V41D-07
The crystal’s view of upper-crustal magma reservoirs
Thursday, 17 December 2015: 09:30
308 (Moscone South)
Kari M Cooper1, Adam JR Kent2, Christian Huber3, Mark E Stelten4, Allison E Rubin1 and Kevin Schrecengost5, (1)University of California Davis, Earth and Planetary Sciences, Davis, CA, United States, (2)Oregon State University, College of Earth, Ocean, & Atmospheric Sciences, Corvallis, OR, United States, (3)Georgia Institute of Technology Main Campus, Earth and Atmospheric Sciences, Atlanta, GA, United States, (4)USGS Alaska Science Center, Anchorage, AK, United States, (5)University of California Davis, Davis, CA, United States
Abstract:
Upper-crustal magma reservoirs are important sites of magma mixing, crustal refining, and magma storage. Crystals residing in these reservoirs have been shown to 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? And how is that view similar or different between plutonic and volcanic records?” Three 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 system (e.g., Yellowstone, WY, and Taupo Volcanic Zone, New Zealand) can show systematic changes in the degree of heterogeneity, consistent with extraction of melts from a long-lived (up to 100s of kyr), heterogeneous crystal mush and in some cases continued crystallization and homogenization of the magma during a short period (< a few kyr) preceding eruption. 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. 3) Comparison of ages of accessory phases in both plutonic blocks and host magmas that brought them to the surface do not show a consistent relationship between the two. In some cases, zircons from plutonic blocks have age spectra much older than zircon in the host magma. In other cases, host and plutonic block zircons have similar age spectra and chemical characteristics, suggesting a closer genetic connection between the two. These observations suggest that crystals in plutonic bodies, if examined at similar spatial and temporal scales to those in volcanic rocks, would show records that are highly heterogeneous in chemistry and age on the scale of a pluton or a lobe of a pluton, but that local regions of limited chemical and age variability may be preserved as remnants of the melt extraction and amalgamation process.
