A Petrologist’s-Eye View of Silicic Magmatic Systems

Abstract:

Decades of petrologic, geochemical, geochronologic, and field studies, in conjunction with laboratory phase equilibria experiments and thermodynamic models have facilitated the reconstruction of the pressure-temperature-composition (P-T-X) profiles of many large silicic magmatic systems from the rock record. Radiometric dating and the more recent developments in diffusion chronometry allow these P-T-X conditions to be set in time, such that we now have the ability to look at P-T-X-t histories of individual silicic magmas. Today we envisage silicic magmatic systems as interconnected crystal-melt mush zones, which exist over 100,000’s-10,000’s of years and experience shorter-lived chemical and thermal perturbations over millennia to hours. Examples from well studies systems will be presented.

Ongoing petrologic research includes resolving the duration of a given set of P-T-X conditions to determine sequences and rates of magmatic processes, as well as relate these observations to those from other disciplines to assess the architecture of magmatic systems, magma dynamics, and causes of eruption. In particular, the relationships between the temperature, crystal content, and the eruptibility of magmatic systems are research foci, and require reconciling thermal histories implied from diffusion chronometry and mineral thermometers. In addition, crystal-scale studies reveal increasing levels of complexity in magmatic systems, with different crystals from the same magma sometimes recording different P-T-X-t trajectories that must be incorporated into models of these systems. Case studies from the Yellowstone and Taupo systems will highlight these areas of active research.

Promising future directions for petrologic research include, (1) the integration of P-T-X-t histories from a variety of silicic magmatic systems with numerical models of magma dynamics, (2) the development of databases that document the frequency of a certain set of magmatic conditions and specific eruption triggers, (3) relating magmatic processes and eruption triggers to monitoring signals at active volcanoes, and (4) exploring how magmatic variables such as tectonic setting, composition, volatile content, and reservoir size correlate with results of (2) and (3) to improve hazard assessments of active systems.