Microprobe dating

Problems with selecting accurate spectra backgrounds, as well as peak overlaps, can also affect concentration calculations, and therefore age accuracy and precision (Williams, pers. Inclusions of quartz, feldspar, iron oxides, possibly Al2Si O5, and accessory minerals are noted to occur in monazites (Parrish, 1990).

Petrographic thin section analysis, backscatter electron imaging, and microprobe x-ray mapping showed few significant inclusions in the dated grains.

For a variety of reasons linked to the minimum melt composition of these leucogranites, these dating studies have not been entirely successful.

Monazites from these migmatites typically show high Th and Pb cores and high Y and U rims.

However, chemical age calculations from electron microprobe analyses at points in both recognized cores and rims of the grains produce similar ages within 1-sigma uncertainty (Figs. Figure 3: X-ray maps showing concentrations of U, Pb, Th, and Y for monazite 57.

) grain may be used to calculate the age of formation of that grain. Examples are shown for a range of natural samples with a wide range of ages.

Comparison with ages derived from isotopic measurements shows the results to be valid.

High-resolution x-ray mapping of monazite reveals zoning patterns in yttrium, uranium, thorium, and lead which suggest multiple stages of growth and resorption (Williams et al., 1999).

Point analyses of each monazite grain were done to determine the elemental proportions of uranium, thorium, and lead.

The method is particularly useful for samples where the minerals and mineral textures record information on more than one event.

Two examples of samples recording information on dual ages are discussed to show how these ages can be related to events in the formation of the host rocks.

Monazite ages obtained from this leucogranite yield two distinct populations, a large number of ages with a normal distribution and mean age of 22.4 ± 0.5 Ma (±2 S.

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