MICROENVIRONMENTAL CONTROLS ON MINERALOGY AND HABIT OF CACO3 PRECIPITATES - AN EXAMPLE FROM AN ACTIVE TRAVERTINE SYSTEM

Analysis of water and associated carbonate precipitates from a small, warm-spring travertine system in SW Colorado, USA, provide an example of the: (i) great variability of the geochemical parameters within these dynamic systems, and (ii) significance of the microenvironment in controlling mineralogy and morphology of carbonate precipitates. Waters emerged from the springs highly charged in CO2, with an initial p(CO2) of 1.2 x 10(5) Pa. Degassing of the CO2 from the waters decreased the pH from 6.1 to 8.0, resulting in an increase of 8 parts-per-thousand in delta-C-13 values downflow in the total CO2 in solution and an increase in the I(SAT) from 2.1 to as high as 63 times supersaturation with respect to calcite. Due to changes in the stable isotopic composition of the waters downflow as well as changes in the degree of supersaturation, stable isotopic analyses range greatly from locale to locale within this small system. Near the spring vents, at relatively low I(SAT) levels, well-developed rhombohedra of calcite formed as biotically induced precipitates around diatom stalks and other algae as well as abiotic crusts. In contrast, near the distal end of the system, very high I(SAT) levels were reached and resulted in the precipitation of skeletal-dendritic crystals of calcite on copper substrates, floating rafts of laterally linked hemispheres of aragonite crystals, and bimineralic carbonate-encrusted bubbles. Microenvironmental parameters control the mineralogy and habit of these precipitates.