EPSS Colloquium winter-2016

A world in a grain of...zircon? What crystal records reveal about magma reservoirs

Upper-crustal magma reservoirs are important sites of magma mixing, crustal refining, and magma storage, and control many aspects of magmatic activity including whether magmas erupt or freeze as plutons in the crust, the chemical composition of erupted magmas or plutons, and the hazard potential for volcanoes. Crystals residing in these reservoirs represent valuable archives of the chemical and physical evolution of reservoirs and the time scales of this evolution. This presentation addresses the question of “What do crystals “see” and record about processes within the upper crust? Two general observations emerge from study of the ages of crystals, combined with crystal-scale geochemical data: 1) Patterns of isotopic and trace-element data over time in zircon crystals from a given magmatic show systematic changes consistent with extraction of melts from a long-lived (up to 100s of kyr), heterogeneous crystal mush. 2) Thermal histories of magma storage derived from crystal records also show that the vast majority of time recorded by major phases was spent in storage as a crystal mush, perhaps at near-solidus conditions. These observations are consistent with a general lack of geophysical observations of large, melt-rich bodies beneath volcanoes and suggests rapid assembly of magma bodies prior to eruption.