New spinel reference materials for oxygen isotopic microanalysis

Abstract

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 black spinel, TanzSP-1 and TanzSP-2, both of which are nearly pure Mg spinels from Tanzania. SIMS analyses show that they have homogeneous oxygen isotopic compositions, with a two-standard deviation of 0.37‰ (N = 105), 0.40‰ (N = 106), and 0.33‰ (N = 106), respectively. Laser fluorination isotope ratio mass spectrometry (IRMS) yields δ¹⁸O 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 Fe²⁺ ranging from 0% to 25%.