TITANITE AS A PETROCHRONOMETER IN ROCKS WITH COMPLEX ZIRCON U-PB SYSTEMATICS

Titanite (CaTiSiO₅) is a common accessory mineral in many igneous and metamorphic rocks whose powerful potential as a geochronometer has been recognized for decades due to its propensity to incorporate significant amounts of U, Th, and Pb, and the pertinence of its closure temperature (>700⁰ C) to igneous and metamorphic processes. However, its full potential as a tool in deconvolving complex geologic histories is just being fully realized. Recent developments in laser-ablation split-stream inductively coupled mass spectrometry (LASS-ICPMS) enable rapid (~25 seconds/analysis) and simultaneous collection of reliable U-Pb isotopic data and elemental abundances from in-situ titanite. This method permits researchers to exploit the wide range of petrologically relevant trace elements incorporated into titanite (e.g. REEs, HFSEs) by providing: 1) a sufficient quantity of data points to calculate U-Pb isochron ages; 2) complementary elemental abundances and isotopic ratios to link ages to geologically meaningful processes; 3) a tool to assess measured ages in cogenetic phases (e.g. zircon). If the sample contains sufficient U (>10 ppm) and has % level variation in common Pb content, this method negates the need to assume a common lead composition – providing accurate U-Pb ages and a common lead composition for the sample. Here, we highlight the usefulness of titanite LASS-ICPMS petrochronology in rocks with cogenetic zircon that yield complex U-Pb data. Data were collected in UCSB’s LASS-ICPMS laboratory from titanite and zircon sourced from Proterozoic alkaline granitoids in the Central Mojave Desert. Zircon in the samples is characterized by highly discordant U-Pb data, abundant inheritance, high/variable LREE, ~800⁰C Ti-in-zircon apparent temperatures, and complete metamictization and/or recrystallization in some cases. In the same samples, titanite yields single U-Pb age populations with demonstrably magmatic REE patterns and higher (~800-925⁰ C) Zr-in-titanite temperatures. In these rocks, titanite yields far more simple and useful petrochronologic data compared to zircon, demonstrating its utility in the study of rocks that would otherwise be difficult to interpret using zircon U-Pb systematics alone.