FRACTIONATION, RECHARGE, AND REMELTING DURING THE EVOLUTION OF RHYOLITIC MAGMATISM AT YELLOWSTONE CALDERA (Invited Presentation)

Over the last decade, important observations about the roles of remelting, fractionation, and recharge in the Yellowstone Plateau volcanic field have resulted from a number of petrologic and isotopic studies of the rhyolites composing the well-documented volcanic stratigraphy of the third and youngest volcanic cycle. All of the rhyolites erupted after the caldera-forming Lava Creek eruption at ~ 631 ka have relatively low δ¹⁸O values, confirming a role for remelting ± assimilation of hydrothermally altered and shallow wallrocks. These rhyolites are distinguished by an older (~520-480 ka) group that has greater degrees of oxygen isotope disequilibria, more crustal radiogenic isotope compositions, and crystal-chemical data indicating a greater role for remelting than for the younger (~255-75 ka) group. The radiogenic isotope compositions of the rhyolites from the younger group generally correlate with their eruption ages, with individual units showing lower ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb, and higher ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf compositions with decreasing age, which together indicate a secular shift toward a mantle-derived signature like that characterizing Yellowstone basalts. The presence of zircons with high ¹⁷⁶Hf/¹⁷⁷Hf compositions suggests that this magmatic evolution included the addition of silicic magmas originating from mantle-derived basalts. Crystallization temperatures for the younger rhyolites are ~725–775°C based on mineral geothermometry and experimental petrology. These relatively low temperatures together with ²³⁸U-²³⁰Th dates for sanidines and pyroxenes that indicate crystallization no more than a few k.y. before eruption can be explained by a caldera-scale body of near-solidus mush that periodically rapidly fractionates batches of rhyolite that are spatially separate. Many of the zircons and chevkinites occurring in the groundmass of these rhyolites yield crystallization ages up to ~ 100 k.y. before eruption, indicating that they are antecrysts recycled from their long-lived mush source. Taken together, these observations for the post-Lava Creek rhyolites suggest that the relative roles of remelting, fractionation, and recharge changed during the volcanic cycle, and that silicic mush contributed to generating the youngest group of Yellowstone rhyolites.