Microprobe Age Dating and Mapping of Monazite

[John Donovan]

The following screen shot shows the raw data (on and off-peak) for a monazite grain (courtesy of Julien Allaz- Univ of Colorado) for U, Th, Pb, Y and La:



Note the "hot spots" in the Pb Ma high off-peak image from sulfur ka, which illustrates the necessity of the multi-point background method of Jercinovic and Allaz, which can dynamically avoid such off-peak interferences from unexpected elements. See this paper for further details:

http://probesoftware.com/download/BoT&A_Jercinovic.pdf

and here for more technical information on using multi-point backgrounds in Probe for EPMA:

http://probesoftware.com/smf/index.php?topic=56.0

After processing in CalcImage for background, matrix and interference (Y and La on Pb) corrections we obtain the following quantitative images:



Note that the Z values are in elemental weight percent values.  By applying the method of Montel (from Allaz) we can calculate the chemical ages using this menu in CalcImage:



We then obtain the U-Th-Pb age images as seen here:



which reveal the chemical age (lower right), Pb from U (upper left), Pb from Th (lower left) and calculated Pb (upper right). The presentation output from the Project menu is seen here:

[John Donovan]

For some excellent discussion on quantification of monazite chemistry, please see this topic by Julien Allaz:

http://probesoftware.com/smf/index.php?topic=186.0

I should note that there are various terms for dating of monazites by EPMA but one term that seems appropriate is Chem Age. I sort of like EPMA Dating.