Compositional and age mapping of monazite
Age maps of monazite are basically produced pixel by pixel using pixel values from high resolution X-ray maps of Th, U and Pb, as equivalent to concentrations, to solve the age equation of Montel et al. (1996). Age maps are constructed and analyzed using a new graphical user interface developed in Pascal using the Delphi programming environment from Borland: AgeMap program.
AgeMap program is available for IBM PC/compatibles:
View some example of the windows interface of the AgeMap program
Examples of compositional maps and associated age map:
Compositional map and age map of a monazite from sample S32D-M7, Snowbird Tectonic Zone, Canada (Mahan et al., 2003) showing a concentric compositional zoning. The size of the image is 84 * 81 µm. The age map constructed with the program AgeMap.exe shows two distinct age domains at about 2.5 Ga and 1.8 Ga. Mean pixel ages were calculated on the three rectangular areas with the AgeMap program.
X-ray maps and age map of monazite An6e-M6, Andriamena unit, North-Central Madagascar. The size of the image is 140 * 210 µm. Mean pixel ages were calculated on the four rectangular areas: (1) 18*40 pixels - 552 Ma, (2) 19*18 pixels - 558 Ma, (3) 20*40 pixels - 1103 Ma and (4) 19*18 pixels - 1065 Ma. A mean age of 614 Ma using 554 pixels was calculated on the young area indicated by the black arrow.
The calculated age map shows basically the same zoning as the Pb image. It consists of a large irregular core, a wide rim and small internal domains showing a prefered orientation. The core yields a mean age of about 1000 Ma, but in detail, the age map clearly indicates that it is not homogeneous in age but shows a continuous spatial variation from about 950 Ma old domains to significantly older areas up to 1650 Ma. The rim and small elongated domains within the core yield consistent ages around 530-600 Ma.
The most conspicuous observation in this age map is the recognition of small young domains (less than 5 µm in width) inside the old core. These raise several questions: What is the mechanism for the formation of these domains? What is the significance of the preferential orientation of these domains?