A new procedure for volcanic glass hydrogen isotope studies of tuffaceous sediments and its application in the North American Cordillera

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Hydrated volcanic glass δD (δDg) values of pure tuffs have been used to reconstruct paleoprecipitation δD in paleoelevation and paleoclimate studies. These hydrated glasses in tuffaceous sediments hold the potential for providing extended and continuous paleoprecipitation δDw records due to their abundance in the geologic record. However, glasses in such materials may be recycled and subjected to surface alteration, and the δDw values of the water that hydrated the glasses can vary across different depositional environments. Here we first develop a method to reduce the influence of recycled glass and surface alteration, then apply the method to the Eocene−Oligocene tuffaceous sediments in the Sage Creek basin, southwestern Montana, USA, to examine the variation of δDg in different depositional environments and test a previous interpretation of early Oligocene surface uplift in southwestern Montana. Our integration of microscopic observations of glass morphology and lab experiments of hydrofluoric acid (HF) abrasion of glasses for various durations (30 s to 30 min) shows that 10% HF abrasion for at least 5 min removes most of the recycled glass in tuffaceous sediments and does not change the δDg values and hydration water contents. Our data also show that pumice and cuspate glasses have higher δDg and water content than blocky glasses, which explains some of the δDg variations. The extended HF abrasion method was applied to 91 tuffaceous sandstone samples from floodplain and alluvial/fluvial channel facies in the Renova Formation in the Sage Creek basin. The results show that the δDg values exhibit greater variations in trunk and avulsion channels and floodplain facies than in paleosols. These variations are likely due to the mixing of waters derived from diverse elevations and sources, with differing evaporation degrees and glass types. This study underscores the significant potential of utilizing hydrated volcanic glass δDg from tuffaceous sediments for paleoclimate and paleoelevation studies. Nonetheless, it is important to assess the influence of depositional environment when interpreting such data. Paleosols are more suitable than other facies for reconstructing basinal precipitation δDw, but it is crucial to evaluate the effect of evaporation on paleosol δDg before interpretation. Additionally, the study reveals an absence of the anticipated early Oligocene negative δDg shift, calling for further investigation of this hypothesis.