Results are presented for in situ simultaneous determination of U-Pb and Sm-Nd isotopes in monazite using the Laser Ablation Split-Stream (LASS) method. This method uses a laser ablation system coupled to a magnetic-sector inductively coupled plasma mass spectrometer (HR) (ICP-MS) for measuring U-Pb isotopes and a multicollector (MC) ICP-MS for measuring Sm-Nd isotopes. Ablated material is split using a Y-connector and transported simultaneously to both mass spectrometers. In addition to Sm and Nd isotopes, the MC-ICP-MS is configured to also acquire Ce, Nd, Sm, Eu, and Gd elemental abundances. This approach provides age, tracer isotope, and trace element data in the same ablation volume, reducing sampling problems associated with fine-scale zoning in accessory minerals and minimizing the material needed for ablation. Precision and accuracy of the U-Pb method (and the precision of the Sm-Nd method) is demonstrated with results from well-characterized monazite reference materials. The LASS results agree within uncertainty with the isotope dilution thermal ionization mass spectrometry (ID-TIMS) U-Pb dates. The accuracy of the Sm-Nd method is assessed by comparing the LA-MC-ICP-MS results with ID-TIMS determinations on a well-characterized, in-house monazite reference material. The LASS method is then applied to monazite from the Birch Creek Pluton in the White Mountains of California as a case study to illustrate the utility of this method for solving geologic problems. The U-Pb ages and Sm-Nd isotopic data from the LASS method support the conclusions drawn from previous results that monazite can record timing and information about the source region(s) of hydrothermal fluids.
A novel approach to creating a trans-vertical grain mount embedded in epoxy has been demonstrated through the creation of a new mounting mould, as well as a polisher adapter and sample holder for the Quanta 400 SEM. These rectangular moulds result in a sample that is 30 mm long × 10 mm high × 17 mm wide, thus leading to the ability to polish 10 samples at once. Up to 14 samples may fit in the SEM holder for analysis. This represents an increase in efficiency of over 50%, and with a slow-speed polishing method, the consumables used are reduced by at least a factor of 4. This has the potential to lead to significant financial savings. A comparison of mounting techniques using a −100+200 mesh size fraction of an iron-ore sample from Labrador demonstrates that this new mounting system removes any bias in analysis resulting from density stratification during the sample preparation. These moulds yield similar modal mineralogy abundances and standard deviation as the two-step trans-vertical method, but they are less labour-intensive to make and more efficient to analyse.
Scanning Electron Microscope-Mineral Liberation Analysis (SEM-MLA) can be used to discriminate between hematite and magnetite in iron ores. However, achieving backscattered electron (BSE) segmentation between the two minerals is difficult for particles ≤75 µm using typical preparation and analysis methods for the MLA method based on a tungsten filament SEM (Quanta 400) with 25 kV high voltage. Preparing iron ore sample mounts using a slow-speed polishing method, and conducting the experiment on a field emission gun SEM-MLA (Quanta 650) with the high voltage setting lowered to 15 kV reduces instrument noise and results in very clean BSE images and segmentation. This method requires new X-ray standards for each mineral at 15 kV because of major changes in the spectra at lower kV. However, once these X-ray spectra are added to the mineral reference list, effective segmentation can be achieved and an accurate analysis obtained.
Single-step trans-vertical moulds have been shown to increase the throughput of automated mineralogy systems. Owing to the small particle size of iron ore grains in the −200 mesh size fraction (<75 µm), not all of the surface area on a mould is needed for measurement in achieving good statistical representation. Trimming the excess area of a mould results in superfluous material being removed, leaving only the necessary analyte. This allows maximum packing in the sample chamber and promotes efficiency by increasing the quantity of samples that can fit in a newly designed trans-vertical adapter for an FEI MLA™ 650 field emission gun SEM. The analysis was completed using a newly designed sample block holder capable of holding 98 samples. This study represents the most expedient method yet reported to prepare well-polished samples and analyse them in a timely manner that can allow for quicker results in process mineralogy.
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