V31F-02:
Using Li Diffusion to Track Thermal Histories within Single Zircon Crystals
Wednesday, 17 December 2014: 8:15 AM
Allison E Rubin1, Kari M Cooper1, Adam JR Kent2, Fidel Costa Rodriguez3 and Christy B. Till4, (1)University of California Davis, Earth and Planetary Sciences, Davis, CA, United States, (2)Oregon State University, College of Earth, Ocean and Atmospheric Sciences, Corvallis, OR, United States, (3)Nanyang Technological University, Earth Observatory of Singapore, Singapore, Singapore, (4)Arizona State University, School of Earth & Space Exploration, Tempe, AZ, United States
Abstract:
Large silicic volcanic centers produce the most catastrophic eruptions on earth, but the processes and timescales of melt generation, storage, and extraction within these systems are not well known. Useful methods for constraining these timescales include 1) U-series dating of single zircons or bulk major minerals, which reveals the time at which a given mineral crystallized, and 2) diffusion profiles, which record the amount of time a single crystal spent above a given temperature prior to eruption. Recent work has directly linked these two techniques in plagioclase by combining diffusion profiles in single crystals with U-series dating of a bulk separate (Cooper and Kent, 2014, Nature v 506). In order to link timescales of crystallization (and thus residence) and diffusion within individual crystals in a large silicic system, we present in situ U-Th ages and diffusion profiles of Li in zircon from the most recent rhyolitic eruption of the Okataina Volcanic Center (OVC), part of the Taupo Volcanic Zone in New Zealand. Li has been experimentally measured to diffuse across distances measureable by NanoSIMS (i.e., microns-submicron) on timescales relevant to magma storage at magmatic temperatures (Cherniak and Watson, 2010, Contrib Mineral Petrol v 160). Combining analyses of Li along NanoSIMS traverses in zircon with surface and interior SHRIMP-RG U-Th age spots reveals that long-lived zircons (with total residence times of up to 100 ka) preserve Li profiles reflecting relatively brief amounts of time (less than hundreds of years) spent at supersolidus conditions. These traverses suggest that only a few percent of the total lifetime of each crystal could have been spent at supersolidus (700-900°C) temperatures. These findings support other data suggesting that small-volume rhyolitic magmas within the OVC may have spent significant proportions of their pre-eruptive existences at near- or subsolidus conditions (e.g., Storm et al., 2012, Contrib Mineral Petrol v 163).
