Monazite: fast or slow growth in metamorphic rocks? Kersley, S.J. скачать в хорошем качестве

Monazite: fast or slow growth in metamorphic rocks? Kersley, S.J.

Monazite: fast or slow growth in metamorphic rocks? Kersley, S.J., Warren C.J., Kunz B.E., Argles T.W. and Roberts, N.M.W. *Correspondence: samuel.kersley@open.ac.uk. ___________________________________________________________________________ Interpretations of geochronological data from metamorphic rocks can be complex due to the preservation of protracted metamorphic evolution and/or later overprinting events throughout geological time. Furthermore, after their growth, geochronometers can further evolve and be isotopically and geochemically modified during prolonged deformation and metamorphism, making differences in the ages recorded by the same chronometer across regions difficult to evaluate. Recent studies show that U-Pb dates from geochronometer minerals can vary widely over an outcrop scale area, often outside of typical analytical uncertainties. This indicates that additional localised P-T-X parameters such as (local) bulk-rock composition, deformation, and dissolution/precipitation likely have an influence over the responses of accessory minerals on hence mineral age variability in different metamorphic environments. As such, assessing a geochronometer's variable trace-chemical behaviours alongside when/why the minerals crystalise in these specific environments is critical to assess what these minerals are capable of recording. Monazite from 8 samples from a single outcrop in Bhutan have been analysed using split-stream LA-ICP-MS for U-Pb age and 17 trace element concentrations simultaneously. These data, combined with optical petrography and internal textures (BSE and Cl images and EDS mapping) have allowed us to identify the presence of two groups. One group with U-Pb ages ranging from 32-15 Ma shows Y-rich rims, while the second group, where the Y-rich rims are absent, shows more clustered ages (ranging from 24-13 Ma). Group one shows younger ages in high-Y rims correlating with higher HREE concentrations, Eu anomaly, Sr concentrations. Group two shows younger ages correlating with higher MREE concentrations and Eu anomaly, but lower Sr concentrations. Preliminary results suggest that subtle variations in the overall bulk composition and petrogenetic assemblages could be the cause of the differences between the two groups of samples within the same outcrop.