Chapter 6 - U–Th–Pb phosphate geochronology by LA-ICP-MS

Abstract

Apatite, monazite, and xenotime are often the dominant carriers of U and Th in crystalline rocks. Apatite is nearly ubiquitous in igneous, metamorphic, and clastic sedimentary rocks, while monazite-xenotime is common in metapelites and alkaline and peraluminous felsic igneous rocks. The availability of U–Th–Pb phosphate age standards has improved significantly, particularly for apatite. This, combined with data reduction protocols accounting for variable common Pb in age standards, means that U–Th–Pb dating of phosphate minerals by LA-ICP-MS has now become routine. Monazite-xenotime in magmatic, metamorphic, and hydrothermal systems exhibits significant textural complexity, and in situ petrological characterization is essential. Simultaneous acquisition of petrogenetically-important trace elements in U–Th–Pb LA-ICP-MS phosphate geochronology (either using quadrupole-ICP-MS or split-streaming) is key. In petrochronology studies, it links dates to specific petrological process(es) and the P-T-t evolution of the rock. In U–Pb provenance studies, it enables source-rock fingerprinting, with detrital apatite and monazite now commonly investigated in ancient basins and modern sediments.