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

Abstract

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, ~sO/~60 studies indicate that fumarolic activity produced a very wide range of 51so values (-0.1 to +12.6; n=32) in the groundmass adjacent to fossil fissure fumaroles. This contrasts sharply with the uniformity of 5180 in the groundmass away from fumarolic conduits (8180=+5.9 to +7.1; n=7) and in all of the feldspar phenocrysts (5180=+6.11 to +7.51 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 1'SO-enriched relative to the others (cloudy plagioclase ~i180=+8.45), and this sample also contained the most 180-enriched groundmass of any of those analyzed (~SJ'sO=+12.6). This preservation of primary magmatic 8180 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 180/160 systematics 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-hydrothermal activity occurred above the zone of intense welding in all three of these ash-flow tufts. This is particularly important, because it provides a direct linkage between the older tufts and the actual observations at the VTTS of steam chemistry, water/rock interaction, circulation geometry, flow velocities, and fumarolic temperaEditorial responsibility: T.H. Druitt tures (up to 645~The 180/160 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 (>450~ fumarolic 1sO-depletion event (groundmass ~5180=-0.1 to +4.8); and (b) a subsequent, much longer-lived, low-temperature (<150~ tSO-enrichment episode (groundmass as high as ~i180=+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 180/160 exchange probably occurred in combination with mineralogical alteration of both the groundmass and the calcium-rich portions of feldspar phenocrysts during the waning (<150~ stages of fumarolic activity (Spilde et al. t993). The slight Iso enrichment of apparently pristine, transparent feldspar phenocrysts (5180=+7.51) in one of the JsO-depleted, meteoric-hydrothermally altered fumarolic samples (whole-rock ~5180=+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 5180 value much...

Paleointensity estimates from ignimbrites: An evaluation of the Bishop Tuff

“…The products of vapor phase alteration, often occurring as precipitates in cavities or as encrusting coatings, may include magnetite as well as lower‐temperature phases such as goethite and hematite [e.g., Keith and Muffler , 1978; Keith , 1991]. Although early studies postulated that the vapor was derived from devitrification and the associated crystallization of anhydrous phases [ Sheridan , 1970], subsequent oxygen isotopic studies indicate that exchange is predominantly with meteoric water, with an initial, short‐lived high‐temperature (>500°C) alteration followed by more protracted exchange at lower temperature (<150°C) [ Holt and Taylor , 1998, 2001].…”

“…Although vapor phase alteration undoubtedly contributes to the complexities in the magnetic mineralogy of ash flows [e.g., Rosenbaum , 1993; Schlinger et al , 1991; Palmer et al , 1996], these effects are likely to be spatially limited. For example, fumarolic cracks in the 1912 ash flow at the Valley of Ten Thousand Smokes are typically narrow and can be traced only meters into the tuff and are accompanied by meter‐scale zonations from leached to unaltered tuff [ Holt and Taylor , 2001; Keith , 1991]. Similarly, devitrification [ Sheridan , 1970] and oxygen isotope depletions [ Holt and Taylor , 1998] are minimal outside of the morphological limit of fumarole mounds in the Bishop Tuff.…”

Hydrothermal Ore Deposits Record the Oxygen Isotope Composition of Meteoric Paleo‐Waters in the San Juan Volcanic Field, Colorado, USA

“…we apply here. Finally, since we estimate the total amount of fluid that had to pass through each system to produce patterns of observed alteration, our work captures a longer time scale and larger spatial scale that complements work focused on ignimbrite-hosted hydrothermal "pipes" (Holt & Taylor, 2001;Holt & Taylor, 1998;Hudak & Bindeman, 2018;Seligman et al, 2018).…”

“…Second, while these studies produced the first map‐view depictions of isotopic “bulls‐eyes,” and their counterparts in vertical cross‐sections, they did not enforce the time‐integrated system‐wide isotopic balance that we apply here. Finally, since we estimate the total amount of fluid that had to pass through each system to produce patterns of observed alteration, our work captures a longer time scale and larger spatial scale that complements work focused on ignimbrite‐hosted hydrothermal “pipes” (Holt & Taylor, 2001; Holt & Taylor, 1998; Hudak & Bindeman, 2018; Seligman et al., 2018).…”

Geomagnetic paleointensity in historical pyroclastic density currents: Testing the effects of emplacement temperature and postemplacement alteration

“…Based on oxygen isotope data of both feldspar phenocrysts and groundmass, the VTTS tuff has experienced a two‐stage fumarolic history [ Holt and Taylor , ]: relatively focused short‐lived (10–15 years), high‐temperature (>450°C) vigorous fumarolic activity followed by more widespread long term (possibly continuing to present day) low‐temperature (<150°C) vapor phase venting. The effects of the high‐temperature stage are localized, and 18 O/ 16 O evidence for interaction with high‐temperature meteoric water is found up to only a few centimeters to a few meters from fumarolic fissures [ Holt and Taylor , ]. There is typically a strong zonation from leached white, to pink and unaltered grey tuff away from the conduit over the scale of centimeters to meters.…”