Zirconium in rutile speedometry: New constraints on lower crustal cooling rates and residence temperatures
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The incorporation of zirconium into the mineral rutile (TiO2) has been both empirically and experimentally calibrated as a measure of rutile crystallization temperatures (Watson et al., 2006). This temperature sensitive system has been employed as a geothermometer with applications to a number of different geologic settings and rock types. Experimentally measured kinetics for Zr diffusion in rutile (Cherniak et al., 2007) indicate that Zr can be lost to temperature dependent diffusion, warranting further investigation of the geologic significance of calculated temperatures. Coupling diffusion kinetics with numerical solutions to the diffusion equation provides a means to forward model the time and temperature dependency of the system. Modeled results indicate a strong dependency of Zr concentration in rutile on both: 1) initial cooling rate following high-temperature metamorphism/crystallization and 2) temperature and duration of long-term geologic residence. Zr concentrations measured in rutile from lower crustal xenoliths that resided at 25-45 km depths for 2000 My, reveal Zr concentrations in the approximate grain center that are consistent with temperatures measured by independent thermometers. Forward models for Zr diffusion show that preserving a Zr record of these initial temperatures in the center of a rutile crystal with a 50 mu m radius requires rapid cooling (>300 degrees C/Ma) from magmatic/metamorphic temperatures followed by a long-term residence (2000 My) at temperatures <550 degrees C. This provides a new way to determine cooling rates between 900 and 500 degrees C and for constraining the temperature of the deep crust. Modeled temperature-time paths using combined rutile Zr and U-Pb geochronological data permit evaluation/refinement of published diffusion kinetics. Properly quantified, this system can be utilized as a high temperature geo-speedometer: a powerful tool for evaluating heat transfer rates at these very high and often unconstrained temperatures. (C) 2011 Elsevier B.V. All rights reserved.