MAGMATIC THERMOMECHANICAL MODELS REVEAL THE ORIGINS OF ISOTOPIC TRENDS IN YELLOWSTONE VOLCANOES

Studies of the isotope geochemistry of the rhyolites of the Yellowstone hotspot track have revealed repeated time-dependent trends in both radiogenic and oxygen isotopes. These include evolution from ancient, Precambrian crust-like radiogenic isotope (Hf, Nd, Sr) compositions to more primitive mantle-like compositions, and the well-documented trend towards lower-δ¹⁸O values in later eruptions at each volcanic center. To provide new insights into the origins of these trends, we present a series of regional-scale, high-resolution magmatic-thermomechanical finite difference models of the basalt intrusion, basalt fractionation, and crustal melting that produce the rhyolitic magmas in a continental hotspot environment. We show that two separate trends in zircon isotope compositions observed throughout the hotspot track leading towards compositions with normal-δ¹⁸O and Precambrian-like radiogenic isotopes and toward compositions with low-δ¹⁸O compositions and juvenile radiogenic isotopes can be traced to separate magma systems which arise in the lower and upper crust, respectively. We determine that the lower crust is likely fairly refactory, with intermittent fertile zones producing the rare rhyolitic with strongly Precambrian-like (ε_Hf<-25) radiogenic isotopes that occur throughout the hotspot track. These zones melt and are depleted before the overriding upper crust, meaning that the normal-δ¹⁸O and isotopically ancient magmas are only important in the early stages of each volcanic center. Similarly, we document the trend towards lower-δ¹⁸O rhyolite, produced by both the successive hydrothermal alteration of the crust overlying the magmatic system, and the tendency for the last rhyolites to erupt in each system to be generated by melts of the upper crust.