THE STRENGTHS AND PITFALLS OF USING APATITE U-PB TO DETERMINE THERMO-TECTONIC HISTORIES: A CASE STUDY FROM THE MESOARCHEAN PILBARA CRATON

Apatite is a ubiquitous mineral in many felsic and mafic rocks. With relatively fast Pb diffusion kinetics, the U-Pb system in apatite is sensitive to mid crustal temperatures (~400°C) and can therefore track exhumation into the upper crust.

As a case study we investigate the thermo-tectonic evolution of granitoid-orthogneiss domes from the Pilbara craton. Their structural relationships and map scale pattern have lead to their origin being interpreted as requiring partial convection within the crust. Theoretical models for the development of these structures make predictions for the thermal histories of granitoid-orthogneiss domes that can be tested with the U-Pb system in apatite

We analyze whole apatite grain U-Pb systematics via Thermal Ionization Mass Spectrometry, as well as depth profiling of the outermost 40 microns with a laser ablation system connected to an Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS). The whole grain apatite results reveal a protracted exhumation history over 400 Myrs that is not easily compatible with the relatively short episode of exhumation predicted by the convective model.

Additionally, we describe complications associated with obtaining meaningful U-Pb ages from apatite. We present U-Pb data with significant reverse concordance, suggesting that apatite is susceptible to U loss during low temperature (50°C) H₂O cleaning. Additionally, we demonstrate how common Pb corrections for laser ablation U-Pb data can be difficult for Mesoarchean apatite grains. Finally, we evaluate how to understand complex grain size-age relationships that do not follow expectations based on volume diffusion.

Looking forward, great opportunities exist to integrate in-situ and whole grain U-Pb apatite data sets with forward models of thermal histories, coupled with geobarometers such as Al in hornblende, to reconstruct terrane scale architecture.