Monazite analysis; from sample preparation to microprobe age dating and REE quantification

Despite the recognized importance of monazite in geochronology and petrology, a range of fundamental analytical and preparational problems remains. For example, chemical Th-U-Pb dating of monazite requires special lead-free sample preparation. This is achieved efficiently and at high quality with specially developed grooved ND-PE polyethylene polishing disks. Techniques useful in locating and characterizing monazite are evaluated. Back scattered electron imaging is an effective way to determine zonation patterns, particularly with respect to thorium. Quantitative analysis of monazite by EMP is delicate and time consuming. A whole series of X-ray peak interferences has been ignored in published work. For example, for monazite containing 12% Th, the commonly disregarded interference of Th hit on Pb hla causes an overestimation of 11% (relative) in Pb. This propagates to an age overestimation of similar to 50 Ma for a sample of 400 to 500 Ma in age. A judicious choice of X-ray peaks used in quantitative EMP analysis avoids or minimises peak overlap for all elements, including REE. Only for U a correction factor is required: U wt%(corrected) = U wt%(measured) - (0.0052 * T wt%(measured)) based on the analytical lines U Mb and Th Ma.