Internal ²³⁸U-series systematics of pumice from the November 13, 1985, eruption of Nevado del Ruiz, Colombia

High-precision mass spectrometer and alpha scintillation measurements of ²³⁸U-series nuclides were obtained for whole-rock pumice and constituent mineral and glass separates to help interpret the timing and nature of magmatic processes that led up to the 1985 eruption of Nevado del Ruiz volcano. Internal isochron diagrams for (²²⁶Ra) Ba vs. (²³⁰Th) Ba and ( ²³⁰Th ²³²Th) vs. ( ²³⁸U ²³²Th) show that data define linear arrays, indicating an average crystallization age of 6.1 ± 0.5 ka (Ra-Th isochron) and 7 ± 6 ka (Th-U isochron). Stratigraphic, petrographic, geochemical, repose time, and eruption volume data for the Holocene eruptive sequence of Nevado del Ruiz indicate that significant changes occurred in the subvolcanic magma chamber between eruptions R9 (≈8.6 ka) and R8 (≈3.0 ka). This coincides with the average crystallization age derived from the U-series data and may represent the injection of a new batch (or batches) of mantle-derived magma into the subvolcanic magma chamber.