ASSESSING THE UTILITY OF DETRITAL MONAZITE AGES FROM MAJOR RIVERS REQUIRES IDENTIFYING AND SEPARATING MONAZITE - NOT SO SIMPLE!

Many studies have demonstrated the utility of detrital zircon ages as an indicator of sediment provenance. However, due to the extreme physical and chemical resilience of zircon it can be recycled multiple times such that in some instances the frequency of specific ages of detrital zircon may be significantly overrepresented compared to the areal extent of crust of that age. Perhaps the best example of age frequency bias is the near ubiquity of zircon between 1.2 – 1.0 Ga in both Modern alluvium and in sedimentary rocks from eastern North America (the so-called ‘curse of the Grenville’).

In an attempt to mitigate the ‘Grenville curse’ we have been investigating detrital monazite ages in alluvium from rivers in the central and southern Appalachians. Monazite can form at much lower temperatures than zircon and is less resistant to physical weathering and thus less likely to survive as many multi cycles as zircon. Current results suggest that the frequency of detrital monazite ages more accurately reflects the abundance of exposed crust of different age domains within the watershed of the rivers studied. It is not known, however, if this the general case for major rivers in North America. We thus have begun an investigation into the utility of detrital monazite from the Mississippi river. The Mississippi was chosen, partly due to its enormous drainage area but also because several recent studies have reported detrital zircon ages from near the mouth of the river.

While identifying and isolating detrital zircon can be done with relative ease, identifying and separating detrital monazite from other magnetic dense minerals has proven difficult. We have employed a number of techniques, ranging from automated rapid magnetic separation of minerals from large volumes of sand to non-traditional heavy liquid based separations. The recovered grains from these processes are then subjected to SEM imaging and EDS analysis. Examples of the separation and identification procedures will be presented as will a photographic ‘atlas’ of the wide variety of morphologies and colors of detrital monazite. This information will be of value to other researchers interested in pursuing detrital monazite age studies.