The monazite grain is pale green and surrounded by a brown halo in the host garnet. This halo is interpreted as resulting from radiation damage , facilitated by a very high Th content in the monazite and at least 2.6 Ga of exposure. No quantitative analyses have been performed, but examination of EDS spectra finds very significant amounts of Th and Si, and even a noticable peak for Pb. Suggestions of heterogeneity in backscatter prompted the collection of a series of very high resolution WDS element maps. In contrast to most of the images presented on these pages which are collected over large areas by rastering the stage, these maps are collected over small areas by rastering the beam. The slight differences in scale and orientation between the upper and lower pairs of images are due to minor changes in instrument conditions on successive acquisitions.
Many notable features are evident in the maps. Most conspicuous is the polyphase growth history clearly represented by each of the four elements shown. The spatial covariance of Th and Si verifies the supposed Th + Si = (LREE) + P substitution mechanism for these elements in monazite. Of some relevance to radiometric dating is the behavior of Pb. The maps of Th and Pb show the same distribution, though Pb is in somewhat less detail owing to its very small concentration. This cuts both ways. That Pb should not migrate detectably from its source Th even in an extremely high temperature setting is very encouraging , as it indicates a very high closure temperature for radiometric dating.. On the other hand, if the very complex growth history shown here is at all common in metamorphic monazites, then one must question what exactly is being dated. There is, of course, the possibility that these different growth zones do not differ significantly in age, but only in their Th content.
For more information about this image contact: Michael
L. Williams