NaCaCl relations in basinal fluids

Abstract

A new mathematical transformation of Na, Ca, and Cl concentrations in numerous basinal fluids around the world produces a linear slope of unity between the mill iequivalencies of Na and Ca cations. The transformation entails a simple milliequivalent comparison between the excess Ca and the Na deficit relative to seawater reference ratios. The relevant parameters are: Ca excess = [ Ca means − ( Ca/Cl) sw Cl means ] 2 40.08 , Na deficit = [( Na/Cl) sw Cl means − Na means ] 1 22.99 , where the concentrations (in mg/L) of the ions measured (meas) in a sample are referred to those in seawater (sw), and the numerical constants convert the results to meq/L. For >800 samples from numerous fluid reservoirs, with Cl concentrations that range from approximately 1–300 g/L and host lithologies from carbonates to granites, a highly correlated regression termed the Basinal Fluid Line (BFL) is found: Ca excess = 0.967 (Na deficit) + 140.3 R = 0.981. The unit slope of the BFL indicates a net cation exchange ratio of 2 Na for 1 Ca. The excess-deficit parameters show no correlation to Mg or K. If a single predominating reaction is presumed to control the BFL, only albitization of plagioclase by 2 Na for 1 Ca exchange is plausible. The BFL offers no support for a predominating reaction involving the 1:1 exchange of Na for Ca that has also been proposed for albitization reactions, nor for the hypothesis that dolomitization produces the elevated Ca contents of basinal fluids. The BFL may incorporate the effects of other water-rock reactions provided that they involve a net exchange of 2 Na for 1 Ca in sedimentary basins. The small y-intercept of 140.3 of the BFL is generally consistent with an origination of the brines from seawater, which would plot at the origin of an excess-deficit graph. However, for regressions derived for fluids from individual basins, the y-intercepts increase with increasing salinity of their fluids, consistent with model predictions for dissolution of halite into either a seawater or freshwater parent, followed by 2 Na for 1 Ca exchange. Because the hydrosphere is dominated by seawater and the upper crust by feldspar minerals, the BFL arguably represents the overall product of cation exchange of high salinity fluids in deep continental environments.