Refining LA-ICP-MS Data Acquisition and Interpretation Methods for the Zircon Geochronometer in Tertiary Magmatic Systems
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2018-10
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Doctoral Thesis
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Abstract
Understanding the mechanisms that govern the evolution of magmatic systems necessitates a firm grasp on their respective timescales. To this end, a variety of mineral chronometers is employed, most notably the U-Pb system in zircon. While high-resolution methods such as isotope dilution thermal ionization mass spectrometry (ID-TIMS) provide unparalleled insight into the temporal evolution of silicic magmatic systems, the time-intensive and specialized nature of these techniques often hinders their utility. Laser ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) is a widely-available technique that combines high throughput with an ever-increasing degree of precision, but suffers some inherent technical and methodological drawbacks. The resolution of these issues and the expansion of LA-ICP-MS as a trusted source of geochronological data in magmatic petrology is considered here.
First, the application of LA-ICP-MS to U-Th zircon dating of young (< 350 ka) zircons has been historically limited due to the difficulty of detecting the low-abundance 230Th isotope (particularly on the backdrop of the much more abundant 232Th), and distinguishing it from isobaric Zr2O3 interferences on mass 230. A novel analytical routine and data reduction scheme are developed, wherein a series of corrections are applied to arrive at an accurate (230Th/238U). These combined methods are applied to a variety of reference materials at secular equilibrium and a series of young zircons, reproducing (230Th/238U) values at secular equilibrium in the former case, and the SIMS-derived ages in the latter.
Next, the application of LA-ICP-MS to U-Pb is considered for relatively young zircons (< 100 Ma; > 350 ka), where age determination is hindered by strong differences in crystal lattice properties relative to older and more radiation-damaged zircon reference materials. A relationship between the α particle radiation dosage and the apparent age offset is systematically characterized on a number of reference materials, and a novel multi-reference material-based data reduction software (Dose_Corrector.ipf) is developed for the commonly-used IgorPro platform.
A detailed discussion of excess variance in LA-ICP-MS U-Pb dating follows, focusing on identifying the sources of excess uncertainty and analytical bias that ultimately limit the inter-session and inter-laboratory reproducibility of the method to ~2% (relative), despite individual analyses having uncertainties as good as 0.5%. A series of guidelines are proposed that are meant to limit the degree of excess uncertainty caused by analytical parameters, and a series of recommendations are made that take a conservative approach to propagating uncertainties in order to increase the reliability of LA-ICP-MS data interpretations.
Lastly, a systematic study of trace elements in thermally annealed and unannealed zircons is carried out by LA-ICP-MS, demonstrating the dependence of Li concentration on thermal perturbation and calling into question the validity of applying Li-in-zircon as a geospeedometer. In particular, it is suggested that the incorporation of Li in zircon is directly linked to H in the crystal lattice, which together work to charge balance trivalent rare earth elements. Both Li and H, present in high concentrations within melt inclusions, are mobilized during thermal events and diffuse through the crystal lattice at rates much faster than previously determined.
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ETH Zurich
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03958 - Bachmann, Olivier / Bachmann, Olivier