Methodology

Line Scans

Trace element compositions of molybdenite grains from Cave Peak samples were measured by LA-ICP-MS on November 17th at the University of Texas at Austin Department of Geosciences. An ESI NWR192 excimer laser ablation system coupled with a Agilent 7500ce ICP-MS. The LA-ICP-MS is equipped with a large format, two-volume, sample cell with fast washout (<1s) that was able to accommodate all of our samples (2 thin sections and 4 billets) along with our three standards (GSE-1G, STD3GL, OREAS-460) in a single cell loading. The system was optimized daily for sensitivity across the AMU mass range and low oxide production (ThO/Th: 0.244 +/- 0.1%) by tuning to a standard (NIST 612), and these parameters checked from trial transects on representative specimens. Following pre-ablation (75µm spot, 5µm/s scan rate, 3.13 J/cm² fluence) to remove any surface contamination present, transects along individual molybdenite grains were performed using a 30 µm diameter spot size at a 5 µm/s scan rate, 3.00 +/- 0.05 J/cm² energy density (fluence), 20 Hz repetition rate, and carrier gas flows of 0.8L/min for Ar and He. All 24 masses (⁹Be, ⁵⁷Fe, ⁵⁹Co, ⁶⁰Ni, ^63-65Cu, ⁶⁶Zn, ⁷⁵As, ⁷⁷Se, ⁹³Nb, ⁹⁵Mo, ¹⁰⁷Ag, ¹¹⁸Sn, ¹²¹Sb, ¹²⁵Te, ¹⁸²W, ¹⁸⁵Re, ¹⁹²Os, ^204-208Pb, ²⁰⁹Bi, and  ²³⁸U with a duty cycle of 0.32300 seconds which corresponds with 80% of measurement time. They were measured in a single sequence divided into two blocks of unknowns with three blocks of our standards; one at the beginning, one at the end, and one in the middle of the unknowns. Using ⁹⁷Mo as our internal standard, the measured intensities of our isotopes were converted to elemental concentrations (ppm) using the Iolite v.4 software. STDGL3, GSE-1G, and OREAS-460, (sulfide standard, nanopellet sulfide and ore and basalt graph, respectively) were used as both our primary calibration and external reference standards; selecting the best values from each based on their recovery times. Recovery times and the data reduction can be found in this finalized excel sheet which includes our data. The total duration of running this experiment came out to be just over 2 hours. 

2D Maps

For 2D maps, two molybdenite grains were imaged using 60 and 8o continuous line traverses. Following pre-ablation (75 µm spot size, 3.13 J/cm² fluence, 20 Hz repetition rate) while transects were run at 50µm/s, 10×10µm aperture, 50 Hz repetition rate, and at 2.9 +/- 0.02J/cm² energy density (fluence), and carrier gas flows of 0.8L/min Ar and He. Baselines were determined from 3 second gas blank measurements. In contrast to the line scans, only 9 masses were measured for both maps (⁵⁷Fe, ⁶⁰Ni, ⁶⁵Cu, ⁶⁶Zn, ⁷⁷Se, ⁹³Nb, ⁹⁵Mo, ¹⁸²W, ²⁰⁴Pb) with a duty cycle of 0.199200 seconds that corresponds with 89 % of measurement time. STDGL3 and GSE-1G were used as both our primary calibration and external reference standards; selecting the best values from each based on their recovery times. The total duration it took to pre-ablate and scan both maps came out to be about an hour and a half. Image conversion were performed using Iolite v.4 software with the following global variables assigned:

Alignment: justify

Map Style: raster

Gradient: jet

Scale Bar: bottom right, Half Bar

Scale type: Linear

Map being created of a zoned grain

Possible Interferences for Trace Elements in Molybdenite Samples

*Interferences that might pose a problem

Above are listed the most likely polyatomic interferences on the isotopes being measured. However, due to this being laser ablation, many of the polyatomic interferences will not be concerns due to lack of a solution/acid that would provide the elements to form them. Some more notable interferences, albeit more minor due to use of laser ablation, will be from sulphur due to higher concentrations of both of these elements within the samples. Nonetheless, the suite of isotopes chosen was done so in order to minimize possible interferences.

Data coming in hot!