Author Response for "improved in Situ Analysis of Lead Isotopes in Low‐pb Melt Inclusions by LA–MC–ICP–MS"

RATIONALE In situ Pb isotope analyses of tiny melt inclusions by laser ablation-multi-collector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) is 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 by 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 by LA-MC-ICP-MS. The Pb isotopic compositions of the Wangjiadashan melt inclusions indicate coexistence of LoMu and EMII+young HIMU components in the mantle source of weakly alkaline basalts.