| Abstract |
The Coso geothermal system, located on the Naval Air Weapons Station, China Lake, California, is developed in a series of Mesozoic granitic and dioritic intrusions. Oxygen isotope measurements on a suite of whole rock samples from wells 33A-7, 68-6 and 73-19 in a high temperature section of the Coso system have revealed varying degrees of 18O/16O depletion from primary igneous δ18O values of 7.5 ‰ down to δ18O values of -4.60 ‰. Near surface whole-rock δ18O values range from +3.7 to +7.6 ‰. Whole-rock δ18O values decrease with depth in all three wells analyzed, with wells 33A-7 and 68-6 having more systematic trends. Whole-rock δ18O values for 33A-7 decrease from 7.6 ‰ near the surface to a minimum of -3.0 ‰ at a depth of 2530 m, then rapidly increase to values between 4 and 5 ‰ down to the base of the well at 3295 m. In well 68-6, whole-rock δ18O values decrease from 7.6 ‰ near the surface to a minimum value of -4.6 ‰ at a depth of 1411 m. In well 73-19 18O/16O depletion is generally less (minimum δ18O = 2.4 ‰), and δ18O values decrease less systematically with increasing depth. The lowest δ18O values in these wells are interpreted to represent alteration zones with higher water/rock ratios, most likely resulting from fracture-controlled increases in rock permeability. In the zones of maximum 18O/16O depletion in wells 33A-7 and 68-6, measured Δr-w fractionations are smaller than equilibrium Δr-w fractionation factors at current temperatures, suggesting that these portions of the Coso geothermal system were hotter and/or characterized by higher permeability in the past. However in well 73-19, measured Δr-w is larger than equilibrium Δr-w at current temperature conditions and the extent of 18O/16O depletion in whole-rock samples is generally smaller than in 33A-7 and 68-6. These differences suggest that isotope exchange in well 73-19 was more limited, either from kinetic barriers or lower permeability (less fluid access) or both. |