Zr-in-rutile thermometry in HP/UHP eclogites from Western China

Abstract

Four Zr-in-rutile thermometry calibrations are applied to eclogites from Western China. Here, we show that if rutile grows in equilibrium with Qtz and Zrn, and is isolated inside garnet, it preserves its Zr composition and does not undergo compositional change due to cation exchange with the host garnet. It thus preserves the composition for the P-T conditions of its formation and the growth zoning of the host garnet. For the HP/UHP metamorphic temperature, the Tomkins et al. (J Metamorph Geol 25:703-713, 2007) calibration yields temperatures that agree well with previous studies, whereas the other three calibrations (Zack et al. in Contrib Mineral Petrol 148:471-488, 2004; Watson et al. in Contrib Mineral Petrol 151:413-433, 2006; Ferry and Watson in Contrib Mineral Petrol in 154:429-437, 2007), which do not include a pressure correction, give systematically lower temperatures. Zr contents of rutile inclusions within garnet show systematic decrease from garnet core to rim. The rutile inclusions in garnet rims contain the lowest Zr content, similar to that in the matrix. Analyses confirm that the pressure plays a significant role in modifying the primary temperature dependence of the Zr content of rutile. Rutiles trapped in garnets are unable to re-equilibrate easily during retrogression, but those in the matrix can do so, providing retrograde P-T path information.