High resolution (5 μm) U–Th–Pb isotope dating of monazite with excimer laser ablation (ELA)-ICPMS

Abstract

Monazite [(LREE)PO 4], a common accessory mineral in magmatic and metamorphic rocks, is complementary to zircon in U–Th–Pb geochronology. Because the mineral can record successive growth phases it is useful for unravelling complex geological histories. A high spatial resolution is required to identify contrasted age domains that may occur at the crystal-scale. Bulk mineral techniques such as ID-TIMS, applied to single monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages. Laser ablation (LA)-ICPMS has been demonstrated to be a useful technique for U–Th–Pb dating of zircons, and this study tests its analytical capabilities for dating monazite. A sector field high resolution ICPMS coupled with a 193 nm ArF excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements. The U–Th–Pb systematic was applied to monazite grains from several samples: a lower Palaeozoic lens from high-grade terrains in Southern Madagascar, Neogene hydrothermal crystals from the Western Alps, a Palaeoproterozoic very high temperature granulite from central Madagascar and a Variscan leucogranite from Spain, directly on a polished thin section. The major aim was to compare and/or reproduce TIMS and EMP ages of monazite from a variety of settings and ages. The three independent 206Pb/ 238U, 207Pb/ 235U and 208Pb/ 232Th ratios and ages were calculated. Isotope fractionation effects (mass bias, laser induced fractionation) were corrected using a chemically homogeneous and U–Pb concordant monazite as external standard. This study demonstrates that excimer laser ablation (ELA)-ICPMS allows U–Th–Pb dating of monazite with a high level of repeatability, accuracy and precision as well as rapidity of analysis. A spatial resolution almost comparable to that of EMP in terms of crater width (5 μm) produced precise 208Pb/ 232Th, 206Pb/ 238U and 207Pb/ 235U ratios for dating Palaeozoic to Precambrian monazites. The advantages of (ELA)-ICPMS isotope dating are precision, accuracy and the ability to detect discordance. In the case of late Miocene hydrothermal monazites from the Alps, a larger spot size of 25 μm diameter is required, and precise and accurate ages were obtained only for 208Pb/ 232Th systematics. Results from the Variscan granite show that in situ U–Th–Pb dating of monazites with (ELA)-ICPMS is possible using a 5 μm spot directly on thin sections, so that age data can be placed in a textural context.