ASSESSING TRACE ELEMENT (DIS)EQUILIBRIUM AND THE APPLICATION OF SINGLE ELEMENT THERMOMETERS IN METAMORPHIC ROCKS (Invited Presentation)

Application of single element thermometers, such as Zr-in-rutile and Zr-in-titanite, inherently assumes that trace element equilibrium was attained at peak thermal conditions. Here we present a comprehensive study of trace element redistribution during rutile replacement by titanite in rocks that experienced high-temperature amphibolite-facies overprinting and those that underwent low-temperature blueschist-facies overprinting from a variety of subduction-related terranes worldwide. Apparent partition coefficients were calculated for the high field strength elements Zr, Nb, Hf, and Ta in 36 rutile-titanite pairs based on in situ trace element concentrations determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We find that trace element distributions approach equilibrium partition coefficients in rocks from amphibolite-facies overprinted terranes, whereas trace element distributions did not approach equilibrium in rocks that experienced blueschist-facies overprinting. Calculated Zr-in-titanite temperatures for amphibolite-facies overprinted rocks are consistent with those reported in the literature, while Zr-in-titanite temperatures for blueschist-facies overprinting consistently overestimate temperature by 50-250 °C, suggesting that Zr does not approach equilibrium distributions during blueschist-facies overprinting. We conclude that single phase thermometers that rely upon slow-diffusing high field strength elements should not be applied to rocks equilibrated at ≤550 °C unless attainment of trace element equilibrium can be demonstrated. We also point out that it is possible to have a cooling history that is both hot and fast such that reaction rates during amphibolite-facies overprinting could outpace matrix diffusion of Zr, resulting in erroneous Zr-in-titanite temperatures.