Na2CO3-bearing fluids: Experimental study at 700°C and under 1, 2, and 3 kbar pressure using synthetic fluid inclusions in quartz

Abstract

Heterogeneous fluid equilibria in the second type H 2 O-Na 2 CO 3 system in the presence of SiO 2 or SiO 2 + NaAlSi 3 O 8 were studied experimentally. Phase diagrams of the second type systems are briefly described. Fluid inclusions in quartz were synthesized by healing of fractures in 1 M Na 2 CO 3 solution at 700°C and under 1, 2, and 3 kbar pressure. Some runs were carried out in the presence of albite gel. The microthermomemtric study of the synthesized inclusions showed that under experimental conditions the fluid was heterogeneous and did not remain inert with respect to quartz and albite. Some inclusions contained a glass like phase, and liquid released from this phase by heating. Having been heated, some inclusions revealed liquid immiscibility. Comparison of the water-silicate-sodium carbonate system with similar sys tems containing sodium sulfate and fluoride Kotel'nikov, 2008, 2010) shows that they have much in common. In all cases, the aqueous salt bearing fluid did not remain inert relative to the quartz under relatively low PT conditions. The inclusions entrapped in the upper heterogeneous region revealed immisci bility in the presence of vapor within a temperature range of 200 to 400°C. The solutions of various concen trations, including oversaturated solutions in the presence of solid phase, underwent recurrent heterogeniza tion. Near 400°C, vapor is either dissolved in one of immiscible liquids or absorbs this liquid. When heating progresses to higher temperature, inclusions commonly become unsealed.

Progresses in Fluid Inclusion Synthesis in Quartz

“…Under such circumstances, the effect of quartz dissolution must be assessed cautiously. Besides the effect, the possible interaction between quartz and saltwater may exert an effect on fluid equilibria, which was discussed by the pioneers as early as in their first publications [40,85] and was confirmed by the observation of glasslike phase in some of the synthetic fluid inclusions [30]. Quartz probably does not remain inert with respect to salt-water system under certain conditions.…”

“…At low temperatures (T < ~800 °C)，the quartz solubility is relatively small and thus, the effect on the fluid could be ignored. However, the solubility of quartz increases drastically at higher temperature (T > ~800 °C), which would become even higher in chloride-bearing solutions than in pure water [30,40,110,111]. In fact, numerous kinds of geofluids in nature are much closer to chloridebearing solutions.…”

“…Quartz probably does not remain inert with respect to salt-water system under certain conditions. [30,36,43,44,47]. Dashed lines are the isopleths of the quartz solubility in pure water, and the values are expressed mole fraction [106].…”

“…Notes: black solid circles show the temperature and pressure conditions of the typical experiments for fluid inclusion synthesis. The related temperature and pressure data are mostly from literatures listed in Tables 1-5 and in the reference list[30,36,43,44,47]. Dashed lines are the isopleths of the quartz solubility in pure water, and the values are expressed mole fraction[106].…”

Transition from carbonatitic magmas to hydrothermal brines: Continuous dilution or fluid exsolution?

“…In contrast, the solubility of Na 2 CO 3 under vapor pressure is retrograde, decreasing from 4.2 mol/kg (30.9 wt %) at 100°C to almost 0 around the critical point of H 2 O ( 36 – 39 ). With further increase in P-T conditions, hydrous melting of Na 2 CO 3 occurs around 500°C and 1.5 kbar, resulting in the coexistence of a Na 2 CO 3 melt with an aqueous fluid (melt-fluid immiscibility) under higher P - T conditions ( 40 , 41 ). Given the much greater abundance of Na over K in carbonatitic magmas ( 2 , 7 ) and associated hydrothermal brines ( 10 , 29 , 42 ), this marked difference between the Na 2 CO 3 -H 2 O and K 2 CO 3 -H 2 O systems hints at the exsolution of synmagmatic brines being very common in the development of carbonatitic intrusions under shallow crustal conditions.…”