NO NUKES: DETERMINING FISSION TRACK AGES FROM ELECTRON MICROPROBE ANALYSIS

Recent investigations of the use of LA-ICP-MS in apatite fission track (FT) analysis have shown that it is possible to obtain FT ages without sample irradiation. Electron microprobe analysis (EMPA) can also be used in FT dating, provided minerals contain sufficient U concentrations for detection by EMPA. U and Pb concentrations in apatite are typically too low to be determined by EMPA for FT applications. Monazite, however, has sufficient quantities of U that can be measured with EMPA. We analyzed U, Th, Y, and Pb in grains from a monazite sand deposit from North Carolina using a JEOL 8900 electron microprobe. Analyses were conducted using a probe current of 200 nA, counting for 300 sec and 150 sec on peak and background, respectively. U was concurrently analyzed on two spectrometers with the average result reported. U was corrected for a Th peak overlap and Pb was corrected for Y and Th peak overlaps. EMPA ages for the monazite range from 350 to 370 Ma, consistent with granitic magmatism in the Inner Piedmont, NC.

In order to calculate an FT age based on these analyses, track density must first be converted to tracks per unit volume analyzed by the EMP, which is approximated using an electron flight simulator program. Tracks per unit volume is then converted to number of daughter atoms per 100 gm. As a test, this calculation was applied to Fish Canyon tuff (FCT) apatite single-grain counts and U concentrations determined from the reactor process. An example of resulting ages are 25.5 and 26.0, corresponding to standard irradiation age determinations of 27.9 and 28.0, respectively. Fission tracks in the monazite grains were etched in boiling 38% HCL for 45 minutes. Track density for these grains ranges from 2.6 x 10⁶ to 2.9 x 10⁶ tracks per cm². UO₂ concentrations obtained by EMPA for these same areas range from 0.51 to 0.60 weight %, which is converted into number of U atoms in 100 gm of sample. Calculated preliminary monazite FT ages for this monazite sand deposit range from 140 to 190 Ma, consistent with other moderate to low temperature FT ages from the region. This preliminary study illustrates the potential offered by monazite FT thermochronology once the general characteristics of fission tracks in monazite are more fully understood.