Improved in situ analysis of lead isotopes in low-Pb melt inclusions using laser ablation-multi-collector-inductively coupled plasma-mass spectrometry

Abstract

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.