RESUMO
RATIONALE: In situ Pb isotope analyses of tiny melt inclusions using laser ablation-multi-collector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) are crucial for exploring the origins of mafic lavas. However, quantitative use of this technique with low-Pb (<10 ppm) melt inclusions is difficult due to their low 204 Pb content and 204 Hg interference. METHODS: Pb isotopic ratios of various reference glasses and olivine-hosted melt inclusions were determined using LA-MC-ICP-MS. Multiple ion counters were used to simultaneously determine signal intensities of all Pb isotopes and 202 Hg. An Hg signal-removal smoothing device reduced its signal in the gas blank by >80%. Instrumental mass bias was corrected using the standard-sample bracketing method. RESULTS: With 24-90 µm diameter laser spots, 2-4 Hz repetition rates, and 2.5-4 J cm-2 energy fluence, the analytical precisions of 20x Pb/204 Pb ratios (x = 6, 7, 8) for standards BHVO-2G, ML3B-G, NIST 614, NKT-1G, T1-G, GOR132-G, and StHs6/80-G were <1.0% (2RSD) when 208 Pb signals >100 000 cps. The Wangjiadashan melt inclusions have 206 Pb/204 Pb = 17.14-18.44, 207 Pb/204 Pb = 15.28-15.66, and 208 Pb/204 Pb = 37.12-38.68. CONCLUSIONS: The described method improves the precision and accuracy of in situ Pb isotope analysis in low-Pb melt inclusions using LA-MC-ICP-MS. The Pb isotopic compositions of the Wangjiadashan melt inclusions indicate the coexistence of LoMu and EMII+young HIMU components in the mantle source of weakly alkaline basalts.
RESUMO
RATIONALE: In situ trace element analysis of melt inclusions by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) provides important geochemistry information. However, the precision and accuracy of this technique are affected by many factors, such as matrix effect, laser conditions, and calibration method. In addition, many previous LA-ICP-MS studies ablated entire melt inclusions along with their host minerals and obtained trace element composition by deconvoluting the mixed ablation signal, which may induce much uncertainty. METHODS: A 193 nm ArF laser ablation system combined with inductively coupled plasma sector field mass spectrometry (ICP-SF-MS) was used to investigate matrix effect, laser conditions, choice of external calibration standards, and data reduction strategy for in situ analysis of 36 major and trace elements in six common silicate reference glasses. The validity of the protocol presented here was demonstrated by measuring trace elements in olivine-hosted melt inclusions. Instead of ablating entire melt inclusions along with their host minerals, melt inclusions were polished to the surface to avoid laser ablating the mineral host. RESULTS: The calibration lines calculated from the calibration standards should cross the coordinate origin, especially for low-concentration elements (<10 ppm). As the laser crater size increased from 17 to 33 µm, the precision was improved from <20% to <8% (2RSD), and accuracy was improved from ±20% to better than ±10%. Most measured trace elements in Dali melt inclusions are consistent with those in their host rocks. For mobile elements (Ba, Sr, Pb), melt inclusions display much smaller variations than their host rocks. CONCLUSIONS: A simple but accurate approach for in situ analysis of trace elements in melt inclusions by LA-ICP-SF-MS has been established, which should greatly facilitate the wider application of in situ trace element geochemistry to melt inclusion studies.
