Oxygen isotope evidence for past and present hydrothermal regimes of Long Valley caldera, California

Abstract

Whole-rock oxygen isotope compositions of cores and cuttings from Long Valley exploration wells show that the Bishop Tuff has been an important reservoir for both fossil and active geothermal systems within the caldera. The deep Clay Pit-1 and Mammoth-1 wells on the resurgent dome penetrate mildly to strongly altered Bishop Tuff with δ 18O WR values as low as −2.6% (vs V-SMOW). The idfu 44-16 well intercepts a thinner Bishop Tuff section with δ 18O WR values of +0.4 to +2.3%. in the western caldera moat, where milder and more sporadic 18O depletions occur in Tertiary volcanic rocks of the western caldera floor ( δ 18O WR = +2.2 to +6.4‰ ). Bishop Tuff samples from deeper parts of the 715 m rdo-8 (Shady Rest) well in the SW moat are also strongly depleted in 18O ( δ 18O WR = −1.5 to +0.6‰ ). Four shallow thermal gradient wells (469–715 m td drilled in the western moat did not penetrate Bishop Tuff, but Early Rhyolites from two of these holes are depleted in 18O ( δ 18O WR = −1.2 to +6.0‰ in plv-1 +4.6 to +5.3%. in mlgrap-1 ), compared to lithologic equivalents from the other two holes ( δ 18O WR = +6.3 to +8.0‰ in plv-2 and mlgrap-2 ). Whole-rock oxygen isotope profiles for the resurgent dome wells are unlike profiles calculated assuming alkali feldspar-H 2O fractionation behavior and total O-isotopic equilibration with −14.3‰ fluids at measured temperatures. The sense of this divergence implies an earlier hydrothermal episode within the central caldera driven by one or more shallow intrusions. Geochemical similarities between an intrusive granophyre at the bottom of the Clay Pit-1 well and a nearby Moat Rhyolite dome with a K/Ar cooling age of ∼0.5 Ma suggest that vigorous hydrothermal activity beneath the central resurgent dome may have occurred as much as ∼0.5 m.y. ago. Calculated and measured O-isotope profiles are similar for deep wells that penetrate the western moat of the caldera, where steep temperature gradients and low δ 18 O WR values in Early Rhyolites from plv-1 are attributed to an active hydrothermal aquifer that has descended slightly from earlier, shallower elevations. Similarly, severe 18O depletions in Bishop Tuff samples from the idfu 44-16 and rdo-8 wells reflect active convection beneath the western moat, whereas milder 18O depletions in Early Rhyolites from mlgrap-1 were apparently caused by hydrothermal alteration at lower temperatures. The O-isotope profiles imply that surface discharge within and around the resurgent dome results from shallow, eastward-directed outflow from a zone of higher enthalpy hydrothermal upflow beneath the western caldera moat. Intrusive magmatic heat source(s) are inferred to exist beneath the western moat, perhaps beneath Mammoth Mountain.