18 O/ 16 O studies of fossil fissure fumaroles from the Valley of Ten Thousand Smokes, Alaska

At three sample sites where there are good exposures of the upper 15 m of the 1912 ash-flow sheet in the Valley of Ten Thousand Smokes (VTTS), Alaska, ^(18)O/^(16)O studies indicate that fumarolic activity produced a very wide range of δ^(18)O values (−0.1 to +12.6; n=32) in the groundmass adjacent to fossil fissure fumaroles. This contrasts sharply with the uniformity of δ^(18)O in the groundmass away from fumarolic conduits (δ^(18)O=+5.9 to +7.1; n=7) and in all of the feldspar phenocrysts (δ^(18)O=+6.11 to +7.5 1 for 11 samples from this study and Hildreth 1987), independent of whether these were collected from fossil fumaroles or from unaltered tuff. Only one sample contained feldspars that were even slightly ^(18)O-enriched relative to the others (cloudy plagioclase δ^(18)O=+8.45). and this sample also contained the most ^(18)0-enriched groundmass of any of those analyzed (δ^(18)O=+12.6). This preservation of primary magmatic δ^(18)O values in the VTTS feldspar phenocrysts is clearly a consequence of the extremely short time span (i.e., 1912 to ≈1923) of vigorous, high-temperature, fumarolic activity in the 1912 ash-flow sheet. These ^(18)O/^(16)O systematica are strikingly similar to those discovered in the 2.8-Ma intracaldera Chegem Tuff (Gazis et al. 1996) and in the fossil fumaroles in the outflow sheet of the 0.76 Ma Bishop Tuff (Holt and Taylor 1998), thus confirming that a similar type of fumarolic meteoric-hydro-thermal activity occurred above the zone of intense welding in all three of these ash-flow tuffs. This is particularly important, because it provides a direct linkage between the older tuffs and the actual observations at the VTTS of steam chemistry, water/rock interaction, circulation geometry, flow velocities, and fumarolic temperatures (up to 645°C). The ^(18)O/^(l6)O effects in the VTTS can all be explained in terms of a two-stage history: (a) an early, 10- to 15-year-long, high-temperature (τ;450°C), fumarolic ^(18)O-depletion event (groundmass δ^(18)O=−0.1 to +4.8); and (b) a subsequent, much longer-lived, low-temperature (<150°C), ^(18)O-enrichment episode (groundmass as high as δ^(18)O=+12.6). Steam in these low-temperature fumaroles probably passed through various parts of the same hydrothermal system associated with the earlier, higher-temperature, fumarolic activity, and a weakened form of this low-temperature hydrothermal circulation continues to the present day (Keith et al. 1992; Lowell and Keith 1991). This low-temperature ^(18)O/^(16)O exchange probably occurred in combination with mineralogical alteration of both the groundmass and the calcium-rich portions of feldspar phenocrysts during the waning (<150°C) stages of fumarolic activity (Spilde et al. 1993). The slight ^(18)O enrichment of apparently pristine, transparent feldspar phenocrysts (δ^(18)O=+7.51) in one of the ^(18)O-depleted, meteoric-hydrothermally altered fumarolic samples (whole-rock δ^(18)O=+4.8) probably indicates that this sample was incipiently altered at low temperatures as fumarolic activity waned, and thus may have had a whole-rock δ^(18)O value much lower than +4.87‰ prior to 1923.