Precise analysis of sub-ppm trace elements in olivine by LA-ICP-MS and application to metamorphic olivine from serpentinite

Abstract

The accurate in situ determination of sub-ppm to ppb-level trace elements, particularly rare earth elements (REEs), in olivine remains a significant analytical challenge due to their extremely low concentrations. This study presents a systematic evaluation and optimization of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for this purpose. We specifically focused on enhancing the performance of a widely accessible quadrupole ICP-MS (Q-ICP-MS) and compared its capabilities with high-resolution sector-field ICP-MS (SF-ICP-MS). A robust analytical protocol was developed for the LA-Q-ICP-MS by employing a large spot size (160 μm) at a high repetition rate (20 Hz). Crucially, a tailored dwell time strategy was implemented to accommodate the contrasting abundances of major elements (Mg, Fe) and ultra-trace REEs, enabling their simultaneous and accurate quantification. The accuracy of the method was rigorously validated using the matrix-matched olivine reference material MongOL Sh11-2, yielding REE concentrations consistent with recommended values down to ng g⁻¹ level. The protocol was subsequently applied to metamorphic olivines from serpentinites in the Dabie-Hong’an Orogen. The metamorphic olivines from Dongjiashan serpentinite exhibit higher concentrations of some incompatible and immobile elements such as Al, Ti, Ga, Y, Zr, and REEs compared to those from Yinshanzhai. Furthermore, some Dongjiashan olivines display U-shaped REE patterns. Collectively, these geochemical signatures provide compelling evidence that the mantle wedge protolith of the Dongjiashan serpentinite experienced a higher degree of cryptic melt metasomatism. This work establishes a high-sensitivity and accessible analytical protocol that significantly expands the applicability of LA-Q-ICP-MS for the determination of low-content trace elements in olivine.