Controls on Mineralogy and Composition of Spelean Carbonates: Carlsbad Caverns, New Mexico

Abstract

Carbonate speleothems and precipitating fluids from Carlsbad Caverns, New Mexico, have been analyzed for their major and minor element and stable isotopic compositions in order to evaluate processes controlling the chemical evolution of cave water and factors determining the mineralogy and composition of cave carbonates. Chemistry and isotopic composition of fluids are determined by rates of CO₂ degassing, evaporation, and carbonate precipitation. Evaporation and calcium carbonate precipitation cause changes in the Mg/Ca ratio of fluids which, coupled with changes in CO₃ ⁼ content, control the minor element chemistry and mineralogy of the precipitating phase.

A broad range of carbonate minerals precipitate from seepage cave fluids, with calcites containing 1.5 to 12.0 mole % MgCO₃; calcite Mg contents exhibit a nonlinear dependence on fluid Mg/Ca ratio, and a linear dependence on fluid CO₃ = content, indicating dual control by both cation and anion con-centrations. Calcite-aragonite polymorphism appears to be largely controlled by elevated fluid Mg/Ca ratios. Combined water and carbonate chemical and stable isotopic data suggest that the most Mg- enriched calcites (10 to 12 mole % MgCO₃) either coprecipitate with aragonite or precipitate from waters with higher CO₂= concentrations than those precipitating aragonite. The range of calcite compositions associated with aragonite suggest that it coprecipitates with Mg-depleted calcite at low fluid CO₃= concentrations, while Mg-enriched calcites form with aragonite at high CO₃= concentrations. Hydromagnesite and huntite precipitate under conditions of extreme evaporation at elevated fluid Mg concentrations and Mg/Ca ratios. Primary dolomite precipitates from waters of moderate Mg/Ca ratio, probably from fluids undersaturated with respect to calcite and aragonite.