GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 27-2
Presentation Time: 8:20 AM


CRUZ-URIBE, Alicia M., Earth and Climate Sciences, University of Maine, 5790 Bryand Global Sciences Center, Orono, ME 04469, FEINEMAN, Maureen, Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, ZACK, Thomas, Department of Earth Sciences, University of Gothenburg, Gothenburg, 40530, Sweden and JACOB, Dorrit E., Department of Earth and Planetary Sciences, Macquarie University, BD Building 4 Research Park Drive, Level 3, CCFS, New South Wales, 2109, Australia,

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.