Solubility of quartz in crustal fluids: experiments and general equations for salt solutions and H₂O–CO₂ mixtures at 400–800°C and 0.1–0.9 GPa

Abstract

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 H 2 O. 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 H 2 O 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 H 2 O 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: log S ¼ log S o þ 3:5 log X H2O :Quartz solubility in binary salt systems (H 2 O-RCl n ) can be fitted to the relationship:where salt concentration mRCl 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 H 2 O 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 catio...

Experimental superheating of water and aqueous solutions

“…First, the solubility of quartz in the different types of trapped liquids at the synthesis conditions strongly varies. As was experimentally demonstrated (Shmulovich et al, 2006), the solubility change with salinity is strongly cation-dependent (Fig. 7).…”

“…This behavior could be expected as quartz solubility in CsCl solution is much higher than in any other salt solution (except alkaline one), up to salting-in effect ( Fig. 7 and Shmulovich et al, 2006).…”

The role of metamorphic fluids in the formation of ore deposits

“…Like shallow waters, however, the make-up of the dissolved load is not particularly 'rock-like'. Experimental studies indicate that levels of Si in silicate-saturated waters are strongly dependent on temperature and moderately dependent on pressure (Newton & Manning 2000), so under hot lower crustal conditions, concentrations can rise to over 1% (Shmulovich et al 2006), although even here the total dissolved load is likely to be much greater than this (Yardley & Graham 2002). Even under modest conditions, sufficient Si enters solution for silicate minerals to be able to segregate readily into fractures to form veins.…”

Brines at high pressure and temperature: Thermodynamic, petrologic and geochemical effects

“…6). Hypotheses explaining this behavior include changes in solvent properties (Xie and Walther, 1993;Shmulovich et al, 2001Shmulovich et al, , 2006 and complexing of salt components with silica ( Anderson and Burnham, 1967;Evans, 2007). However, as noted by Newton and Manning (2010), neither is fully satisfactory and the modeling of quartz solubility in saline solutions at deep-crustal conditions remains empirical (Fournier et al, 1982;Fournier, 1983;Shibue, 1996;Akinfiev and Diamond, 2009).…”

“…6; Shmulovich et al, 2006;Newton and Manning, 2010). At 5 kbar and 800 • C, quartz salts out at all X salt if the salt is CaCl 2 or NaCl, but exhibits initial salting in to a maximum before salting out if the salt is KCl or CsCl.…”