He diffusion and (U-Th)/He thermochronometry of rutile

Rutile is a primary accessory mineral in many HT and HP metamorphic rocks (e.g., blueschist, eclogite, and granulite) and some plutonic rocks. In this study, we present new experimental data constraining the He diffusion kinetics of rutile. We have undertaken He diffusion experiments on compositionally well-characterized rutile from a variety of metamorphic and plutonic environments. The majority of rutile He diffusion experiments show well- behaved Arrhenius behavior characterized by activation energies of 48-51 kcal/mol and log(D0/a2) of 6.5-7.1 s-1, translating into closure temperature estimates of ~220-235°C (-dT/dt = 10°C/myr). However, several rutile diffusion experiments have shown significant deviation from linear Arrhenius behavior either during early portions of the heating schedule or during higher temperature heating steps above 500°C. Previous studies attribute this low diffusivity during early heating steps to the presence of small near-surface He diffusion domains (e.g, defects, cracks, etc.). Higher temperature deviation from well-behaved volume diffusion is irreversible and we are investigating whether this phenomenon is related to phase instability under in-vacuo heating conditions (low fO{2}). Furthermore, we are also studying potential microstructural effects, such as ilmentite exsolution lamellae and the influence of major (e.g. Sn substitution) and minor (e.g., Nb, Cr, Zr and U) element composition on diffusion kinetics. In order to demonstrate the accuracy of rutile (U-Th)/He dating, we have dated fast cooled rutile from mantle and lower-crustal xenoliths from Tertiary volcanic rocks from several locations in the western United States. These rutile (U-Th)/He ages tend to be in excellent agreement with both 40Ar/39Ar and zircon (U-Th)/He data. Very low U contents in eclogitic rutile, however, commonly require TIMS or MC-ICP-MS analysis to obtain single-grain age precision comparable to zircon (U-Th)/He dating (<10%). In conclusion, rutile is characterized by distinct He diffusion kinetics and a closure temperature of ~220-235°C. Rutile (U-Th)/He dating, therefore, offers the possibility of quantifying an important portion of the thermal history in metamorphic rocks that is often difficult to constrain, if carried out in a well- characterized petrological context.