High-Resolution Numerical Modeling of Heat and Volatile Transfer from Basalt to Wall Rock: Application to the Crustal Column beneath Long Valley Caldera, CA

We present a high-resolution numerical model of the thermal evolution of the crustal column beneath Long Valley caldera, California, from which >800 km(3) rhyolite erupted over the last 2.2 Myr. We examine how randomly emplaced basaltic sills of variable thickness (10, 50, or 100 m) at various depth intervals (10-25 km) and at variable emplacement rates (5-50 m/kyr) gradually heat the crust and lead to a variably mixed crustal lithology (solidified mafic sills and preexisting granitoid). We additionally explore the time scales over which dissolved water (similar to 3 wt%) in a newly emplaced basaltic sill exsolves during crystallization and is transferred to adjacent wall rock that is undergoing partial melting. We employ a finite-difference-based technique, with variable spatial (>= 1 m to >= 10 km) and temporal (<100 and > 10(6) years) resolution, that enables dense analysis within and directly adjacent to a newly emplaced sill. Our results show that once ambient crustal temperatures reach similar to 500-600 degrees C, subsequent injections of basaltic sills lead to significant partial melting of adjacent wall rock (granitoid and solidified mafic sills) on time scales (10(1)-10(2) years) that match those of exsolution of H2O-rich fluid from basaltic sills. Large volumes of fusion (>10%) during fluid-undersaturated partial melting, combined with the preexisting occurrence of aplite dikes, facilitates the development of melt-filled fractures that exceed the critical length for self-propagation. The advection of wall-rock partial melts (with a combined mantle-derived and crustal geochemical signature) to shallower depths will alter both the thermal and compositional profile of the middle-upper crust.