Supplementary tables.xlsx

Spinel, a key mineral in mafic-ultramafic rocks, is highly sensitive to petrologic and geochemical processes. The oxygen isotopic composition of spinel provides valuable insights into diverse geological processes, for which accurate and precise in-situ analysis is a fundamental requirement. A major challenge for microanalytical techniques, such as secondary ion mass spectrometry (SIMS), is the scarcity of matrix-matched reference materials for correcting instrumental mass fractionation and monitoring analytical accuracy. Here, we report three new working reference materials for in-situ oxygen isotopic analysis: MadSP (Mg₀.₇₅Fe₀.₂₅Al₂O₄), a Madagascar spinel, TanzSP-1 and TanzSP-2, both of which are pure Mg spinels from Tanzania. SIMS analyses show that they have homogeneous oxygen isotopic compositions, with two-standard deviations of 0.37‰ (N = 105), 0.40‰ (N = 106), and 0.33‰ (N = 106), respectively. Conventional fluorination isotope ratio mass spectrometry (IRMS) yields δ¹⁸O_VSMOW values of 11.94 ± 0.43‰ for MadSP (2SD, N = 5) , 23.40 ± 0.20‰ for TanzSP-1 (2SD, N = 4), and 13.29 ± 0.48‰ for TanzSP-2 (2SD, N = 6). Our results show that there are no significant matrix effects for magnesia-alumina spinels with Fe ranging from 0% to 25%.