The presence of sulfate-rich fluids in natural magmatic hydrothermal systems and some carbonatite-related rare earth element (REE) deposits is paradoxical, because sulfate salts are known for their retrograde solubility, implying that they should be insoluble in high=temperature geofluids. Here, we show that the presence of quartz can significantly change the dissolution behavior of Na₂SO₄, leading to the formation of extremely sulfate-rich fluids (at least 42.8 wt% Na₂SO₄) at temperatures >~330 °C. The elevated Na₂SO₄ solubility results from prograde dissolution of immiscible sulfate melt, the water-saturated solidus of which decreases from ≥~450 °C in the binary Na₂SO₄-H₂O system to ~270 °C in the presence of silica. This implies that sulfate-rich fluids should be common in quartz-saturated crustal environments. Furthermore, we found that the sulfate-rich fluid is a highly effective medium for Nd mobilization. Thermodynamic modeling predicts that sulfate ions are more effective in complexing REE(III) than chloride ions. This reinforces the idea that REEs can be transported as sulfate complexes in sulfate-rich fluids, providing an alternative to the current REE transport paradigm, wherein chloride complexing accounts for REE solubility in ore fluids.

Carbonatites are the most important primary sources for the rare earth elements (REEs). While fractional crystallization of carbonate minerals results in the enrichment of volatiles, alkalis, and REEs in the remaining melts, the transition from carbonatitic magmas to hydrothermal brines remains unclear. Here, we investigated the pressure-temperature-composition (P-T-X) properties of the Na₂CO₃-H₂O system up to 700°C and 11.0 kbar using a hydrothermal diamond anvil cell and a Raman spectrometer. Our results show that Na₂CO₃becomes increasingly soluble under highP-Tconditions, leading to the disappearance of melt-fluid immiscibility and the continuous transition from Na₂CO₃melts to hydrothermal brines under deep crustal conditions. Given the abundance of Na₂CO₃in highly evolved carbonatitic systems, we suggest that the continuous melt-fluid transition in deep-seated carbonatites results in REEs being sufficiently concentrated in the brine-melts to form economic ore bodies, whereas in shallow systems, REEs preferentially partition into carbonatitic magmas over synmagmatic brines and disperse in carbonatite rocks that underwent limited fractionation.

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.

Orogenic gold deposits contribute the largest proportion of the world’s gold reserves, and the source of their ore-forming components has been recognized as the metamorphic devolatilization of metapelites or metabasites across the greenschist- to amphibolite-facies transition. However, hypozonal orogenic gold deposits represent an enigma in this context. Some of these apparently formed in higher-grade metamorphic rocks when temperatures were beyond the wet solidus of quartz-feldspar−bearing rocks; it is therefore puzzling how these fluids were generated in the source and migrated through the crust without causing partial melting. Here, we show that devolatilization of hydrated komatiites, a volumetrically significant lithological unit in Precambrian greenstone belts, is a viable model that can plausibly lead to gold mineralization at amphibolite-facies conditions. Our thermodynamic simulations indicate that subsolidus metamorphic devolatilization of komatiites at ∼700 °C (upper amphibolite facies) can unlock significant amounts of gold via dehydration of talc and chlorite. This genetic model is supported by the geochemical characteristics of, and estimated pressure-temperature (P-T) formation conditions of, hypozonal gold deposits and the intimate spatiotemporal association between hypozonal deposits and komatiites in greenstone belts. This work expands the P-T range of the metamorphic devolatilization model and enhances its robustness in explaining gold mineralization in metamorphic terranes.

Hydrothermal H₂S is an important energy source for hydrothermal ecosystems. However, it is difficult to obtain accurate hydrogen sulfide concentrations in high‐temperature hydrothermal fluids because they are highly susceptible to oxidation and compositional variability with mixing. In this study, a new in situ approach for measuring H₂S, HS⁻ and pH in hydrothermal fluids was developed and applied to the detections of Okinawa Trough hydrothermal activities. The in situ total H₂S concentrations in the Jade and Biwako fluids were determined to be 31.4 and 76.7 mmol/kg, respectively. The in situ measured pH of the Jade fluids was determined to be 6.3, which has exceeded that of a neutral fluid at a specific temperature and pressure, indicating that the pH of Jade fluids is weakly alkaline. The pH transition of hydrothermal fluids from alkaline to acidic may be attributed to the thermal decomposition of organic matter and sulfide precipitation.