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Accurate Li isotope analysis of silicate glasses using secondary ion mass spectrometry (SIMS) is challenging due to the matrix effect, which causes instrumental mass fractionation (IMF). The characteristics of the matrix effect vary for each element, necessitating empirical correction methods. Here, we present Li isotopic measurements on a set of silicate glasses spanning a broad compositional range from ultramafic to highly siliceous using a Cameca NanoSIMS 50L. A radio-frequency (RF)-O primary beam with an intensity of 1 nA was rastered over a 5 × 5 μm2 area on the sample, and the secondary ions 6Li, 7Li and 30Si were simultaneously detected. We observed a significant matrix effect on δ7Li in the silicate glasses, which yielded an IMF of up to 19‰, exceeding the analytical precision of <2.5‰ (1SD). The IMF is strongly correlated with the chemical compositions of the silicate glasses. To correct the matrix effect, we employed various correction schemes, including univariate and multivariable compositions. All results consistently indicated that silica exerted a dominant influence on the IMF. We then propose a straightforward "online" method to correct this silica-related matrix effect by utilizing simultaneously measured 30Si signals. This strategy improves the accuracy of the Li isotope measurements to within 3‰ and can be widely applied to anhydrous subalkaline magmatic glasses with compositions ranging from ultramafic to highly siliceous. [ABSTRACT FROM AUTHOR]

Ilmenite occurs as a common accessory mineral in igneous and metamorphic rocks and is a major constituent in lunar basaltic rocks. The distributions and concentrations of major and trace elements in ilmenite have potential to record temporal changes in melt chemistry and crystal fractionation processes. However, such data are scarce because of the limitations of in situ microanalytical technology, the lack of reference materials for trace elements, and the difficulties in analyzing the small needle-like shape of ilmenite. In this study, a natural ilmenite sample derived from the Panzhihua Fe–Ti–V oxide ore deposit was characterized for its major and trace elements in relation to the potential use as reference material. The homogeneity of the studied ilmenite (PZH12-09) was assessed by electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). It is homogeneous with relative standard deviation (RSD) values within ±3.76% for major elements (Fe, Ti, Mg, and Mn) and ±12.6% for trace elements (Sc, V, Cr, Co, Ni, Zr, Nb, Hf, and Ta), respectively. The relative deviations for major and trace elements determined by solution-mode ICP-MS and in situ microanalysis range from 0.11% to 7.71%, suggesting that the PZH12-09 ilmenite can be used as a reference material for in situ microanalysis. Furthermore, we developed a high-precision EPMA method to simultaneously determine the major and trace elements of ilmenite. The disadvantage of high detection limits and poor accuracy in trace element analyses were overcome by optimizing the analytical conditions, including the accelerating voltage (20 kV), beam current (200 nA), use of large Bragg crystals (LPET and LLIF), aggregate intensity counting, peak overlap and secondary standard corrections. The measured concentrations of most trace elements are consistent within 10% relative deviation compared with the reference values. The RSD values were within ±15% at various concentrations of trace elements (except for Ni and Nb), indicating the high accuracy and precision characteristics of the EPMA methodology. Detection limits for the trace elements vary from 11 to 27 μg g−1 (3σ). The developed method should provide robust trace element data for both extraterrestrial and terrestrial ilmenite samples at high spatial resolution (1–2 μm). [ABSTRACT FROM AUTHOR]

Titanium (Ti) isotopes are emerging as a power tool for studying magmatic processes on the Earth and other planets. Pioneering studies carried out bulk-rock Ti isotopic measurements by conventional solution nebulization multi-collector inductively coupled plasma mass spectrometry (SN-MC-ICP-MS) and in situ Ti isotopic analysis via secondary ionization mass spectrometry (SIMS), which sacrificed spatial resolution and had relatively low analytical precision, respectively. In this work, a novel and robust method for in situ Ti isotopic analysis of titanium-bearing minerals (i.e., rutile and ilmenite) was presented, based on a femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometer (fs-LA-MC-ICP-MS). Very stable isotopic signals can be ensured after careful optimization of the parameters of fs-LA (e.g., fluence, spot size, and frequency), and thus a high analytical precision has been obtained for Alfa-Ti (an ultrapure Ti metal rod) under a high spatial resolution of spot diameter = 30 μm. The within run and external reproducibility for δ49/47Ti (49Ti/47Ti isotopic ratio, reported as δ49/47Ti notation, in parts in 103) measurement on Alfa-Ti are 0.05‰ (2SE, internal precision of within-spot analysis) and 0.07‰ (2SD, external reproducibility of spot-to-spot analysis), respectively. A series of titanium-bearing minerals, including five potential rutile U–Pb chronological standards and four potential ilmenite Fe isotope standards, were assessed for Ti isotopic homogeneity on a 30 μm-scale and precision of the measurement. The most homogeneous minerals were subsequently used to comprehensively evaluate the analytical accuracy and potential matrix effect. Our results show that in situ Ti isotopic analysis is susceptible to matrix effects when using fs-LA and accurate δ49/47TiOL-Ti values (calibrated against the OL-Ti reference material developed in the Origins Laboratory of the University of Chicago) can be obtained when a matrix-matched reference material is used as a bracketing standard. Therefore, well characterized matrix-matched reference materials are necessary for in situ Ti isotopic analysis. KNW rutile and PZH12-15 ilmenite characterized in this study show potential as suitable reference materials for micro-beam Ti isotopic analysis. [ABSTRACT FROM AUTHOR]