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Carbonate has often been identified in aqueous carbonic inclusions in spodumene-bearing and other pegmatites, but its origin remains unclear. Here, the conditions at which carbonate and hydrogen carbonate can be generated from spodumene, CO2 and H2O, were studied using a hydrothermal diamond-anvil cell (HDAC) and Raman spectroscopy. In all experiments, spodumene persisted in aqueous carbonic solution up to the maximum temperature (600 to 800 °C). Heating of hydrogen carbonate/oxalate solutions produced CO2- and HCO3−-rich peralkaline fluids, which resulted in strong corrosion of spodumene (and polylithionite-trilithionite) and, in one run, formation of zabuyelite [Li2(CO3)] crystals at low temperatures. The experiments indicate that the reaction of spodumene with CO2 and H2O requires a peralkaline fluid to proceed rapidly. In addition, they show that spodumene crystallizes upon the heating of quartz, muscovite, and aqueous lithium carbonate solution. We conclude that if the aqueous fluid was rich in alkali hydrogen carbonate, zabuyelite in fluid inclusions in pegmatites can form both via a subsolidus reaction of CO2-bearing fluid inclusion with the spodumene host or by trapping a peralkaline fluid early in the evolution of simple or complex pegmatites. The results of our experimental study strengthen the conclusion that, although counterintuitive, hydrogen carbonate-rich peralkaline fluids may be involved in the evolution of peraluminous granitic pegmatites, in which peralkaline minerals are normally absent or very rare.

Melt inclusion (MI) homogenization experiments are essential for determining the pressure-volume-temperature-composition (P-V-T-X) parameters of magma systems. The hydrothermal diamond-anvil cell (HDAC) is currently the only equipment that can exert external pressure on MIs while allowing in situ observation of MI phase changes during heating. The HDAC’s pressure potentially prevents the MI diffusion that, under heating at one atmosphere, produces artificially elevated measurements of phase transition temperatures. It is important to compare the phase transition temperatures measured using HDAC at elevated external pressure with those obtained using conventional equipment at one atmosphere. Such a comparison not only helps assess the reliability of HDAC phase transition temperatures but also helps determine phase transition temperatures that are naturally occurring in MI during the natural history of cooling. In this study, we homogenized MIs hosted in quartz from the granitic porphyry in the Yixingzhai Au deposit, China, using HDAC at an elevated pressure of ~(140–230) MPa. We compared our experimental results with published data measured using a Linkam TS1500 stage at one atmosphere. The experiments show that the initial melting temperature (TIniM), total melting temperature (TTotM), and total homogenization temperature (ThTot) of the MIs are 695 ± 20, 780 ± 15, and 833 ± 17 °C, respectively. These phase transition temperatures are as much as 374 °C lower than the corresponding values measured at one atmosphere using the Linkam stage. Moreover, the temperatures measured using HDAC agree with actual values estimated using the linear extrapolation method based on correlations of MI size with phase transition temperatures measured using the Linkam stage. Based on the experimental HDAC results, we estimate that MIs in the Yixingzhai Au deposit were trapped at ~140 Ma and contained ~2 wt% H2O. These values are consistent with previously estimated emplacement pressures and H2O contents of granitic magmas in granitic porphyry-type Cu-Au deposits. Our results demonstrate that MI-homogenization experiments using HDAC at suitably elevated pressures can yield reliable naturally occurring phase transition temperatures in MI during the melt cooling process.

Ore-forming fluids enriched in alkali carbonate are commonly observed in natural melt and fluid inclusions associated with tin mineralization, particularly in granitic pegmatite. However, the roles of alkali carbonates remain unclear. Hence, to investigate the roles of alkali carbonate, herein, cassiterite (SnO2) crystallization experiments in SnO2–Li2CO3–H2O and SnO2–Na2CO3–H2O systems were conducted using a hydrothermal diamond-anvil cell. The results showed that SnO2 could dissolve into the alkali carbonate aqueous solution during heating, and long prismatic cassiterite crystals grew during the subsequent cooling stage at average rates of 0.61 × 10–6 to 8.22 × 10–6 cm/s in length and 3.40–19.07 μm3/s in volume. The mole fraction of cassiterite crystallized from the SnO2–Li2CO3–H2O system ranges from 0.03 to 0.41 mol%, which depends on the Li2CO3 content dissolved in the aqueous solution. In situ Raman analysis of the alkali carbonate-rich aqueous solution in the sample chamber suggests that the dissolution of SnO2 can be attributed to the alkaline conditions produced by hydrolysis of alkali carbonate in which Sn(OH)62− may be a potential tin-transporting species. The cassiterite crystallization conditions obtained in our SnO2–alkali carbonate–H2O systems primarily fell within the 400–850 °C and 300–850 MPa temperature and pressure ranges, respectively; furthermore, cassiterite crystallization ended in rare metal pegmatite-forming conditions. These crystallization features of cassiterite are similar to those formed in tin-mineralized granitic pegmatites. It indicates that an alkali carbonate-rich aqueous solution or hydrous melt can work as a favorable transport medium for tin and provides the necessary conditions for cassiterite crystallization in granitic pegmatite, bearing the roles in decreasing the viscosity of hydrous melts and enhancing the solubility of SnO2 in ore-forming melts or fluids. These roles of alkali carbonate can also be extended for the mineralization of other rare metals (e.g., Li and Be) in granitic pegmatite.