18O/ 16O mapping and hydrogeology of a short-lived (≈10 years) fumarolic (>500°C) meteoric–hydrothermal event in the upper part of the 0.76 Ma Bishop Tuff outflow sheet, California

Abstract

18O/ 16O data from the 200-m-thick, 0.76 Ma Bishop Tuff outflow sheet provide evidence for a vigorous, short-lived (≈10 years), high-temperature, fumarolic meteoric–hydrothermal event. This is proved by: (1) the juxtaposition in the upper, partially welded Bishop Tuff of low- 18O groundmass/glass ( δ 18O=−5 to +3) with coexisting quartz and feldspar phenocrysts having magmatic δ 18O values (+8.7±0.3; +7.5±0.3); and (2) the fact that these kinds of 18O/ 16O signatures correlate very well with morphological features and mapped zones of fumarolic activity. Profiles of δ 18O with depth in the Bishop Tuff within the fumarole area define a 40- to 50-m-thick, low- 18O, stratigraphic zone that is sandwiched between the essentially unwelded near-surface portion of the tuff and an underlying, densely welded black tuff that displays magmatic 18O/ 16O values. Shallow-dipping columnar joints and other fumarolic features (i.e., subhorizontal tubular conduits and steep fissures) correlate very well with these pervasively devitrified, low- 18O zones. The base of the low- 18O zone is extremely sharp (∼3‰ per meter) and is located directly above the transition from partially welded tuff to densely welded black tuff. The observed average whole-rock 18O-depletions within this low- 18O zone are about 6–7‰, requiring meteoric water/rock ratios in excess of 0.24 in mass units. Rainfall on the surface of the tuff would not have been high enough to supply this much H 2O in the short lifetime of fumarolic activity, suggesting that some recharge must have been from groundwater flow through the upper part of the tuff, above the sloping (1°–5°) top of the impermeable lower zone. This is compatible with the observation that the fumarolic areas roughly correlate with the preeruptive regional drainage pattern. Some of this recharge may in part have been from the lake that filled Long Valley caldera, which was dammed by the Bishop Tuff up to the level of this boundary between the partially and densely welded zones (≈7000 ft, the elevation of the highest Long Valley Lake shorelines). Gazis et al. had previously shown that the 2.8-Ma intracaldera Chegem Tuff from the Caucasus Mountains exhibits exactly the same kind of 18O-signature that we have correlated with fossil fumaroles in the Bishop Tuff outflow sheet. Although not recognized as such by McConnell et al.; 18O/ 16O data from drill-hole samples from the intracaldera Bishop Tuff in Long Valley also display this characteristic 18O signature (i.e., analogous δ 18O-depth profiles, as well as low- 18O groundmass coexisting with high- 18O feldspar phenocrysts). This fumarolic 18O/ 16O signature is observed to much greater depths (≈650–750 m) in the intracaldera tuffs (≈1500 m thick) than it is in the ≈200-m-thick Bishop Tuff outflow sheet (≈80 m depth).