Lead isotope ratios of volcanic glass by laser ablation inductively-coupled plasma mass spectrometry: Application to Miocene tephra beds in Montana, USA and adjacent areas

Abstract

Recent improvements in spatial resolution and sensitivity of laser ablation inductively-coupled plasma mass spectrometry allow exploration of the tephrochronologic potential of Pb isotope ratios of volcanic glass, in particular, their use in characterization and provenance detection of distal tephra beds. Measurements have good accuracy and precision, as determined by comparison with reference materials. A test suite of samples was taken from Miocene tephra beds in Montana and SW Saskatchewan, all of which come from the Yellowstone hotspot track (YHST), but only those tephra beds of known age were used because of the need to track any temporal changes in the Pb isotope ratios. Samples of a few Quaternary tephra beds from the Yellowstone Plateau volcanic field were also included. The Pb isotope ratio analyses form three distinct clusters on a 207Pb/ 208Pb – 206Pb/ 208Pb plot with ages of 0.6–2.0 Ma, 5.5–7.8 Ma, and 7.4–11.1 Ma. The same Pb isotope–age clusters were reproduced when published Pb isotope information on dated rocks from the YHST is used. Rocks from the Heise volcanic field and from a core recovered between the Heise and Picabo volcanic fields all fall within the younger Miocene group, whereas rocks from the Bruneau-Jarbidge volcanic field plot in the older Miocene group. Therefore, Pb isotope ratios of glass shards do help in provenance detection of distal tephra beds – at least in the tectono-volcanic setting of the YHST – and will be a useful addition to the criteria necessary to establish a reliable Miocene tephrostratigraphic framework for Montana and adjacent areas.