Solubility of quartz in crustal fluids: experiments and general equations for salt solutions and H 2 O–CO 2 mixtures at 400–800°C and 0.1–0.9 GPa

The solubility of quartz has been measured in a wide range of salt solutions at 800°C and 0.5 GPa, and in NaCl, CaCl 2 and CsCl solutions and H 2 O–CO 2 fluids at six additional P – T conditions ranging from 400°C at 0.1 GPa to 800°C at 0.9 GPa. The experiments cover a wide range of compositions along each binary. At P – T conditions where the density of pure water is low (0.43 g cm −3 ), addition of most salts produces an enhancement of quartz solubility at low to moderate salt concentrations (salt‐in effect), although quartz solubility falls with further decrease in X H2O . At higher fluid densities (0.7 g cm −3 and greater), the salt‐in effect is generally absent, although this depends on both the cation present and the actual P – T conditions. The salt‐in effect is most readily produced by chloride salts of large monovalent cations, while CaCl 2 only produced a salt‐in effect at the most extreme conditions of high‐T and low‐P investigated (800°C at 0.2 GPa). Under most crustal conditions, the addition of common salts to aqueous fluids results in a lowering of quartz solubility relative to that in pure water (salt‐out effect). Comparing quartz solubility in different fluids by calculating X H2O on the basis that all salts are fully associated under all conditions yields higher quartz solubility in solutions of monovalent salts than in solutions of divalent salts, absolute values are also influenced by cation radius. Quartz solubility measurements have been fitted to a Setchenow‐type equation, modified to take account of the separate effects of both the lowering of X H2O and the specific effects of different salts, which are treated as arising through distinct patterns of non‐ideal behaviour, rather than the explicit formation of additional silica complexes with salt components. Quartz solubility in H 2 O–CO 2 fluids can be treated as ideal, if the solvation number of aqueous silica is taken as 3.5. For this system the solubility (molality) of quartz in the binary fluid, S is related to its solubility in pure water at the same P – T conditions, S o , by:Quartz solubility in binary salt systems (H 2 O–RCl n ) can be fitted to the relationship:where salt concentration m RCl n is expressed as molality and the exponent b has a value of 1 except under conditions where salting‐in is observed at low salt concentrations, in which case it is <1. Under most crustal conditions, the solubility of quartz in NaCl solutions is given to a good approximation by:We propose that quartz solubility in multicomponent fluids can be estimated from an extended expression, calculating X H2O based on the total fluid composition (including dissolved gasses), and adding terms for each major salt present. Our experimental results on H 2 O–NaCl–CO 2 fluids are satisfactorily predicted on this basis. An important implication of the results presented here is that there are circumstances where the migration of a fluid from one quartz‐bearing host into another, if it is accompanied by re‐equilibration through cation exchange, may lead to dissolution or precipitation of quartz even at constant P and T , with concomitant modification of the permeability structure of the deep crust.