Assessing the natural performance of felsic tuffs using the Rb-Sr and Sm-Nd systems-A study of the altered zone in the Topopah Spring Member, Paintbrush Tuff, Yucca Mountain, Nevada

Core samples of the Topopah Spring Member of the Paintbrush Tuff from drill hole UE25A#1 were analyzed for Sr and Nd isotopes and selected minor and trace elements to constrain the origin of a zcolitized zone associated with the lower vitrophyre. The Sm-Nd radiogenic isotope system was unperturbed by the alteration, but the Rb-Sr system was severely disturbed in the altered zone. The thick, densely welded high-silica rhyolite is remarkably uniform in composition with only a slight decrease in initial ⁸⁷Sr/⁸⁶Sr ratios with depth (0.7129–0.7125). Quartz latite in the upper part of the Topopah Spring has significantly lower initial Sr isotope ratios (0.7095–0.7114). Locally in the altered zone, Sr and Ca were gained in substantial amounts, and K, Rb, and Y were removed. Ti, Zr, Nb, Ba, La, Ce, Sm, and Nd were relatively immobile during the alteration. Sr added to the altered zone had an isotopic composition of 0.7119 ± 0.0002 and was probably derived from dissolution of glass in the bulk Topopah Spring (high silica rhyolite + quartz latite) and transported downward by infiltration.