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Determination of Pb isotopes in melt inclusions by Laser Ablation -Multiple Collector-Inductively Coupled Plasma-Mass Spectrometry (LA-MC-ICP-MS) is limited to samples formed later than Mesozoic because U and Th signals are not monitored. In this study, Pb isotopic compositions of two reference glasses and ancient melt inclusions which have undergone significant U-Th decay are determined by LA-MC-ICP-MS. The effect of the U/Pb and Th/Pb analytical errors on the calculation of initial Pb isotopic ratios was also evaluated. In the experiment USGS NKT-1G is used as the external calibrant and a standard-sample-standard bracketing method is used to correct instrument drift and mass bias. Analytical precisions of 208Pb/206Pb and 207Pb/206Pb of USGS standards, BHVO-2G and TB-1G, are better than 0.30% (2RSD) and both agree within 0.30% of the preferred values. However, 232Th/206Pb and 238U/206Pb ratios are highly variable and yield an external precision of 5%. Error propagation calculations indicate that the ages of the samples have significant effects on the errors of the initial lead isotopic ratios. For Phanerozoic samples that are earlier than 540 Ma, even the differences between the measured 232Th/206Pb and 238U/206Pb ratios and the real values reach 10%, yet the age-corrected initial lead isotopic ratios are still within 0.80% deviations relative to the real values. Therefore, the proposed method in this study can determine the lead isotopic compositions of melt inclusions in samples from Cenozoic to Paleozoic, expanding the geological application.

Compared to the traditional strontium isotope analytical method, a micro-sample strontium isotope system study can reveal the heterogeneity of analyzed samples and is widely used in various fields of geology, such as crust-mantle interaction, genesis and evolution of magmas, open magma systems, tracing provenance of sedimentary basin and climate research. Accurate measurement of Rb-Sr isotopic ratios is the premise to apply Sr isotope in these research fields. Two detailed and widely used analysis methods for micro-sample Sr isotope measurement are microdrilling-TIMS/MC-ICPMS and LA-MC-ICPMS. These two methods have different advantages and disadvantages in sample preparation, interference correction and measurement in mass spectrometry. The greatest advantage of microdrilling-TIMS/MC-ICPMS is that this technique can acquire high quality data (external precision is better than 100×10-6, 2SD). However the tedious chemical separation is very time-consuming (about 10 days) and the chemical procedure blank must be critical to low level. The large sampling depth (100-2000 μm) and diameter (200-2000 μm) reduce the spatial resolution. However, with chemical separation this method can effectively measure samples with high Rb content. Simple sample preparation and fast analysis are the most outstanding advantages of LA-MC-ICPMS which can analyze lots of samples in a much shorter time than that of microdrilling-TIMS/MC-ICPMS. The laser spot size is mainly from 60 to 300 μm. Based on Sr contents in samples, the method of LA-MC-ICPMS improved spatial resolution, compared to microdrilling-TIMS/MC-ICPMS technique. However, without chemical separation, many kinds of interferences (e.g. Rb, Ca, Kr and REEs) hinder this technique to obtain high quality data (external precision is about 200×10-6, 2SD and accuracy is about 150×10-6). LA-MC-ICPMS is only suitable for high Sr and low Rb samples, instead of samples with high content of interferences. Further studies are required for the microdrilling-TIMS/MC-ICPMS method to improve the efficiency of chemical separation, also microdrilling technology with increasing of spatial resolution by reducing spot diameter and depth. Meanwhile, a better solution for interferences of Kr, Rb and REEs is needed for LA-MC-ICPMS in order to improve precision and accuracy of high interference samples. Instrumental sensitivity improvement is also needed to satisfy Sr isotope measurement for samples with relative low Sr content.

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