Grain size matters: Implications for element and isotopic mobility in titanite

The U-Pb isotopic signature of titanite collected across an exhumed refractory lower crustal block within the Albany-Fraser Orogen, Australia, records thermal overprints not apparent in a suite of other U-Pb chronometers. This helps to reconcile a dichotomy within the geochronological record of two adjacent zones within the orogen. The zircon U-Pb record for the older Biranup Zone preserves widespread overprinting at 1225-1140 Ma (Stage II), whereas the younger Fraser Zone records an older 1330-1260 Ma (Stage I) tectonothermal event. Titanite in the Fraser Zone also predominantly records a U-Pb age of 1299 ± 14 Ma, reflecting the interval of closure to radiogenic Pb mobility. Nonetheless, small titanite grains reveal subsequent overprinting with a mean reset age of 1205 ± 16 Ma. By contrast, titanite from metasedimentary rocks within the adjacent Biranup Zone principally record U-Pb ages of 1200-1150 Ma, interpreted as dating cooling after prolonged Stage II metamorphism. Interestingly, titanite also preserves domains with old apparent ages. These domains have a statistically significant association with lower U content and also indicate reduced Sm/Yb ratios and are interpreted to have lost U but acquired HREE (e.g. Yb) more rapidly than MREE (e.g. Sm). The old apparent ages are interpreted as artefacts of a Stage II U redistribution process, leading to unsupported radiogenic Pb. In addition, titanite grain size has a strong effect on the preservation or resetting of metamorphic U-Pb ages. Thermochronological modelling based on apparent age versus grain size relationships indicates that complete resetting of small titanite grains requires overprinting temperatures of 695-725 °C during Stage II in the Fraser Zone. This result is similar to estimates from the Biranup Zone based on phase equilibrium modelling that indicates pressures and temperatures of 6.5-8.5 kbar and 675-725 °C. An in situ U-Pb analysis strategy for titanite that targets a range of grain sizes has the potential to reveal differential resetting and place important controls on thermal history.