T83 Geochemical Signatures of Fluid-Rock Interaction: Earth Surface Weathering to Hydrothermal Systems I

Tuesday, 27 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

ORIGIN OF SODIUM-BICARBONATE GROUNDWATERS BY SILICATE HYDROLYSIS

Paper number: T83-5
Program: Technical Sessions
Time: 2:20 PM – 2:35 PM

Authors

Abstract

Groundwaters dominated by dissolved sodium-bicarbonate represent an important end-member continental water type. Host rocks for such waters include siliciclastic aquifers, granodiorites, basalts, and anorthosites. The sources of CO2 incorporated in bicarbonate are largely thought to be microbial, although mantle CO2 has been invoked in one study. Two principal hypotheses address the sources of dissolved Na: 1) the base exchange hypothesis, where dissolution of CaCO3 by carbonic acid releases dissolved Ca which displaces Na adsorbed on clay minerals; 2) the silicate hydrolysis hypothesis, where incongruent dissolution of Na-silicate minerals produces dissolved Na and HCO3 and solid reaction products.

The Southern Hills aquifer system of Mississippi and Louisiana, USA is an excellent natural laboratory for studying the role of silicate hydrolysis in the origin and evolution of Na-HCO3 groundwaters in siliciclastic aquifers. There is a large body of water analyses, and the physical hydrogeologic setting over flow paths of 120 km is well established. The waters vary systematically in composition down dip from barely altered meteoric waters having compositions of 0.1 mM HCO3, pH = 5.5; to highly evolved waters with 3.8 mM HCO3, pH = 8.4. The down-dip waters then mix with saline waters produced by the subsurface dissolution of salt domes.

The concentration of dissolved HCO3 serves as a reaction progress variable. From HCO3 = 0.1 to 1.5 mM, in the up-gradient part of the system, there are approximately linear increases in dissolved silica, Na, and Ca with increasing HCO3, consistent with dissolution of detrital plagioclase feldspars having an average bulk composition of approximately Ab0.70An0.30 and the precipitation of kaolinite. With increases in HCO3 > 1.5 mM, the Na/H activity ratio increases significantly, but silica and Ca concentrations now decrease. The plagioclase dissolution reaction can now be described as: plagioclase + kaolinite + dissolved silica + carbonic acid yields (Ca>Na)smectite + dissolved (HCO3 + Na>>Ca). Downdip precipitation of calcite is another possible sink for Ca. Theoretical reaction path modeling calculations of incongruent silicate weathering have been presented in the literature for over 50 years. Other than this study, however, there have been few documented field examples over such a wide range of HCO3 concentrations.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-353931

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