Experimental calibration of the vapour–liquid phase relations and lithium isotope fractionation in the system H2O–LiCl at 400°C/20–28 MPa

Abstract

AbstractWe experimentally determined the vapour–liquid two‐phase field and the vapour–liquid fractionation of the lithium isotopes in the system H2O–LiCl along the 400°C isotherm at 20–28 MPa. The critical point of the 400°C isotherm is at 27.6 MPa/750 mmol kg−1 LiCl, i.e. at slightly lower pressure but higher salt concentration than in the H2O–NaCl system. At a given pressure, the vapour in the H2O–LiCl system has higher salt concentrations compared to the vapour in the H2O–NaCl system whereas salt concentrations in coexisting liquid are comparable in both systems. The vapour–liquid two‐phase field in the H2O–LiCl system therefore opens less with decreasing pressure. The available data indicate that adding LiCl to a predominantly H2O–NaCl fluid may notably increase salt concentrations in the vapour. The lithium isotope fractionation between vapour and liquid is only minor with a slight preference of 7Li for the liquid. The data do not allow quantifying Δ7Livapour‐liquid precisely, but they suggest Δ7Livapour‐liquid ≤ −0.5‰. Even for extreme Rayleigh fractionation in an open system the lithium isotopic signature of vapour, liquid, and bulk system will not be altered by more than 1‰. The lithium isotopes are a conservative tracer during aqueous fluid phase separation and lithium isotope fractionation observed in natural hydrothermal systems reflects the combined effects of source reservoir and fluid–mineral interactions.