Скачать или смотреть (Un)reliability of the Zr-in-titanite thermometer at low temperature (less than 600°C)

(Un)reliability of the Zr-in-titanite thermometer at low temperature (less than 600°C)
Gosselin, E., Soucy La Roche, R., and Larson, K.
Titanite is a powerful mineral to link time and temperature in pressure-temperature-time paths as it can be used both as a U-Pb geochronometer and as a thermometer. The Zr-in-titanite thermometer was calibrated over the range 600–1000 °C [1]. To test its reliability below the calibration temperature range, we investigated the age, microstructural characteristics and Zr content of titanite in a ca. 1075 Ma mangerite sample from Quebec (Canada). The sample contains a sinistral-sense, 4–8 mm thick ultramylonite that formed at the end of the late Mesoproterozoic Ottawan phase of the Grenvillian orogeny. The ultramylonite overprints an older, high-temperature metamorphic foliation. At the margins of the ultramylonite, quartz exibits dynamic recrystallization through subgrain rotation and plagioclase is characterized by kinked albite twins and undulose extinction. K-Feldspar porphyroclasts in the ultramylonite display undulose extinction, fractures with incipient bulging recrystallization and flame perthite. Chlorite and epidote in the ultramylonite indicate retrograde upper greenschist to lower amphibolite facies metamorphic conditions during shearing. These observations are consistent with a deformation temperature of ~400–500 °C. Sillimanite, cordierite and andalusite in paragneiss from the area provide a maximum pressure constraint of ~0.4 GPa during regional metamorphism, which likely predated shearing.
In the low-strain portions of the section, titanite occurs as rare, small (100–200 μm), randomly oriented lobate grains associated with magnetite and rimming ilmenite. Titanite generally exhibits homogenous lattice orientations and rare subgrain boundaries. The scarcity of titanite combined with the replacement texture are consistent with absence of igneous titanite in the mangerite. U-Pb data define lower intercept of 1016 ± 11 Ma in Tera-Wasserburg space, which we interpret as the age of metamorphic replacement of ilmenite by titanite at upper greenschist to lower amphibolite facies. In contrast, titanite in the ultramylonite is abundant, large (200–500 μm), and elongate parallel to the foliation. Grains are commonly sigmoidal with asymmetric magnetite wings consistent with a sinistral sense of shear. Titanite displays deformation twins and variation in crystal lattice orientation from core to tip paired with increased density of misorientations. Grain size, abundance, morphology, and lattice orientations in titanite in the ultramylonite are inconsistent with pre-shearing and post-shearing titanite growth. Titanite grains in the ultramylonite yield a lower intercept age of 1002 ± 10 Ma, which we interpret as the timing of shearing.
Applying the Zr-in-titanite thermometer to the syn-kinematic titanite yields temperatures of 670–765 °C, which are higher than those inferred from quartz and feldspar microstructures and qualitative metamorphic conditions (400–500 °C). This discrepancy is larger in magnitude than can be attributed to uncertainty on pressure (0.3 ± 0.2 GPa ≈ ± 22 °C), aTiO2 (0.75 ± 0.25 ≈ ± 19 °C) and thermometer calibration (± 20 °C), and to variations in titanite Zr concentrations (150–300 ppm). Using geologically plausible parameters favoring the lowest temperature results (P = 0.1 GPa; aTiO2 = 0.5), even grains with the lowest Zr content (150 ppm) yield crystallization temperatures in excess of 600 °C. Our results indicate that the Zr-in-titanite thermometer may overestimate the temperature of crystallization of titanite that formed below 600 °C. Although they do not invalidate its use within the calibration range, the possibility that the Zr content of low-temperature titanite may be equivalent to that of high-temperature titanite is a concern. We therefore recommend using the Zr-in-titanite thermometer with caution, in addition to other thermometers and microstructural observations pointing to a crystallization temperature above 600 °C.