¹⁸O/¹⁶O Studies of Short-Lived (10-25 Year), Fumarolic (>500°C) Meteoric-Hydrothermal Events in the Outflow Sheets of Ash-Flow Tuffs

Citation

Holt, Elizabeth Warner (2000) ¹⁸O/¹⁶O Studies of Short-Lived (10-25 Year), Fumarolic (>500°C) Meteoric-Hydrothermal Events in the Outflow Sheets of Ash-Flow Tuffs. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8vdb-7r67. https://resolver.caltech.edu/CaltechTHESIS:11282022-190922104

Abstract

¹⁸O/¹⁶O data from the 0.76 Ma Bishop Tuff outflow sheet provide evidence for a vigorous, short-Jived (≈10-25 years), high-temperature (400°-650°C), fumarolic meteoric-hydrothermal event immediately following eruption. This is proved by: (1) the juxtaposition in the upper, partially welded Bishop Tuff of low-¹⁸O groundmass/glass (δ¹⁸O = -5 to +3) with coexisting quartz and feldspar phenocrysts having magmatic δ¹⁸O values (+8.4 ± 0.3; +7.2 ± 0.3); and (2) the fact that these types of ¹⁸O/¹⁶O signatures correlate very well with the morphological features and mapped zones of fumarolic activity. Ten detailed δ¹⁸O-depth profiles in various parts of the Bishop Tuff outflow sheet show evidence for two types of fumarolic meteoric-hydrothermal circulation systems in the upper part of the tuff; there is a broadly based, stratigraphically bound, 20- to 40-m-thick Upper Low-¹⁸O Zone (ULZ) that straddles the contact between the Tableland Unit and the Gorges Unit, and there is also a set of localized, 40- to 80-m-thick zones in the partially welded tuff in which extremely ¹⁸O-depleted rock (δ¹⁸O as low as -6.5) characterizes a Deep Fumarolic System (DFS). This DFS is spatially associated with steep fissures, tubular conduits, and shallow-dipping columnar joints in cylindrical volumes of rock lying beneath more than 1000 fumarolic mounds scattered across the surface of the tuff. Both of these ¹⁸O-depleted zones are concentrated over areas where the underlying Densely Welded Zone (DWZ) is particularly thick, namely above pre-Bishop Tuff paleodrainages. The DWZ remained largely hot, ductile, and impermeable during fumarolic activity, which prevented the heated meteoric fluids from penetrating the densely welded tuff until it cooled sufficiently so that throughgoing vertical fractures provided access to the base of the ash-flow sheet (at which time the fumarolic activity appears to have rapidly terminated). Whole-rock δ¹⁸O values of samples collected from the surface of the 1912 ash-flow sheet at the Valley of Ten Thousand Smokes (VTTS), Alaska show analogous ¹⁸O/¹⁶O systematics and a similar range in δ¹⁸O values (-0.1 to +12.6) to surface samples collected from the Bishop Tuff outflow sheet (+2.5 to +16.7).

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