Monazite Geochronology

Monazite is a light rare earth element (LREE)-bearing phosphate mineral.  Crystals typically contain distinct chemical domains, each of which represent successive growth thru geologic history.  Electron microprobe analysis can characterize the geometry and U-Th- total Pb age for each domain. This kind of data allow the growth of monazite to be related to geologic events affecting the host rock.

Monazite is common in pelitic and psammitic metamorphic rocks at greenschist facies and above where it is often recognized as inclusions in porphyroblasts but may also be in direct connection with the matrix. Locating monazite grains can be done on standard geological thin sections via x-ray compositional mapping. Figure 7A reveals a cerium x-ray map on a quartzite sample from the Cheyenne belt. Peaks in cerium content may correlate to large monazite grains. Figure 8 shows compositional zonation on selected grains identified in figure 7. These individual domains likely represent successive generations of monazite growth and can be targeted for crystallization dates. Dating monazite follows the U-Th total Pb age method which is described elsewhere in the 5690 website. I will not discuss the specifics of this system but I have included the equation from which an age is determined:

Monazite crystallizing from igneous material may be concentrically zoned due to evolving melt composition. Metamorphic monazite typically shows extensive and complex compositional variation. Interfingering of domains, cross-cutting of igneous domains and multiple metamorphic domains with complex relationships are typical of metamorphic monazite.

The interpretation of compositional and geochronologic datasets from compositionally zoned monazite are of most value when element diffusion is negligible. The closure temperature, “the temperature at which solid-state diffusion of elements in the crystallographic lattice effectively ceases to be statistically meaningful at the resolution of analytical technique” is at 800˚C for prolonged time periods.

Electron microprobe geochronology of monazite allows for in situ microanalysis of individual growth domains. This technique involves two critical aspects: obtaining adequate detection limits and errors relative to the questions asked and obtaining accurate concentrations. If done properly this method can allow the analyst to calculate ages of each domain. These domains in turn can be related to processes affecting the host rock and potentially regional tectonic events. Figure 9 illustrates one possible approach to attaining ages of compositional domains within a monazite crystal from the Cheyenne belt. This grain shows complex zoning of Ca, Y, Th and U and each zone may represent either initial igneous growth or subsequent metamorphic growth.