High-resolution numerical modeling of mechanisms and timescales of heat and volatile transfer from basalt to granitoid crust: implications for the origin of voluminous rhyolite

Abstract:

High-SiO₂ (75-77 wt% SiO₂) rhyolite is the most differentiated magma type on Earth and constitutes some of the largest explosive eruptions (100-1000’s km³) over the last 1 Myr, including those from Long Valley, CA. Most voluminous high-SiO₂ rhyolites are found in regions of continental extension, often in association with basalt (i.e., bimodal volcanism). In this study we explore the mechanisms by which heat and volatiles are transferred from the basalt to the granitoid crust to induce partial melting. Several previous studies have numerically modeled the thermal evolution of the crust as a function of basalt influx and examined how changes in basalt flux rates affect thermal profiles. Here, we build on this previous work, although our study differs in the following four ways. First, the 1-D non-homogeneous transient thermal diffusion equation is solved for temperature and melt fraction using the method of lines and adaptive 3/4 Runge-Kutta time stepping. This allows for high-spatial (1 m) resolution in injected magmas and the surrounding wall rock, tracking crystallization and partial melting, and high-temporal (< week) resolution during the time it takes for injections to drop below the solidus. Second, emphasis is placed on the random emplacement of basaltic sills within the middle crust rather than a fixed depth of emplacement. This leads to a heated region of the middle crust with a mixed lithology of pre-existing crust and newly emplaced, solidified mafic sills. Partial melting of this mixed lithology provides a mechanism for mantle-derived basalt to materially contribute to the formation of silicic melt without necessarily involving extraction of residual melt from crystallizing basaltic intrusions. Third, attention is paid to the inevitable formation of localized partial melts along the wall-rock margins of basalts within the first few hundred years of their emplacement, even when long-term (>1 kyr) ambient temperatures are sub-solidus (i.e., < 700°C). Fourth, a detailed examination is made of the mechanisms and timescales by which water is transferred from basaltic sills to surrounding crust.