Natural Ti-Rich Mineral (Ilmenite, Titanite and Perovskite) Reference Materials for In Situ Ti Isotopic Measurement by LA-MC-ICP-MS

Abstract

Iron-titanium oxides such as ilmenite (FeTiO₃), titanite (CaTiSiO₅) and perovskite (CaTiO₃) are the common Ti-rich mineral phases crystallised during magmatic and metamorphic processes on Earth. Depending on magma types or conditions of phase equilibria, formation of these Ti-rich minerals can result in Ti isotopic fractionation in the range of -1.52‰ to +2.90‰ on δ⁴⁹Ti_OL-Ti (i.e., the per mil difference of ⁴⁹Ti/⁴⁷Ti ratio relative to the OL-Ti reference material), making the Ti isotope ratios of these minerals potential tracers for conditions of magmatism and metamorphism. Due to their resistance to aqueous alteration, these Ti-rich accessory minerals are also commonly present as pristine, detrital phases in sedimentary rocks, which offer an opportunity to study the evolution of magmatism and metamorphism throughout the Earth's geological history. Here we have developed a novel technique for in situ Ti isotopic measurement in ilmenite, titanite and perovskite using femtosecond laser ablation multi-collector inductively coupled plasma-mass spectrometry (fs-LA-MC-ICP-MS) under wet plasma conditions. Samples were ablated with different laser spot sizes (15–50 μm) and different laser energy densities (0.6–4.2 J cm^-2) to obtain adequate Ti signal intensity at a fixed laser repetition rate of 2 Hz. When ⁴⁹Ti signal intensity of samples ranged from 0.3 to 3.7 V, no significant signal-dependent Ti isotopic fractionation was observed under wet plasma conditions. Repeated measurements on twelve Ti-rich minerals using different analytical protocols provided comparable δ⁴⁹Ti_OL-Ti values within uncertainties, confirming the accuracy of the proposed fs-LA-MC-ICP-MS method. With one exception (i.e., RUS1), all analysed minerals are homogeneous in their Ti isotopic compositions between individual chips with an intermediate precision of ±0.13‰ to ±0.17‰ (2s) on δ⁴⁹Ti_OL-Ti. Collectively, these minerals record a significant δ⁴⁹Ti_OL-Ti variation ranging from -0.46‰ to +2.12‰. These warrant the suitability of these materials as Ti isotopic reference materials for in situ Ti isotopic measurement.