Methodology

The amphibole grains were prepared by first removing them from the bulk rock. This was fairly simple as they were large enough to be picked out by hand. Grains were selected based on size and shape. The larger and more euhedral the grain the better in order to ensure that the analysis would have plenty of space on a representative cross section of an amphibole. Five to seven grains were placed in crystal bond on each slide and were polished down to expose the surface.

Trace element signatures of amphiboles from Mt. Vesuvius were measured by LA-ICP-MS in a single session (November 20, 2024) consisting of two runs at the University of Texas at Austin Department of Geosciences using an ESI NWER193 excimer laser ablation system (193nm, 4ns pulse width) coupled to an Agilent 7500ce ICP-MS. The LA-ICP-MS system is equipped with a large format, two-volume, sample cell with fast washout (<1s) that accommodated all samples and standards in a single cell loading. The system was optimized daily for sensitivity across the AMU mass range and low oxide production (ThO/Th: 0.38+/-0.01%) by tuning on a standard (NIST 612), and these parameters checked from trial spots on grains otherwise free of pre-planned laser spots and lines.

Following pre-ablation (125µm spot, 3.41J/cm^2 fluence, 2Hz repetition rate) to remove shallow surface contaminants, multiple spot ablations were performed on four selected grains for each sample type totaling twelve amphiboles analyzed, using a 100µm spot, 3.41J/cm^2 energy density (fluence), 10Hz repetition rate, and carrier gas flows of 0.8L/min for Ar and He. Baseline intensities before each standard and sample analysis were determined from 30s gas blank measurements made while the laser was off and all masses were scanned by the quadrupole. The quadrupole time-resolved method measured 21 masses with 20ms integration times (Si29, Ca43-44, Sc45, Rb85, Y89 Ba137, La139, Ce140, Pr141, Nd146, Sm147, Eu153, Gd157, Tb159, Dy163, Ho165, Er166, Tm169, Yb172, Lu175). The quadrupole duty cycle of 0.4582s corresponds with 91% measurement time. Measured intensities were converted to elemental concentrations (ppm) using the Iolite software (Paton et al., 2011), with Si29 as the internal standard and a Si index concentration value of 47.5 wt%. It should be noted that EMPA or other in-situ quantitative chemical analysis was not conducted on these grains. The Si wt% value is based on an average value for Si content in typical amphiboles. NIST 612 (USGS synthetic glass) was used as the primary calibration standard. NIST 610 (USGS synthetic glass) and BHVO-2G (Hawaiian basalt) were used as external reference standards. Standards were analyzed in triplicate over a period of 405s before and after the unknowns with a dwell time for each individual spot of 45s. The total average of secondary standard recovery fractions for all elements was within 3% of the reference values for NIST 610 and 6% for BHVO-2G (https://georem.mpch-mainz.gwdg.de/). Typical concentrations of all three amphibole grain types were 1000 to 100 times higher than the limits of detection for all elements with the exception of Rb and Ba. The concentrations for these elements were typically below the limits of detection so it will not be considered in the discussion and major findings for the spot data. Because the signals for the two Ca isotopes were identical, the higher abundance isotope of Ca44 will be used for the purposes of data quality and results tables.

The second run consisted of the linear transect portion of this laser ablation analysis. Following pre-ablation (125µm spot with 50µm/s scan rate, 3.41J/cm^2 fluence, 10Hz repetition rate) to remove shallow surface contaminants, linear transects were performed on five total selected grains deemed large and clean enough using a 50µm spot with 5µm/s scan rate, 3.41J/cm^2 energy density (fluence), 10Hz repetition rate, and carrier gas flows of 0.8L/min for Ar and He. All settings regarding the elements analyzed are the same as the spot portion of the analysis. The duty cycle and measurement time are also the same as the spots resulting in linear sampling rates of 2.291µm. Measured intensities were converted to elemental concentrations (ppm) using the Iolite software (Paton et al., 2011), with Si29 as the internal standard and a Si index concentration value of 47.5 wt% just as with the spots. NIST 612 (synthetic basalt glass) was used as the primary calibration standard here as well. NIST 610 and BHVO-2G (synthetic basalt glasses) were used as external reference standards. Standards were analyzed before and after the run in triplicate before and after the unknowns. The total average of secondary standard recovery fractions for all elements was within 10-15% of the reference values for NIST 610 and 1-3% for BHVO-2G. Typical concentrations of all three amphibole grain types were 1000 to 100 times higher than the limits of detection for all elements. The derived elemental time-series were smoothed by consecutive moving median and average filters using a 7-point boxcar width (5µm equivalent distance), resulting in a smooth, locally weighted, signals free from high-frequency outliers. Signals were converted to distance (µm) based on the scan rate and duty cycle.