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 olivine trace elements analyses. We focused on enhancing the performance of a widely accessible quadrupole ICP-MS (Q-ICP-MS) and compared its capabilities with a 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 abundance of major elements (Mg and 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 the ng g⁻¹ level. The protocol was subsequently applied to metamorphic olivines from serpentinites in the Dabie-Hong'an Orogenic Belt. The metamorphic olivines from the Dongjiashan serpentinite exhibit higher concentrations of some incompatible and immobile elements such as Al, Ti, Ga, Y, Zr, and REEs than those from Yinshanzhai. Furthermore, some Dongjiashan olivines display U-shaped REE patterns. Collectively, these geochemical signatures provide compelling evidence supporting that the mantle wedge protolith of the Dongjiashan serpentinite experienced a higher degree of cryptic melt metasomatism. This work establishes a highly sensitive and widely accessible analytical protocol that significantly extends the capabilities of LA-Q-ICP-MS for measuring low-content trace elements in olivine.