Micro-scale (1 μm) Mg isotope analysis of olivine by NanoSIMS with online matrix correction and its application to Chang'e−5 sample

Abstract

In-situ stable Mg isotope analysis of olivine, the most common mineral in igneous and metamorphic rocks, provides critical insights into their formation and timescales. However, accurate correction of significant matrix effects is hampered by the lack of olivine reference materials with forsterite (Fo) content below 80. More importantly, current techniques using secondary ion mass spectrometry (SIMS) or laser ablation-multi collector-ICP-MS (LA-MC-ICP-MS) are insufficient to decipher geological processes occurring at fine scales (<10 μm). Here, we report potential olivine reference materials with Fo contents ranging from nearly 0 to 90.8, covering a wide range and displaying homogeneous compositions within individual samples. These reference materials were developed through high-temperature experiments and the collection of extraterrestrial meteorites. Using these materials, we established a method for Mg isotope analysis on a CAMECA NanoSIMS 50L, for the first time achieving a high spatial resolution of ∼1 μm with a precision of 0.7–0.8 ‰ (1SD). We found that the matrix effect is best modelled by a BiHill equation with the ²⁴Mg/(²⁴Mg + ⁵⁶Fe) ratio. Simultaneous detection of ²⁴Mg–²⁶Mg–²⁸Si–⁵⁶Fe by NanoSIMS enable us to calibrate the matrix effect online for olivine Mg-isotope analysis. This method was successfully applied to Chang'e−5 (CE5) lunar chemically-zoned olivine crystals, revealing substantial variation of δ²⁶Mg (>4 ‰) on a micron scale (<100 μm).