Controls on Calcite Solubility in Metamorphic and Magmatic Fluids

Abstract:

Calcite is an important hydrothermal alteration product in a wide range of environments. The role of calcite in hydrothermal alteration depends on its solubility in geologic fluids, especially H₂O. At ambient T and P, calcite solubility is low and it exhibits well-known declining, or “reverse”, solubility with rising T. However, experimental and theoretical studies show that increasing P yields higher solubility and restricts the region of reverse solubility behavior to higher temperature. At 0.2 GPa the reverse solubility region lies at T>600°C; at 0.5 GPa, >800°C. Thus, whereas calcite possesses relatively low solubility in pure H₂O in shallow hydrothermal systems (typically <10 ppm C), it is substantially more soluble at conditions of middle and lower crustal metamorphism and magmatism, reaching concentrations ≥1000 ppm. At the higher P of subduction zones, aragonite solubility in H₂O is even greater. Thus, neglecting other solubility controls, calcite precipitation is favored as crustal fluids cool and/or decompress. However, the solubility of calcite in H₂O also depends strongly on other solutes, pH, and fO₂. Sources of alkalinity decrease calcite solubility. In contrast, sources of acidity such as CO₂ and Cl increase solubility. Crustal fluids can be enriched in alkali halides such as NaCl. Calcite solubility increases with increasing salt content at a given P and T. From approximately seawater salinity to salt saturation, the fluid behaves as a dilute molten salt and calcite solubility increases as the square of the salt mole fraction regardless of the alkali (Li, Na, K, Cs) or halogen (F, Cl, Br, I) considered. Similar behavior is seen in mixed salt solutions. At lower salinities, solubility behavior is as expected in dilute electrolyte solutions. The transition from dilute electrolyte to molten salt is fundamental to the properties of crustal fluids. Reduction of carbonate species or CO₂ in the fluid to CH₄, which is common during serpentinization of peridotite, will also increase calcite solubility. Consideration of these effects can yield high CaCO₃ solubility and generate complex mobility and deposition patterns in environments ranging from the middle crust to the upper mantle.