Interpretation of monazite ages obtained via in situ analysis

Abstract

Monazite grains from Nepal and Vietnam were compositionally analyzed with an electron microprobe and dated (Th-Pb) with an ion microprobe. Five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population: (1) Pb loss due to diffusion, (2) dissolution/reprecipitation reactions along a retrograde path, (3) analytical uncertainties, (4) analyses of overlapping age domains, and (5) episodic monazite growth. The influence of these factors is sample-dependent, but can be evaluated: (1) using peak metamorphic conditions and X-ray element maps to assess potential polymetamorphism or retrogression, (2) obtaining other geochronologic data including previous work or dating other minerals in the sample, (3) evaluating any method-related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses, and (4) ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element (REE) oxide or allanite. Chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms. Instead of relying on monazite chemical composition, major (Mn, Fe, Mg, Ca) and minor (Y) element garnet-zoning patterns and peak P-T conditions should be used to facilitate age interpretation. This thermobarometric data records the sample's thermal history, changes in garnet growth rate and mechanisms, and accessory mineral breakdown.