Oxygen isotope compositions of intracaldera rocks: hydrothermal history of the Long Valley Caldera, California

Abstract

The Bishop Tuff and associated rocks filling Long Valley Caldera, California represent a case where thick and relatively uniform geologic units of known initial compositions have been subjected to a strong geothermal fluid flux within an enclosed basin. Oxygen isotopic compositions within individual components of this ‘flux plate’ are used to characterize a magmatically driven paleohydrothermal system. Oxygen isotope ratios of silicate components analyzed with a laser-probe/mass-spectrometer system clearly illustrate the isotopic heterogeneity in the hydrothermally altered felsic volcanic rocks. The alteration resulted from moderately high-temperature hydrothermal activity. Hydrothermal activity is linked to periods of post-caldera-collapse volcanism and resurgence in the central caldera. In general, 18O depletion in the Bishop Tuff resulting from exchange with hydrothermal fluids proceeds as pumice matrix sanidine quartz due to progressive resistance to alteration. Samples from the Long Valley Exploratory Well (LVEW) centered on the resurgent dome show downward increasing exchange and disequilibrium over the well's 2000-m depth. A flanking well shows the opposite pattern over 1300 m depth. Oxygen isotope isopleths along a W-E vertical cross section of the caldera reveals convective circulation upwards and outwards from the central resurgent dome. A paleotemperature maximum of ~ 350 °C in LVEW and a geothermal gradient of ~ 130 °C/km are deduced from feldspar/water oxygen isotope fractionation equations using the pumice and sanidine components of the intracaldera volcanic rocks. This is consistent with the degree of alteration and the secondary mineralogy. Present-day temperature maximum in the well is ~ 100 °C, and the geothermal gradient is 50 °C/km. Fossil hydrothermal water, trapped in fluid inclusions in hydrothermal vein quartz hosted by intracaldera rocks, has a calculated oxygen isotope composition of − 10.2%. based on chemistry and trapping temperature. This is 4%. heavier than the average value for the present-day hydrothermal water and consistent with higher temperatures and water/rock ratios. Intrusive activity was initiated in the resurgent dome area soon after caldera collapse (760 ka), but migrated toward the western margin of the caldera, the locus of Holocene volcanism. Thus, the central hydrothermal system matured and waned as the heat sources from intrusions shifted westward. Geophysical evidence of renewed intrusive activity located beneath the resurgent dome may presage a future renewal of the cycle of hydrothermal activity in the central caldera region. Although the hydrothermal activity is complex and transitory, this history is normal for a large, relatively active caldera system such as Long Valley Caldera.