Abstract
AbstractThe first step in most geochronological studies is to extract dateable minerals from the host rock, which is time consuming, removes textural context, and increases the chance for sample cross contamination. We here present a new method to rapidly perform in situ analyses by coupling a fast scanning electron microscope (SEM) with Energy Dispersive X‐ray Spectrometer (EDS) to a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LAICPMS) instrument. Given a polished hand specimen, a petrographic thin section, or a grain mount, Automated Phase Mapping (APM) by SEM/EDS produces chemical and mineralogical maps from which the X‐Y coordinates of the datable minerals are extracted. These coordinates are subsequently passed on to the laser ablation system for isotopic analysis. We apply the APM + LAICPMS method to three igneous, metamorphic, and sedimentary case studies. In the first case study, a polished slab of granite from Guernsey was scanned for zircon, producing a 609 ± 8 Ma weighted mean age. The second case study investigates a paragneiss from an ultra high pressure terrane in the north Qaidam terrane (Qinghai, China). One hundred seven small (25 µm) metamorphic zircons were analyzed by LAICPMS to confirm a 419 ± 4 Ma age of peak metamorphism. The third and final case study uses APM + LAICPMS to generate a large provenance data set and trace the provenance of 25 modern sediments from Angola, documenting longshore drift of Orange River sediments over a distance of 1,500 km. These examples demonstrate that APM + LAICPMS is an efficient and cost effective way to improve the quantity and quality of geochronological data.
Highlights
Geology, like other fields of Science, greatly benefits from the ever increasing pace of technological progress
Since thin sections are needed for petrographic purposes anyway, the only additional sample preparation step was the application and removal of the carbon coating before and after Automated Phase Mapping (APM) analysis, respectively
The most significant limitations of our metamorphic study were the absence CL-images to guide the placement of the laser spot, and the use of a single collector quadrupole Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LAICPMS), which precluded the use of a 204Pb-based common Pb correction
Summary
Like other fields of Science, greatly benefits from the ever increasing pace of technological progress. Continuous improvements in the miniaturization, automation, and affordability of mass spectrometers have provided geologists with unprecedented access to precise chronological data, and have opened up new research fields such as detrital geochronology. It is common for studies to comprise thousands of U-Pb ages in dozens of samples, something that would have been prohibitively expensive just a decade ago. Geochronological/isotopic data are increasingly combined with compositional measurements (Engi et al, 2017). Sedimentary provenance studies increasingly employ a combination of detrital geochronology, bulk geochemistry, and heavy mineral analysis (Vermeesch & Garzanti, 2015). Mineral separation is a bottleneck that prevents these exciting new developments from being more widely adopted
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