A new reference material for O–Sr isotope determination in epidote using a micro-beam

Abstract

A new natural epidote reference material, PSV, sourced from Alchuri village, Shigar valley, Haramosh mountains, Gilgit, Pakistan, is characterized in this study for O and Sr isotope composition analysis by micro-beam analytical techniques. Comprehensive studies including thermal ionization mass spectrometry (TIMS), solution nebulization multi-collector inductively coupled plasma-mass spectrometry (SN-MC-ICP-MS), laser ablation multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS), secondary ion mass spectrometry (SIMS), and laser fluorination have been performed on PSV epidote to assess the homogeneity of O and Sr isotope compositions and provide recommended values for δ¹⁸O and the ⁸⁷Sr/⁸⁶Sr ratio. The mean δ¹⁸O value measured by the laser fluorination method is 12.58 ± 0.07‰ (2s, n = 12), recommended as the reference value for micro-beam analysis. Seventy SIMS analyses were performed on PSV epidote, resulting in a mean δ¹⁸O value of 19.67 ± 0.03‰ (2s, n = 70) obtained without external calibration utilizing matrix-matched reference materials. This finding emphasizes the necessity of employing external calibration with matrix-matched reference materials for accurate determination of oxygen isotope ratios in epidote utilizing the SIMS method. The mean ⁸⁷Sr/⁸⁶Sr ratio derived by TIMS and SN-MC-ICP-MS is 0.707672 ± 0.000013 (2s, n = 2) and 0.707679 ± 0.000007 (2s, n = 10), respectively. The in situ Sr isotopic analysis utilizing LA-MC-ICP-MS revealed homogeneous Sr isotopic compositions at a micro-scale of 40 to 85 μm. The weighted mean ⁸⁷Sr/⁸⁶Sr ratio was determined to be 0.70768 ± 0.00001 (2s, n = 336; MSWD = 2.0), which is in agreement with the solution value within the analytical uncertainty. In conclusion, despite the chemical composition variability, the PSV epidote exhibits reproducible O and Sr isotope compositions, making it a suitable reference material for in situ O–Sr isotope ratio analysis utilizing micro-beam methods.