Disturbance of the monazite U-Th-Pb chronometer by fluids: A study combining stable isotopes and in situ dating

Abstract : The monazite U-Th-Pb chronometer is extensively used to date metamorphic rocks because it is usually considered to be resistant to diffusive Pb loss at temperatures as high as 800°C. However, age perturbations are sometimes documented in monazite grains showing distinct compositional and age domains. The mechanism by which Pb is lost is not well understood. Fluid-assisted recrystallization has been shown to constitute an efficient mean to reset the monazite chronometer in experimental conditions (Seydoux-Guillaume et al. 2002). Whether the same holds true in natural systems remains to be documented. Here we present in situ U-Th-Pb ages on monazite from foliated pegmatitic veins from highly strained levels of the Central Rhodope metamorphic pile (Greece, Bulgaria), together with oxygen isotope characterization and 40Ar-39Ar data. U-Th-Pb ages were obtained by (LA)-ICPMS on 8 monazite grains from three samples. In one sample from Bulgaria, the ages are concordant at 35 Ma, and no chemical zonation is observed. Quartz and feldspar have ..18O values consistent with equilibration at high temperatures. Microfabrics in the vein and the host rocks indicate high grade conditions of deformation. In the two other samples, from Greece, monazite grains show distinct chemical domains and yield discordant U-Th-Pb ages ranging from 52 to 36 Ma. Strong oxygen isotope disequilibrium between quartz and feldspar argues for a complex fluid history: high-..18O fluids first invaded rocks and were followed by low-..18O fluids at lower temperatures. Low temperature fluid circulation at around 36 Ma is consistent with 40Ar-39Ar dating of both samples at 33-34 Ma (mica plateau ages) and with microfabrics documenting a transition from high grade to greenschist facies conditions of deformation during intense shearing. These data confirm the capacity of fluids in disturbing significantly the monazite chronometer.