Trace elements in titanite: A potential tool to constrain polygenetic growth processes and timing

Abstract

The petrological information preserved in the trace element signature of titanite is a valuable complement to in situ U/Pb geochronology, and can be used to refine interpretations on the growth of this mineral. We present trace element and U/Pb isotopic compositions of titanite grains in five amphibolite- to granulite-facies samples from the east Albany-Fraser Orogen of Western Australia. Chondrite-normalised rare earth element (REE) abundance patterns discriminate between titanite populations and correlate with backscatter electron (BSE) zonation. In two samples, titanite REE composition correlates with proximity to garnet, and in one sample titanite composition correlates with gneissic compositional banding. The Dy/Yb ratio can be used as a geochemical indicator to link titanite growth to garnet growth. Titanite that crystallised in a garnet-bearing assemblage is HREE-depleted with Dy/Yb > 2, whereas titanite that crystallised in a garnet-free assemblage typically has flat HREE patterns with Dy/Yb < 2. The titanite grains investigated in this study have a wide range of Eu and LREE signatures, with no obvious correlation to mineralogy, lithology, or growth environment. Furthermore, the Th/U ratio is not uniquely diagnostic of metamorphic or magmatic titanite. For the five samples reported here, Eu, LREE and Th/U are useful to discriminate between titanite populations, but not necessarily as growth process indicators. By integrating the REE signatures with U/Pb data, the five new titanite U/Pb ages can be linked to a range of processes: magmatic titanite crystallisation, metamorphic titanite growth, garnet growth, and/or cooling through the closure temperature for Pb diffusion in titanite.