Methodology

Concentrations of REEs and important geochronometric elements were measured by LA-ICP-MS on 11/18/2024 at UT Austin’s Department of Geosciences, using an ESI NWR193 excimer laser ablation system (193nm, 4ns pulse width) paired with an Agilent 7500ce ICP-MS. This LA-ICP-MS system utilizes a large two-volume sample cell capable of fast washout (<1s) that accommodated all samples and standards in a single cell. The system was tuned with NIST 612 and produced low oxide concentrations (.344% +/-.01%). These parameters were checked on a representative sample specimen as well. After pre-ablation (50μm spot, 50μm/s scan rate, 3.9J/cm2 fluence) to remove shallow surface contaminants, transects from rim to opposite rim were performed on each specimen (30μm spot, 5μm/s scan rate, 4.47J/cm2 fluence, 20Hz repetition rate, and carrier gas flows of .8L/min for Ar and He). Baseline intensities before each standard and sample analysis were determined using a 30s gas blank, scanning all masses while the laser was off. The quadrupole time-resolved method measured 22 masses with 20-30ms integration times (Si-29, Sc-45, Ti-49, Ti-50, Y-89, La-139, Ce-140, Pr-141, Nd-146, Sm-147, Eu-153, Gd-157, Tb-159, Dy-163, Ho-165, Er-166, Tm-169, Yb-172, Lu-175, Pb-208, Th-232, and U-238). A sampling period of .5322s corresponds to 92.07% measurement time. Element intensities were converted to PPM using Iolite software (Paton et al. 2011), using Si-29 as the internal standard element at an estimated concentration value of 32% for the unknowns. NIST 612 was used as the primary calibration standard, and NIST 610 was used as an external reference standard. Standards were analyzed hourly in triplicate for 60s. Line scans were conducted with a size of 30μm, and lines of spots were conducted with a spot size of 30μm and spot spacing of 35-50μm. The average fluence of the run was 4.49 ± 0.01 J/cm^2. The grand average of secondary standard (n=59) recovery fractions for all elements was typically within 4% of reference values (via GeoRem). Typical concentrations over specimen transects were 10s to 1000s times higher than detection limits, except for Ti, La, Pr, and Md, which were mainly below detection.

Click here to view a short time lapse of data collection when conducting a line of spots.