First attempt to determine oxygen isotopes in oxygen by MC-ICP-MS

Abstract

Oxygen is the key component of crustal and mantle rocks and fluids. The oxygen isotopic composition is a key tool to understand Earth's geological history and processes, such as continental formation, magmatic-hydrothermal processes, and crust–mantle interactions. The oxygen isotopic analysis is commonly implemented by Isotope Ratio Mass Spectrometry (IRMS) and Secondary Ion Mass Spectrometry (SIMS); however, its wide application is limited by the high cost and serious matrix effect. LA-MC-ICP-MS has been the method of choice for in situ isotopic analysis due to its relatively low cost, high analysis speed, high spatial resolution, and the low matrix effect. The determination of oxygen isotope using Ar-ICP has two limitations. Firstly, the exposure of Ar-ICP to the atmosphere may result in atmospheric interference, leading to an increase in the blank of oxygen isotopes and a reduction in the signal-to-blank (S/B) ratio. Secondly, the presence of doubly-charged Ar ions introduces interference that affects the accuracy of oxygen isotopic analysis. In order to investigate whether MC-ICP-MS can be used in the determination of oxygen isotopes, we attempt to use three approaches (¹⁸O¹⁶O/¹⁶O¹⁶O, ¹⁸O/¹⁶O and ¹⁸O¹H₂/¹⁶O¹H₂) to determine oxygen isotopes in oxygen, and the applicability of three approaches is assessed based on interference, peak width, sensitivity, and stability. With our built methods, the obtained long-term productivity of δ¹⁸O measured by ¹⁶O¹⁸O/¹⁶O¹⁶O was greater than 0.16‰ (2 SD), and the measured results for oxygen were consistent with those obtained by IRMS and MC-MIP-MS within the uncertainty limit. This demonstrates the feasibility of our method and also lays the foundation for the realization of in situ oxygen isotope analysis using LA-MC-ICP-MS.