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 pCO2 of 1.2 × 105 Pa. Degassing of the CO2 from the waters decreased the pH from 6.1 to 8.0, resulting in an increase of 8%‰ in δ13C values downflow in the total CO2 in solution and an increase in the ISAT 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 ISAT 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 ISAT 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.

[1]  J. Drever,et al.  The geochemistry of natural waters , 1988 .

[2]  H. Richnow,et al.  Travertine formation in Plitvice National Park, Yugoslavia: chemical versus biological control , 1987 .

[3]  R. Amundson,et al.  The chemistry and mineralogy of a CO2-rich travertine depositing spring in the California Coast Range , 1987 .

[4]  J. Herman,et al.  CO2 outgassing and calcite precipitation in Falling Spring Creek, Virginia, U.S.A. , 1987 .

[5]  I. Barnes.  Geochemistry of Birch Creek, Inyo County, California a travertine depositing creek in an arid climate , 1965 .

[6]  R. Bathurst.  Carbonate Sediments and Their Diagenesis , 1972 .

[7]  C. Panichi,et al.  Isotopic disequilibrium in travertine deposition , 1968 .

[8]  J. Dandurand,et al.  Kinetically controlled variations of major components and carbon and oxygen isotopes in a calcite-precipitating spring , 1982 .

[9]  D. Suarez,et al.  Calcite supersaturation and precipitation kinetics in the Lower Colorado River, All‐American Canal and East Highline Canal , 1983 .

[10]  M. P. Eastman,et al.  The coprecipitation of Sr2+ with calcite at 25°C and 1 atm , 1986 .

[11]  R. Jacobson,et al.  Geochemical controls on a calcite precipitating spring , 1975 .

[12]  J. Turner.  Kinetic fractionation of carbon-13 during calcium carbonate precipitation , 1982 .

[13]  I. Friedman.  Some investigations of the deposition of travertine from Hot Springs-I. The isotopic chemistry of a travertine-depositing spring , 1970 .

[14]  Samuel Epstein,et al.  REVISED CARBONATE-WATER ISOTOPIC TEMPERATURE SCALE , 1953 .

[15]  C. Hendy,et al.  The isotopic geochemistry of speleothems—I. The calculation of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as palaeoclimatic indicators , 1971 .

[16]  P. Southgate.  Cambrian skeletal halite crystals and experimental analogues , 1982 .

[17]  B. Chalmers.  Principles of Solidification , 1964 .

[18]  E. Usdowski,et al.  Partitioning of /sup 13/C and /sup 12/C on the degassing of CO/sub 2/ and the precipitation of calcite - Rayleigh-type fractionation and a kinetic model , 1985 .

[19]  J. Hoefs,et al.  Relationship between13C and18O fractionation and changes in major element composition in a recent calcite-depositing spring — A model of chemical variations with inorganic CaCO3 precipitation , 1979 .

[20]  James W. Ball,et al.  WATEQ4F - a personal computer Fortran translation of the geochemical model WATEQ2 with revised data base , 1987 .

[21]  F. W. Gilcreas,et al.  Standard methods for the examination of water and waste water. , 1966, American journal of public health and the nation's health.

[22]  J. Herman,et al.  The chemical evolution of a travertine‐depositing stream: Geochemical processes and mass transfer reactions , 1988 .

[23]  J. Herman,et al.  Calcite precipitation rates in the field; measurement and prediction for a travertine-depositing stream , 1988 .

[24]  James W. Ball,et al.  WATEQ2—A Computerized Chemical Model for Trace and Major Element Speciation and Mineral Equilibria of Natural Waters , 1979 .