Authigenic monazite and xenotime Pb-Pb/U-Pb dating of siliciclastic sedimentary rocks

Abstract

Authigenic xenotime (YPO4) and monazite ((Ce, La, Nd)PO4) are common trace components in siliciclastic sedimentary rocks, where they can form under diverse geological processes such as low-temperature early diagenesis or hydrothermal activities or later low grade metamorphism. Authigenic xenotime and monazite could carry distinctive geochemical fingerprints. By virtue of their commonly high U and Th concentrations (commonly 100–1000 ppm) and high closure temperature for U/Th-Pb isotope systems (700–800 °C), dates from authigenic phosphate are frequently used to constrain the depositional age of sedimentary rocks. Their tiny size (commonly <20 μm but can be occasionally up to about 1 mm in diameter for large monazite nodules) requires SEM-BSE imaging to locate for in situ high spatial and mass resolution SIMS U-Pb/Pb-Pb dating analyses. Their variable chemical compositions, particularly xenotime, require multiple (at least three) matrix-matching standards to correct UPb fractionation. Interpretation of radiometric ages sometimes encounters difficulties because of diverse origins of phosphates including diagenetic, hydrothermal, and detrital origins. An integrated petrographic and geochemical study can aid in telling apart diagenetic xenotime/monazite from their hydrothermal and detrital counterparts, thereby correctly applying them to constrain the depositional age of sedimentary rocks.