Monazite dating using the electron microprobe has become increasingly popular over the past decade, especially as a tool for constraining the age of deformation or metamorphic events. The high spatial resolution, and in-situ, non-destructive nature of the technique allows unique access to age information directly from the same thin sections from which petrologic or microstructural relationships are obtained. Detailed compositional mapping of individual monazite grains reveals micro-domains wich can often be linked to microstructures, metamorphic minerals, or metamorphic reactions, allowing specific timing constraints to be placed on stages in the structural or metamorphic history. Although core domains in some grains may be large enough to be dated by ion probe or other isotopic means, dates on narrow rim domains or irregularly shaped internal domains require the micron-range resolution that only the electron microprobe can provide. Although less precise than isotopic methods, microprobe dating can therefore contribute substantially to a complete understanding monazite geochronology.

At UMass, we are carrying out monazite dating as part of mapping and tectonic analysis of the Western Churchill Province, Canada, in the Southwestern US, and collaboratively in a number of other areas around the world. In Canada, this research is allowing us to map out the fundamental domains of >3.0 Ga, 2.6 Ga, and 1.9-1.8 Ga tectonism, and to identify important tectonic boundaries. This work is already having a substantial impact on our understanding of the evolution of the Canadian Shield. In the southwestern US, monazite research is revolutionizing models of 1.65 Ga vs. 1.4 Ga tectonics and the proterozoic assembly of southern Laurentia.

Microprobe monazite dating is much more than a reconnaissance tool. Because of the in-situ nature and very high spatial resolution, microprobe dating can provide insight not obtainable by other methods. Three specific types of applications are clear: 1) timing metamorphic events; 2) timing deformational events; and 3) providing insight into companion geochronologic data. Follow the links above to see more...

Microprobe dating requires the measurement of Th, U, and Pb, along with Y and any other elements required for interference correction. Although Th and Y can be present as “major” elements, U and Pb concentrations in monazite are typically on the order of 100s to 1000s of ppm (i.e. trace components) and are a challenge for electron probe microanalysis (EPMA). Because uncertainties associated with microprobe dates are particularly dependent on precise and accurate trace element analyses, a significant amount of attention is now paid to analytical issues such as interference correction, background estimation and other detailed analytical issues.