Department of Solid Earth Geochemistry (DSEG), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-Cho, Yokosuka 237-0061, Japan E-mail: [email protected] Fax: 81-46-867-9625 Tel: 81-46-867-9765 We present an improved U–Pb age dating method for zircon and monazite crystals using 193 nm excimer laser ablation and 200/266 nm femtosecond laser ablation (200/266Fs LA) multiple-Faraday collector inductively coupled plasma-mass spectrometry (MFC-ICP-MS).Optimization of a 266 fs laser beam enabled an analysis of U ratios with an in-run precision of 1–2% from a crater of dimensions 50 μm × 10 μm (diameter × depth) at a repetition rate of 2 Hz for 30 s.The access to the FIB facility was possible thanks to the French RENATECH network.
Analyses along a grain boundary can yield chemical concentrations that are not of pure monazite, due to the overlap of the analysis volume and the matrix surrounding the monazite grain (Fig. Analytical error in electron microprobe analysis may also be problematic in finding reliable age domains for monazite.Thompson Mine and Monangotory standard monazites, dated using a 44069 monazite crystal (424.9 ± 0.4 by ID-TIMS) as a standard, also reproduced the U–Pb ages determined by ID-TIMS/LA-MFC-ICP-MS, but at a sample volume four times smaller than that required for zircons.Zircon and monazite ages are accurate given the small offsets from ID-TIMS ages, 0.15–0.7% for zircons and 0.2–0.7% for monazite well within internal precision from the primary standard in the analytical session and competitive with an internal precision of 0.43–0.6% for zircon and 0.2–0.8% for monazite.It is useful for geochronology because it contains relatively large amounts of uranium and thorium, which decay radioactively to lead.Typically, there is negligible non-radiogenic lead in monazite.These three domains yield distinct isotopic U–Pb ages of 1034 ± 6, 1005 ± 7, and 935 ± 7 Ma, respectively.Uranium–Th–Pb EPMA dating independently confirms these ages.Inheritance of earlier (detrital or igneous) monazite crystals is noted to be possible, but uncommon in monazites in igneous and metamorphic rocks.In metamorphosed pelites, this would be signified by older monazite cores of possibly earlier metamorphic or igneous origin with younger overgrowths of metamorphic monazite (Parrish, 1990). Matrix monazites were typically aligned in the dominant foliation, S2 (Fig.Sources of analytical error include differing x-ray absorption properties of different local minerals as well as fluorescence or excitation of additional x-rays in surrounding minerals through which the x-rays pass.Problems with selecting accurate spectra backgrounds, as well as peak overlaps, can also affect concentration calculations, and therefore age accuracy and precision (Williams, pers. Inclusions of quartz, feldspar, iron oxides, possibly Al2Si O5, and accessory minerals are noted to occur in monazites (Parrish, 1990).