SOURCES AND FRACTIONATION PROCESSES INFLUENCING THE ISOTOPIC DISTRIBUTION OF H, O AND C IN THE LONG VALLEY HYDROTHERMAL SYSTEM, CALIFORNIA, USA

The isotopic ratios of H, O and C in water within the Long Valley caldera, California reflect input from sources external to the hydrothermal reservoir. A decrease in δD in precipitation of 0.5‰ km-1, from west to east across Long Valley, is caused by the introduction of less fractionated marine moisture through a low elevation embayment in the Sierra Nevada Mountain Range. Relative to seasonal fluctuations in precipitation (-158 to -35א.), δD ranges in hot and cold surface and groundwaters are much less variable (-135 to -105א.). Only winter and spring moisture, reflecting higher precipitation rates with lighter isotopic signatures, recharge the hydrological system. The hydrothermal fluids are mixtures of isotopically heavy recharge (δD = - 115‰, δ18O = - 15‰) derived from the Mammoth embayment, and isotopically lighter cold water (δD = -135‰, δ18O = -18‰). This cold water is not representative of current local recharge. The δ13C values for dissolved carbon in hot water are significantly heavier (- 7 to - 3‰) than in cold water (-18 to -10‰) denoting a separate hydrothermal origin. These δ13C values overlie the range generally attributed to magmatic degassing of CO2. However, δ13C values of metamorphosed Paleozoic basement carbonates surrounding Long Valley fall in a similar range, indicating that hydrothermal decarbonization reactions are a probable source of CO2. The δ13C and δ18O values of secondary travertime and vein calcite indicate respective fractionation with CO2 and H2O at temperatures approximating current hydrothermal conditions. © 1990.