Experimental resetting of the U–Th–Pb systems in monazite
Résumé
Abraded fragments (200–400 µm) of a large, chemically homogeneous, and non-metamict Brazilian monazite crystal, characterised by a concordant U–Pb ages of 474 +/- 1 Ma (208Pb/206Pb = 19.5), were hydrothermally treated at varying temperatures with solutions of different compositions. Product monazites were analysed with Scanning Electron Microscope (SEM), Electron Microprobe (EMP), Secondary Ion Mass Spectrometer (SIMS) and Isotope Dilution–Thermal Ionisation Mass Spectrometer (ID-TIMS). Experiments with pure water over a temperature range of 800–1200°C, at 700 MPa and durations ranging from 5 to 60 days showed that even at 1200°C any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain. Neither Pb diffusion at the EMP scale, nor significant discordancy were observed. We performed experiments at 800 and 1000°C for different durations using different fluid compositions at quartz saturation: a 10 wt.% CaCl2 fluid, a 10 wt.% SrCl2 fluid, a 10 wt.% NaCl fluid and a fluid containing NBS 982 Pb standard (208Pb/206Pb = 1). For all runs, EMP traverses revealed no Pb-diffusion profiles. Significant overgrowths of newly formed monazite are documented by SEM analyses. They occurred only in the 1000°C experiments when either CaCl2 or Pb-bearing fluids were present. In the CaCl2 experiment, two zones could be distinguished within the crystal: a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution/precipitation, which was enriched in Ca and Pb-free. ID-TIMS dating of single grains treated with SrCl2 and CaCl2 solutions at 1000°C are significantly discordant. Experiments employing the NBS Pb-standard produced sub-concordant monazite, for which the 207Pb/206Pb apparent age has become older than prior to the experiment (544 Ma at 800°C and 495 Ma at 1000°C). The newly grown monazite rim had obviously incorporated Pb from the fluid. None of our reaction products contained a detectable diffusion profile. The only resetting mechanism we detected involved dissolution/precipitation. The extent of the dissolution/precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks.