Oxygen isotope and trace element evidence for three-stage petrogenesis of the youngest episode (260–79 ka) of Yellowstone rhyolitic volcanism

Abstract

We report the first high-precision δ18O analyses of glass, δ18O of minerals, and trace element concentrations in glass and minerals for the 260–79 ka Central Plateau Member (CPM) rhyolites of Yellowstone, a >350 km3 cumulative volume of lavas erupted inside of 630 ka Lava Creek Tuff (LCT) caldera. The glass analyses of these crystal-poor rhyolites provide direct characterization of the melt and its evolution through time. The δ18Oglass values are low and mostly homogeneous (4.5 ± 0.14 ‰) within and in between lavas that erupted in four different temporal episodes during 200 ka of CPM volcanism with a slight shift to lower δ18O in the youngest episode (Pitchstone Plateau). These values are lower than Yellowstone basalts (5.7–6 ‰), LCT (5.5 ‰), pre-, and extracaldera rhyolites (~7–8 ‰), but higher than the earliest 550–450 ka post-LCT rhyolites (1–2 ‰). The glass δ18O value is coupled with new clinopyroxene analyses and previously reported zircon analyses to calculate oxygen isotope equilibration temperatures. Clinopyroxene records >900 °C near-liquidus temperatures, while zircon records temperatures <850 °C similar to zircon saturation temperature estimates. Trace element concentrations in the same glass analyzed for oxygen isotopes show evidence for temporal decreases in Ti, Sr, Ba, and Eu—related to Fe–Ti oxide and sanidine (±quartz) crystallization control, while other trace elements remain similar or are enriched through time. The slight temporal increase in glass Zr concentrations may reflect similar or higher temperature magmas (via zircon saturation) through time, while previosuly reported temperature decreases (e.g., Ti-in-quartz) might reflect changing Ti concentrations with progressive melt evolution. Multiple analyses of glass across single samples and in profiles across lava flow surfaces document trace element heterogeneity with compatible behavior of all analyzed elements except Rb, Nb, and U. These new data provide evidence for a three-stage geochemical evolution of these most recent Yellowstone rhyolites: (1) repeated batch melting events at the base of a homogenized low-δ18O intracaldera fill resulting in liquidus rhyolite melt and a refractory residue that sequesters feldspar-compatible elements over time. This melting may be triggered by conductive "hot plate" heating by basaltic magma intruding beneath the Yellowstone caldera resulting in contact rhyolitic melt that crystallizes early clinopyroxene and/or sanidine at high temperature. (2) Heterogeneity within individual samples and across flows reflects crystallization of these melts during preeruptive storage of magma at at lower, zircon-saturated temperatures. Compatible behavior and variations of most trace elements within individual lava flows are the result of sanidine, quartz, Fe–Ti oxide, zircon, and chevkinite crystallization at this stage. (3) Internal mixing immediately prior to and/or during eruption disrupts, these compositional gradients in each parental magma body that are preserved as melt domains distributed throughout the lava flows. These results based on the most recent and best-preserved volcanic products from the Yellowstone volcanic system provide new insight into the multiple stages required to generate highly fractionated hot spot and rift-related rhyolites. Our proposed model differs from previous interpretations that extreme Sr and Ba depletion result from long-term crystallization of a single magma body—instead we suggest that punctuated batch melting events generated a sanidine-rich refractory residue and a melt source region progressively depleted in Sr and Ba.