ZIRCON DIVERSITY AND NUMERICAL MODELING ON DIFFERENT SCALES TO UNDERSTAND MAGMA GENESIS IN AREAS OF HOT SPOT-CRUST INTERACTION WITH EMPHASIS ON YELLOWSTONE PLUME (Invited Presentation)

Recent studies involving in situ investigation and improved single-crystal U-Pb geochronologic studies of zircon and other minerals have revealed that large volumes (1000s of km³) of low and high silica rhyolites erupt as hot, 800 to >900°C near-liquidus magmas. The estimated time between formation and eruption of the magma is recently recognized to be very short with timescales of a few 1000s of years. This presents significant challenges to heat and volume mass balances and defys conventional models of magma genesis. Additional δ¹⁸O and ε_Hf heterogeneity within single zircons in magma products of several permil and many epsilon units suggests pre-eruptive batch assembly of (re)melts of diverse hydrothermally altered and unaltered Archean crusts on timescales faster then zircon diffusive solution-reprecipitation timescales of 10’s to a few 1000s of years. We attempt to integrate a series of numerical models that range from a millimeter scale zircon survivability study, through melting and mixing in medium 50-1000 m scale, to large scale (300x1000 km), fully coupled thermomechanical numerical models that investigate million-year timescales. We arrive to the conclusion that any significant surface rhyolitic magmatism requires crust-wide reworking and thermomechanical reorganization of the entire crustal column. Bimodality of volcanism is related to a series of thermomechanical reasons including the rheological contrasts near crustal boundaries. The crustal fertility plays an important role in magmatic productivity. Numerical experiments with rhyolite-rhyolite mixing suggest that compositional convection accompanies melting and results in effective homogenization of rhyolitic products on required timescales: to preserve strongly diverse in δ¹⁸O and ε_Hf zircons in partially homogenized glass and assemble large volumes of eruptable magmas.