Calibration of the calcite-water oxygen-isotope geothermometer at Devils Hole, Nevada, a natural laboratory

The delta O-18 of ground water (-13.54 +/- 0.05 parts per thousand) and inorganically precipitated Holocene vein calcite (+ 14.56 +/- 0.03 %) from Devils Hole cave #2 in southcentral Nevada yield an oxygen isotopic fractionation factor between calcite and water at 33.7 degrees C of 1.02849 +/- 0.00013 (10001n alpha(calcite-water) value at 28.09 +/- 0.13). Using the commonly accepted value of partial derivative(alpha(calcite-water))/partial derivative T of -0.00020 K-1, this corresponds to a 10001n alpha(calcit-water) value at 25 degrees C of 29.80, which differs substantially from the current accepted value of 28.3. Use of previously published oxygen isotopic fractionation factors would yield a calcite precipitation temperature in Devils Hole that is 8 degrees C lower than the measured ground water temperature. Alternatively, previously published fractionation factors would yield a delta O-18 of water, from which the calcite precipitated, that is too negative by 1.5 % using a temperature of 33.7 degrees C. Several lines of evidence indicate that the geochemical environment of Devils Hole has been remarkably constant for at least 10 ka. Accordingly, a re-evaluation of calcite-water oxygen isotopic fractionation factor may be in order. Assuming the Devils Hole oxygen isotopic value of calcite-water represents thermodynamic equilibrium, many marine carbonates are precipitated with a delta O-18 value that is too low, apparently due to a kinetic isotopic fractionation that preferentially enriches O-16 in the solid carbonate over O-18, feigning oxygen isotopic equilibrium. Published by Elsevier Ltd.