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