18O/16O evidence for an early, short-lived (∼10 yr), fumarolic event in the Topopah Spring Tuff near the proposed high-level nuclear waste repository within Yucca Mountain, Nevada, USA

18O/16O analyses of rock samples from the Exploratory Studies Facility (ESF) and from three other locations in the vicinity of Yucca Mountain, Nevada, suggest that the uppermost 5–20 m of the 12.8 Ma Topopah Spring Tuff displays a characteristic 18O/16O pattern that is indicative of short-lived (∼10 yr), high-temperature (∼500°C), fumarolic meteoric-hydrothermal activity. Hydrothermally altered rock is characterized by 1–3-m-thick zones of 18O-depleted groundmass (down to δ18O=+5.5) located above and locally within the upper vitrophyre, as well as complementary, 2–4-m-thick, overlying zones of 18O-enriched (as high as δ18O=+13.7), mineralogically altered, groundmass. Thirty-four samples of the Topopah Spring Tuff collected below the upper vitrophyre are not hydrothermally altered and correspondingly display a more uniform set of δ18O values (δ18O=+8.6 to +11.1). Sanidine phenocrysts from fumarolically altered tuff in the upper cooling unit are petrographically pristine and also display uniform δ18O values (δ18O=+7.80±0.45), which are in isotopic equilibrium with other phenocryst phases at magmatic temperatures. The 18O/16O signature displayed by the upper part of the Topopah Spring Tuff is analogous to that produced by short-lived (10–25 yr), high-temperature (500°C), fumarolic activity in the Bishop Tuff and in the 1912 ash-flow sheet in the Valley of Ten Thousand Smokes, Alaska. The remarkable preservation of a fragile, fumarolic 18O/16O signature in the upper part of the Topopah Spring Tuff indicates that no subsequent hydrothermal alteration of this part of the tuff has occurred.