Methodology

Major and trace elements in olivine grains and serpentine veins hosted by mantle xenoliths in the form of thin sections were measured by LA-ICP-MS on Nov. 20, 2024 at the University of Texas at Austin Department of Geosciences, using an ESI NWR193 excimer laser ablation system (193 nm, 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 accommodates 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: 0.284 ± 0.097%) by running on a standard (NIST 612), and these parameters checked from trial transects on representative specimens. Following pre-ablation (75 µm spots for serpentine veins and standards and 140 µm spots for olivine grains, 2 Hz repetition rate, 1s dwell time, 800 ml/min He flow rate, 3.04 J/cm² fluence) to remove shallow surface contaminants, spot analysis were performed on each specimen, using a 125 µm diameter spot for the olivine grains and a 50 µm diameter spot for the serpentine veins, 3.16 J/cm² energy density (fluence) for olivines and 3.37 J/cm² for serpentine veins, 10 Hz 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 20s gas blank measurements, made while the laser was off and all masses were scanned by the quadrupole.

The measurements were mass resolved and all masses were scanned by the quadrupole. The measurements were time-resolved mode measured 17 masses with 10 ms integration times (23Na, 24Mg, 25Mg, 27Al, 29Si, 39K, 43Ca, 44Ca, 45Sc, 47Ti, 51V, 55Mn, 57Fe, 59Co, 60Ni, 66Zn, 90Zr), seven masses with 30 ms integration times (53Cr, 88Sr, 139La, 140Ce, 146Nd, 147Sm, 180Hf), and one mass with 50 ms integration time (31P). The quadrupole duty cycle of 0.4752s corresponds to 90% measurement time.

Measured intensities were converted to elemental concentrations (ppm) using Iolite software (Paton et al., 2011), using 24Mg as the internal standard and a Mg index concentration value of 29.8 wt% for San Carlos olivine with a FO# = 90. GSC 1G was used as the primary calibration standard and NIST 612, BHVO 2G, and NIST 610 were used as external reference standards. Standards were analyzed hourly in triplicate for 45s. Over the session, the grand average of secondary standard (n=25) recovery fractions for all elements was typically within 10% of reference values.

For olivine, typical concentrations were 1 to 10s times higher than limits of detection for Al, P, Ca, Sc, Ti, V, Cr, and Zn. Typical concentrations were 1000s to 100000s times higher than limits of detection for Mg, Si, Mn, Fe, Co, and Ni. Na, K, Sr, Zr, La, Ce, Nd, Sm, and Hf concentrations were mainly below detection, and are not subsequently considered. For serpentine veins, typical concentrations were 1 to 10s times higher than limits of detection for Na, Al, K, Ca, Sc, Ti, V, Cr, and Zn. Typical concentrations were 100s to 10000s times higher than limits of detection for Mg, Si, Mn, Fe, Co, Ni, and Sr. P, Zr, La, Ce, Nd, Sm, and Hf concentrations were mainly below detection, and are not subsequently considered.