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Microanalysis of carbon and magnesium isotopes in dolomite is gaining increasing attention as a valuable tool in various fields of earth sciences. However, the matrix effect poses a major challenge for determining C and Mg isotopes using laser ablation multi-collector ICP-MS, particularly due to the wide range of chemical compositions found in natural dolomite. A preferred solution is to use a matrix-matched reference material with homogeneous isotope compositions at the micron scale. In this contribution, we established a new protocol for preparing a dolomite reference material for isotopic microanalysis through pressureless sintering. The effects of particle size and sintering temperature on the isotopic homogeneity of sintered dolomite powder pellets (SDPPs) were carefully evaluated. Our results indicate that a smaller d90 particle size improves isotope homogeneity, and SDPPs with a d90 of 3.17 μm show the best laser ablation repeatabilities of δ26Mg and δ25Mg (0.11‰ and 0.09‰) that are 2–4 times better compared to the repeatability of those with a d90 of 10.37 μm. Higher sintering temperature enhances particle cohesion and pellet hardness, which are crucial for laser ablation performance; Vickers hardness (HV) is improved to 41.58 kgf mm−2 when SDPPs are heated up to 400 °C below decomposition temperature. These SDPPs show the best δ13C, δ26Mg and δ25Mg homogeneity with a twice standard deviation value of 0.37‰, 0.11‰ and 0.06‰ (n = 15), respectively. Of note, in situ carbon and magnesium isotope characterization of SDPPs using LA-MC-ICP-MS shows negligible isotope fractionation against natural dolomites. This further confirms the reliability and robustness of the proposed protocol for dolomite isotopic reference preparation via pressureless sintering. [ABSTRACT FROM AUTHOR]

Robust U–Pb dating of accessory minerals (e.g., apatite, titanite, wolframite, and cassiterite) by LA-ICP-MS is often impeded by variable contents of common Pb in matrix-matched reference material (RM). Therefore, common Pb needs to be corrected in these RMs analyses that are used as standards to calibrate isotope fractionation during data reduction. In this study, a novel stand-alone computer program (called Isoclock) focused on the deduction of common Pb from RM, and the corrected RM were used for isotope fractionation. A new common Pb correction method was introduced into the program that allows the deduction of common Pb not only in 206Pb and 207Pb, but also in 208Pb, so that both U/Pb and Th/Pb age calculations can be performed. The software contains several processing steps for the raw files of different mass spectrometers. The processing interface contains data import and view, background correction and filtering of outliers, calculation of common Pb for RMs, fractionation calibration, and age calculation. Isoclock is written using the free and open-source Python language and can either run the code directly or by using a graphical interface. Finally, we discuss the main advantages of this software and its geological application using six examples of apatite, cassiterite, titanite, monazite and wolframite U–Pb dating. [ABSTRACT FROM AUTHOR]

In situ magnesium isotope compositions of carbonates play an important role in tracing geological and biological processes. Matrix effects of carbonates with distinct chemical and physical properties are the main factors affecting the accurate and precise determination of Mg isotopes by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). This study investigates Mg isotope microanalysis of magnesite, dolomite and siderite under wet and dry conditions. The results demonstrated that the main isobaric interference of 48Ca2+ on 24Mg+ in carbonates with high Ca/Mg ratios (up to 6.79) can be significantly suppressed and downhole isotopic fractionation can be reduced with the addition of water vapor in front of the ablation cell (e.g., under wet conditions). MGS-3, DOL-8, DOL-9 and SD-5 show homogeneous in situ Mg isotope compositions for the investigated pieces (>1 cm2) with intermediate precision better than 0.14‰ and 0.15‰ for δ26Mg and δ25Mg, and are adopted as in-house standards. The obtained in situ δ26Mg and δ25Mg values of matrix-matched samples were consistent with those determined by solution nebulization (SN)-MC-ICP-MS with uncertainties of 0.16–0.27‰ and 0.12–0.25‰ (2SD) under dry conditions and 0.15–0.18‰ and 0.06–0.10‰ (2SD) under wet conditions. This further indicates that wet-laser analyses improved the analytical precision of Mg isotope determinations by 1.1 to 2.5 times. Inaccurate Mg isotope data were obtained under dry conditions when carbonate samples were measured against a physical property and/or chemical composition of a different standard, and such deviations can be largely improved in wet-laser analyses. [ABSTRACT FROM AUTHOR]