The role of mantle-derived magmas in the isotopic evolution of Yellowstone's magmatic system

Injection of mantle-derived magmas into the Earth's crust provides the heat necessary to develop and maintain large silicic magmatic systems. However, the role of mantle-derived magmas in controlling the compositional evolution of large silicic systems remains poorly understood. Here we examine the role of mantle-derived magmas in the postcaldera magmatic system at Yellowstone Plateau, the youngest magmatism associated with the Yellowstone hotspot. Using microbeam techniques, we characterize the age and Hf isotope composition of single zircon crystals hosted in rhyolites from the most recent eruptive episode at Yellowstone Plateau, which produced the Central Plateau Member rhyolites. We place these zircon data into context by comparing them to new solution Hf isotope data for the Central Plateau Member glasses, Yellowstone basalts, and potential local crustal sources. Zircons in the Central Plateau Member rhyolites record a wide range of Hf isotope compositions relative to their host melts and extend from values similar to previously erupted Yellowstone rhyolites to values similar to Yellowstone basalts. Most zircons (similar to 90%) are in isotopic equilibrium with their host melt, but a significant proportion show eHf values higher than their host melt, thus providing the direct evidence that silicic derivatives of mantle-derived basalts have recharged Yellowstone's magmatic system. Mixing models confirm that the isotopic characteristics of the youngest Yellowstone rhyolites can be explained by recharge of Yellowstone's magma reservoir with silicic derivatives of underplating, mantle-derived basalts (similar to 5-10% material added by mass). This process helps drive the long-term isotopic evolution of Yellowstone's magmatic system.