V43B-3156
Numerical Simulation of Magma Reservoirs to Interpret Chrono-Chemical Signal
Thursday, 17 December 2015
Poster Hall (Moscone South)
Oscar M Lovera, University of California Los Angeles, Earth and Space Sciences, Los Angeles, CA, United States, Mark Harrison, University of California Los Angeles, Los Angeles, CA, United States, Axel K Schmitt, Universität Heidelberg, Institut für Geowissenschaften, Heidelberg, Germany, Matthew M Wielicki, UCLA-Earth & Space, Los Angeles, CA, United States and Casey R Tierney, Northern Arizona University, Flagstaff, AZ, United States
Abstract:
We have developed a 2-D finite difference thermokinetic model to describe the evolution of open-system magma reservoirs incorporating crustal assimilation, melt recharge and fractional crystallization. The model is based on a T-crystallization relationship coupled to a zircon growth model calibrated from zircon solubility and a crustal T-assimilation curve from the EC-RAFC models of Spera and Bohrson (2004). Our model takes into account the latent heat of melting and/or solidification and features temperature-dependent thermal diffusivity. Trace element abundances in the melt are calculated through conservation of mass and isotopic speciation allowing prediction of the distribution of εHf values in zircons. Applications to model the evolution of deeply emplaced large granitoids (i.e., ~25km, ~15000 km3) show that steady recharge yields a zircon population that records the full spectrum of εHf in the system whereas no recharge yields a much narrower range. . Insights gained from modeling reinforce our view that the relationship between assimilation and geothermal structure can be used to estimate past crustal thickness of convergent margins. Modeling of shallow, initially small, subvolcanic magma reservoirs (i.e., ~7 km, ~200 km3) permits insights into zircon age and compositional variability for large silicic volcanic fields and associated calderas. Thermal modeling indicates that substantial recharge is required to maintain magmatic temperatures in the core of an intrusive complex where zircon remains saturated for periods of 100’s of kiloyears. Coupled with previously developed statistical methods, zircon rim-ages predicted by the model were compared to the U-Th rim ages measured from five distinct lava domes of the Altiplano-Puna Volcanic Complex erupted between ca. 87 and 120 ka. The fitting constrains the amount of recharge to ~10-3 km3/a between the time of initial intrusion (>500 ka) and the eruption age (75-100 ka). Thus zircons may have the potential to preserve a record of the magnitude and timing of recharge events.
