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Published July 1998 | public
Journal Article

^(18)O/^(16)O 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


^(18)O/^(16)O 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-^(18)O groundmass/glass (δ^(18)O=−5 to +3) with coexisting quartz and feldspar phenocrysts having magmatic δ^(18)O values (+8.7±0.3; +7.5±0.3); and (2) the fact that these kinds of ^(18)O/^(16)O signatures correlate very well with morphological features and mapped zones of fumarolic activity. Profiles of δ^(18)O with depth in the Bishop Tuff within the fumarole area define a 40- to 50-m-thick, low-^(18)O, 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 ^(18)O/^(16)O 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-^(18)O zones. The base of the low-^(18)O 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 ^(18)O-depletions within this low-^(18)O 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 ^(18)O-signature that we have correlated with fossil fumaroles in the Bishop Tuff outflow sheet. Although not recognized as such by McConnell et al.; ^(18)O/^(16)O data from drill-hole samples from the intracaldera Bishop Tuff in Long Valley also display this characteristic 18O signature (i.e., analogous δ^(18)O-depth profiles, as well as low-^(18)O groundmass coexisting with high-^(18)O feldspar phenocrysts). This fumarolic ^(18)O/^(16)O 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).

Additional Information

© 1998 Elsevier Science B.V. Received 13 August 1997; accepted 19 December 1997. We wish to thank John Holt for helping with the field work for this study and Robert E. Criss, Wes Hildreth, and Fraser Goff for their very helpful reviews of an earlier version of this manuscript. We acknowledge the Los Angeles County Department of Water and Power for their cooperation in allowing us to collect rock samples in Owens River Gorge and the Sierra Nevada Aquatic Research Laboratory for use of their facilities. Greg Holk provided patient instruction in the use of the Caltech laser fluorination line, and Vic Nenow assisted with laboratory repairs. Discussions with Carey Gazis, Fraser Goff, Roy Bailey, John Holt, Peter Lipman, Dave Broxton, Bob Sharp, Don Burnett, Brian Hausback, Gordon Keating, Chip Lesher, Claudia Lewis, Jason Mayfield, Leah Moore, Andrei Sarna-Wojcicki, Peter Schiffman, Craig Scrivner, Michael Sheridan, Bob Smith, Joann Stock, and Ingrid Ukstins were also very helpful. Funding for this research was provided by the National Science Foundation, Grant No. 9317036.

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October 23, 2023