ELECTRON MICROPROBE DATING OF MONAZITE: DISSECTION OF A MULTIVARIABLE ANALYTICAL PROTOCOL, WITH APPLICATION TO "TRIPLE-POINT" ROCKS, NORTHERN PICURIS MOUNTAINS, NEW MEXICO

Detector Gas:Energy differentials between second order La and Ce L a and Ar K a generate escape peaks that overlap Pb voltage pulses; Ce and La L X-ray peaks close to Pb M X-ray peaks are filtered only at the expense of nearly all Pb M-peak X-ray counts for gas flow detectors. The energy differential between Xe and second order La and Ce peaks generates escape peaks outside of the Pb voltage window, resulting in greatly diminished second order Ce and La peaks in the Pb M region for sealed X-ray detectors. Additionally, scans of the Pb region with P10 detectors and various collimator settings reveal anomalous spectral background curvature, complicating placement of Pb background collection position. For these reasons, use of Xe detectors for Pb analysis is recommended.

Pb Analytical line and interferences:For standard monazite analytical conditions, single point Poisson statistics for monazite with ~1500 ppm Pb average 5-7% (±90 ppm) for Pb M b vs 3-4 % (± 50 ppm) for Pb M a; for ~7500 ppm Pb, counting errors decrease to 1.3-1.6 % (± 100 ppm) for Pb M b and 0.7-0.9 % (± 50 ppm) for Pb M a. Grouped age standard errors (n > 25) for homogeneous domains in monazite are comparable to standard errors in counting statistics. On Xe detectors, Pb M a is subject to Th (M z) and Y (L g) interference, whereas Pb M b is subject to only U (M z) interference; correction factors are derived from analysis of Pb lines in Pb-free Y, Th, and U standards, and applied to uncorrected Pb k-ratios prior to ZAF correction. Error propagation on corrected counts introduces little additional error (+0.001-0.003%) to uncorrected counting errors, implying that Pb M a is the superior analytical line.

Picuris Monazite:Monazites from 4 ky+sil±and-bearing samples are found as matrix grains and as inclusions in all three Al-silicates. U+Y zoning defines 2-3 domains per grain, some with irregular, high-Th cores. Except for some high-Th and/or high-S domains, M a and M b ages are in general agreement. Applying the U/Pb vs Th/Pb technique of Cocherie and Albarede (2001) yields precise weighted core +rim U-Th-Pb M a ages of 1428±25 (1409±40 for M b) My to 1359±22 My for the 4 samples. No evidence of ~1650 My metamorphism is recorded by monazites from these samples.