Accurate determination of Ti stable isotopes in Ti-rich minerals using nanosecond LA-MC-ICP-MS

Abstract

Mass-dependent titanium (Ti) isotopic variations in Ti-rich minerals as geological tracers are commonly measured by laser ablation (LA) coupled with multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). A high-precision method for in situ Ti stable isotopic analysis in Ti-rich minerals was developed using nanosecond (ns) LA-MC-ICP-MS. Analytical performances of ns-LA-MC-ICP-MS were contrasted with femtosecond (fs) LA-MC-ICP-MS for Ti isotopic analysis in Ti-rich minerals and Ti metal. Matrix effects, more pronounced in ns-LA-MC-ICP-MS, were observed using non-matrix-matched calibration. Wet plasma conditions mitigated these effects compared to dry plasma conditions, effectively eliminating them between rutile and Ti metal using fs-LA-MC-ICP-MS. However, matrix effects persisted between rutile and other Ti-rich minerals (ilmenite, titanite and perovskite), even under wet plasma conditions. Subsequent Ti isotopic analyses of five Ti-rich minerals, using matrix-matched calibration protocols with both techniques, yielded internal precision (δ⁴⁹Ti, 2SE) of ≤0.08‰ at ⁴⁹Ti signal intensities >0.6 V with 10¹¹ Ω resistors. External reproducibility (δ⁴⁹Ti, 2SD) ranged from ±0.11‰ to ±0.17‰ across the five minerals analyzed by ns-LA-MC-ICP-MS, consistent with the precision achieved by fs-LA-MC-ICP-MS. Results obtained via ns-LA-MC-ICP-MS agreed with those from both fs-LA-MC-ICP-MS and solution nebulization MC-ICP-MS, except for Ti metal, confirming the accuracy of the ns-LA-MC-ICP-MS method. Isotopically homogeneous ilmenite GER16 and titanite MAD12 are proposed as bracketing and/or quality control standards.