garnet, are datable, with smaller cost implications than ICP-MS trace element analysis.…”

Magmatic‐hydrothermal Evolution and Mineralization Mechanisms of the Wangjiazhuang Cu (‐Mo) Deposit in the Zouping Volcanic Basin, Shandong Province, China: Constraints from Fluid Inclusions

“…If the volatile content is sufficient, the vapor pressure may eventually exceed the system confining pressure and the aqueous fluid will separate from the melt or parent magma. This phase separation is termed 'second boiling' (Burnham, 1979;Roedder, 1992;Cui et al, 2021). Trace amounts of metals and other elements can enter fluids.…”

“…Fluid immiscibility (or boiling) is an important mechanism leading to the precipitation of metal sulfides (Roedder, 1992;Cui et al, 2021). This process typically occurs during the cooling and decompression of hydrothermal fluids.…”