In Situ U-Th-Pb Dating and Sr-Nd Isotope Analysis of Bastnasite by LA-(MC)-ICP-MS

Bastnasite is the end member of a large group of carbonate-fluoride minerals with the common formula (REE) CO3F center dot CaCO3. This group is generally widespread and, despite never occurring in large quantities, represents the major economic light rare earth element (LREE) mineral in deposits related to carbonatite and alkaline intrusions. Since bastnasite is easily altered and commonly contains inclusions of earlier-crystallised minerals, in situ analysis is considered the most suitable method to measure its U-Th-Pb and Sr-Nd isotopic compositions. Electron probe microanalysis and laser ablation (multi-collector) inductively coupled plasma-mass spectrometry of forty-six bastnasite samples from LREE deposits in China, Pakistan, Sweden, Mongolia, USA, Malawi and Madagascar indicate that this mineral typically has high Th and LREE and moderate U and Sr contents. Analysis of an in-house bastnasite reference material (K-9) demonstrated that precise and accurate U-Th-Pb ages could be obtained after common Pb correction. Moreover, the Th-Pb age with its high precision is preferable to the U-Pb age because most bastnasites have relatively high Th rather than U contents. These results will have significant implications for understanding the genesis of endogenous ore deposits and formation processes related to metallogenic geochronology research.