Removal of Hg interferences for common Pb correction when dating apatite and titanite by LA-ICP-MS/MS

Abstract

LA-ICP-MS U–Pb dating is one of the most commonly used geochronology methods for dating minerals such as zircon, apatite, monazite and titanite. However, many of these U-bearing minerals also contain non-radiogenic common Pb, which requires correction to accurately determine the age of the mineral. Correction methods include: determining the ²⁰⁷Pb/²⁰⁶Pb common-Pb ratio; the iterative Andersen (2002) method using the measured ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U and ²⁰⁷Pb/²⁰⁶Pb; using ²⁰⁸Pb for low Th-bearing minerals; or using the non-radiogenic ²⁰⁴Pb. To date, the ²⁰⁴Pb correction method has been limited by the isobaric interference from ²⁰⁴Hg, which requires mathematical correction by measuring ²⁰²Hg. In this study we demonstrate that for LA-ICP-MS analysis, the Hg interference can efficiently be removed by a charge transfer reaction using NH₃ gas in the reaction cell of an Agilent 8900 ICP-MS/MS, without any additional interference correction required. We demonstrate the applicability of the ²⁰⁴Pb correction method using a range of common Pb bearing apatites and titanites. The use of LA-ICP-MS/MS enables accurate ²⁰⁴Pb correction at ²⁰⁴Pb sensitivity lower than could be corrected previously.