^(18)O/^(16)O mapping and hydrogeology of a short-lived (<25 yr) fumarolic (>500°C) meteoric-hydrothermal event in the upper, partially-welded, 0.76 Ma Bishop Tuff

^(18)/^(16)O data on feldspar, quartz, glass, and whole-rock samples from the 200-m-thick Bishop Tuff outflow sheet are strikingly similar to those at Chegem caldera, Caucasus Mtns. (Gazis et al., 1996), confirming that these isotopic systematics are the result of a short-lived, high-temperature, fumarolic meteoric-hydrothermal event whose surface manifestations were like those of 1912-1925 at Valley of Ten Thousand Smokes, Alaska. This is proved by: (1) the juxtaposition in the upper, partially-welded Bishop Tuff of low-^(18)O groundmass/glass (δ^(18)O= -5 to +3) with coexisting quartz and feldspar phenocrysts having magmatic δ^(18)O values (+8.0±0.3; +7.1±0.4); and (2) the fact that these kinds of ^(18)/^(16)O signatures correlate very well with morphological features and mapped zones of fumarolic activity (Sheridan, 1970).