Apatite Laser Ablation Lu-Hf Geochronology: A new approach to date mafic rocks and detrital sediments
Date
2025
Authors
Kharkongor, Melissa Bok Kharmujai
Editors
Advisors
Glorie, Stijn
Mulder, Jack
Kirkland, Christopher (Curtin University)
Mulder, Jack
Kirkland, Christopher (Curtin University)
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Abstract
The ability to accurately constrain the formation and subsequent thermal history of mafic igneous rocks is key in understanding a wide range of Earth system processes, from crust formation to thermal and tectonic evolution. Zircon has been widely used as the U-Pb geochronometer of choice, for several reasons: (1) having a high closure temperature to diffusion (~900oC), which makes it more robust in retaining primary crystallisation ages and (2) its robustness to mechanical and/or chemical weathering processes. However, zircon does not often crystallise to datable grain sizes in mafic rocks, and is, therefore, only rarely applicable to constrain the age of mafic rocks. Apatite, however, is a common accessory mineral in many igneous lithologies that can be dated with multiple radioactive decay systems. This study investigates the potential of apatite for dating mafic rocks, using the application of laser-ablation Lu-Hf and U-Pb geochronology. With a closure temperature of ca. 350-550oC for the U-Pb system and ca. 660-730oC for the Lu-Hf system, apatite provides a unique opportunity to capture the timing of both the primary magmatic history (apatite Lu-Hf) and subsequent thermal events (U-Pb). Through case studies, U-Pb and Lu-Hf geochronology was applied to mafic intrusions with different tectonic settings (eg: Brasiliano and Svecofonnian orogeny convergent setting, Sudbury impact crater etc). as well as four major Layered mafic intrusions (LMI). The case studies highlight that Lu-Hf dating of apatite is a robust method for determining primary crystallisation ages of mafic igneous rocks whereas the U-Pb system is often affected to post-magmatic processes. Pb diffusion is a key factor in U-Pb systematics, with smaller apatite crystals often yielding younger ages due to Pb diffusion, while the Lu-Hf systematics seem to not readily reset. Trace element compositions provide useful indicators to identify secondary disturbances, where if mafic apatites do not retain their mafic source composition, Lu-Hf dates record the timing of deformation or high temperature metamorphism. In this thesis, we also explore the potential of apatite laser ablation Lu-Hf dating in detrital studies. The dearth of zircon in mafic rocks intrinsically biases the detrital record towards felsic sources, while the contribution from mafic sources is underrepresented. The ability to obtain primary age constraints on apatite through the Lu-Hf method can help address this bias, as the SiO2 concentration of the host-rock exerts less control on the apatite mode in detrital systems. Through case studies from the Pilbara Craton (Western Australia) and Iran, Lu-Hf and U-Pb isotopic data was simultaneously collected within the same ablation spot. For the Pilbara study, the Lu-Hf dates reveal primary ages up to ~ 3.84 Ga—the oldest apatite yet recorded, while the U-Pb dates appear mainly reset. Sr/Y ratios indicate most Eoarchean apatite were derived from TTG type felsic sources and the felsic-to mafic source ratio remained relatively constant throughout the Archean. This indicates that the proto-crust of the Pilbara Craton had a significant early TTG component, akin to the present-day exposures. For the Cambrian Lalum Formation in central Iran, a major Lu-Hf age peak at ~630 Ma was obtained, supplemented by second-order older age peaks at ~1800 Ma and ~2500 Ma, and two apatites were dated at 3.4 Ga and an exceptionally old apparent 4.1 Ga grain. Geochemical analysis (Sr/Y, LREE) indicates mafic-ultramafic sources dominate the detrital spectra, which are likely derived from the Arabian-Nubian Shield. This study also explores correlations between Sr/Y ratios, Lu content, and age uncertainty, revealing that filtering of datasets for elevated uncertainties biases the detrital apatite record to higher Sr/Y ratios and thus directly favours less mafic sources. Ultimately, a multi-mineral and multi-method approach is favourable to reduce lithological discrimination in provenance studies.
School/Discipline
School of Physics, Chemistry and Earth Sciences
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Physics, Chemistry and Earth Sciences, 2025
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