WHAT’S GOING ON BENEATH YELLOWSTONE? A GEOCHEMICAL PERSPECTIVE

Bob Smith and his many students and colleagues have enlightened us for over 50 years with their studies of the geophysics and geodynamics of the Yellowstone magmatic-hydrothermal system. Their thorough and comprehensive work provides a major portion of our current knowledge of this globally notable locale. Our research complements that of Smith and his group, by offering a geochemical and petrological perspective on Yellowstone that allows us to continue to refine our understanding of the system.

Yellowstone is one of Earth’s most prolific sources of carbon dioxide, similar to major volcanoes such as Kilauea and Etna. The flux ~45 kton/day and the isotopic composition of both CO₂ and He require that over half of the CO₂ comes from the mantle, via basaltic magma that intrudes the crust and transfers gas during decompression and crystallization-induced degassing. The magma flux compares with that of Kilauea. Above the basalt sits a sizeable rhyolitic magma reservoir that impedes transfer of the subjacent basalt and interacts with the overlying hydrothermal system, generating fluid-pressure transients that assist in generating Yellowstone’s continual seismicity and surface deformation.

The story of Yellowstone is also one of crustal transformation. As basalt invades the crust, it inevitably causes melting, metamorphism and hybridization with existing crustal rocks, most of which are >2-billion-year old Beartooth granites and gneisses. Evidence for the long-term transformation is clear from Ar and He isotope systematics of present-day Yellowstone gases. The mass flux of radiogenic He, produced by breakdown of radioactive U and Th in crustal rocks, cannot be supported by steady-state degassing of crustal or mantle rocks. The mass flux can only be supported if the crust, for billions of years, remained, cold, dead, and locked with respect to release of He and other gases. Thus, ⁴He accumulated within old crust due to breakdown of U and Th. Two-million years ago, the Yellowstone hotspot reached the formerly inactive craton beneath Yellowstone. The result was radical fracturing, metamorphism and hydrothermal purging of the crust of retained radiogenic gas, and hydrocarbons from Mesozoic and Tertiary organic-rich sediments. Similar histories also must have taken place further west at Heise, Picabo, etc.