Development of laser ablation collision/reaction cell Lu-Hf geochronology
Files
(Thesis)
Date
2023
Authors
Simpson, Alexander David
Editors
Advisors
Glorie, Stijn
Hand, Martin
Spandler, Carl
Hand, Martin
Spandler, Carl
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Volume Title
Type:
Thesis
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Abstract
The work conducted in this thesis has led to the successful development of a micro-analytical approach to Lu-Hf geochronology. In order to achieve this, the novel method utilises inductively coupled plasma quadrupole tandem mass spectrometry ICP-Q-MS/MS (in this case and Agilent 8900 ‘QQQ’ was used), coupled to a laser ablation system. The use of a reaction gas within the reaction cell of the mass-spectrometer allows for the chemical separation of same-mass isotopes (of different elements). In the case of the Lu-Hf system, NH3 gas is introduced, which reacts differently with hafnium (Hf) compared to the Heavy Rare Earth Elements (HREEs), including lutetium (Lu) and ytterbium (Yb). In more detail, the chemical reaction between Hf and NH3 produces a reaction product that is proportional to Hf, which is the radiogenic daughter isotope in the Lu-Hf dating method. The analytical method has been optimised to promote production of this reaction product at a rate of 50 – 60 % of total Hf, allowing for sufficient sensitivity to measure radiogenic Hf isotopes in a range of minerals, including relatively Lu-poor apatite and carbonate minerals. Most importantly, this reaction product can be measured free from isobaric interferences of 176Lu and 176Yb, which forms the backbone and novelty of the developed method. In this work, it is demonstrated that the laser ablation approach allows for accurate in situ Lu-Hf ages to be rapidly and efficiently determined for a variety of minerals while maintaining the geological context in which they formed.
Garnets, which are one of the most important metamorphic minerals, have been dated using the laser-ablation approach with precisions up to ~1.5 % (2σ uncertainty, excluding Lu rich garnets from pegmatites). Given that sample preparation is trivial and that a large amount of data (>1000 analyses) can be collected in a single analytical session, this method has the potential to allow for ‘terrane scale’ age mapping of metamorphism, something that would be prohibitively difficult with previous Lu-Hf geochronological methods. Further, the spatial resolution and age precision are sufficient to distinguish core-rim age differences in polymetamorphic garnets. Consequently, the development of the laser ablation Lu-Hf method allows for a range of new studies focusing on metamorphic garnets to be pursued.
The ability to determine in situ Lu-Hf dates of apatite and calcite has also been demonstrated, with uncertainties as low as 0.6% (2σ). Both apatite and carbonate Lu-Hf dating has been applied to ore deposits, demonstrating that this technique has the potential to produce more reliable ages compared to the U-Pb system in these minerals. Consequently, Lu-Hf dating of both apatite and calcite has great potential to be used to determine the age of mineralisation, as both often form with clear petrological relationships with ore minerals.
Several potential matrix-matched reference materials have been identified, with independent constraints on crystallisation ages. Consequently, this thesis has developed LA-ICP-Q-MS/MS Lu-Hf geochronology to the point that it can be applied to solving geological problems.
School/Discipline
School of Physical Sciences
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 2023
Provenance
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