Abstract
In this work, a novel method for high spatial resolution U-Pb dating of zircon by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at 10–16 μm spot diameter has been proposed. This was achieved by introducing 2% (v/v) water-ethanol vapours into ICP in combination with the shielded torch system to increase the sensitivity and suppress the isotopic fractionation effect. Precise and accurate concordant U-Pb ages for Plešovice, GJ-1, and 91500 zircons were obtained using the proposed method, and the results agreed well with the isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and LA-ICP-MS within 2σ, except for the ages obtained using dry plasma at 10 μm spot diameter. Additionally, the effects of plasma condition (dry plasma or 2% (v/v) ethanol plasma) and spot size (10, 16, 24, or 32 μm) on precision (RSD), accuracy (RE), and uncertainty (2σ) of the 206Pb/238U ages have been studied. The results indicated that increasing the spot diameter and introducing 2% (v/v) water-ethanol vapours into ICP significantly improved the precision, accuracy, and uncertainty for small spot diameters (10 and 16 μm), while it exhibited little influence for intermediate spot diameters (24 and 32 μm). Furthermore, the effects of spot size and plasma condition on the precision, accuracy, and uncertainty strongly depended on the sensitivity of analytes.
Highlights
Fine elemental and isotopic composition information of minerals at microscale are beneficial to decode the crystallization environment, the experienced geochemical process, and the history of geological body evolution
The capability of LA-ICP-MS was significantly improved by technological progress in laser ablation system and ICP-MS instrument, high spatial resolution analysis by LA-ICP-MS was yet hampered by low sensitivity with an inferior precision and perceptible isotopic fractionation
The results showed that single analysis was poor under dry plasma compared to ethanol plasma, under which the concordance was less than 90% and far away from the concord line in Figures 2(b) and 3(b)
Summary
Fine elemental and isotopic composition information of minerals at microscale are beneficial to decode the crystallization environment, the experienced geochemical process, and the history of geological body evolution. The capability of LA-ICP-MS was significantly improved by technological progress in laser ablation system and ICP-MS instrument, high spatial resolution analysis by LA-ICP-MS was yet hampered by low sensitivity with an inferior precision and perceptible isotopic fractionation. In quest of enhancing the analytical sensitivity of LAICP-MS, high laser ablation repetition rate and large spot diameter are often employed as a common LA-ICP-MS strategy. Such a strategy increases the consumption of sample and the chance of elemental fractionation during laser ablation. The addition of small amounts of a few active matrixes, such as helium [5], nitrogen [6, 7], hydrogen [8], methane [9], ethanol [10, 11], and water vapours [12], into traditional argon plasma has been studied for improving signal
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