Chemical Abrasion Thermal Ionization Mass Spectrometry (CA-TIMS) Method Development for Monazite

Because of the demonstrated benefits of analyzing zircon with the chemical abrasion (CA) TIMS method, we are evaluating the method's utility for monazite. Six monazite samples with known TIMS ages of 45 to 1395 Ma were selected for analysis on the basis of their suitability for use as standards. An aliquot of each sample was annealed at 1000°C for 48 hours prior to step-wise dissolution; a control aliquot of three of these samples was analyzed by step-wise dissolution without annealing.

Prior to analysis, we tested the effect of acid strength and temperature on monazite dissolution rates. Our data demonstrate that 70–90% of un-annealed monazite dissolves in a single dissolution step using 6N HCl for 12 hours at 120°C. Annealed monazite dissolves at slower rates—30–50%—under the same conditions. Experiments with 6N HCl for 12 hours at 80°C yielded dissolution rates of ~10% per step for annealed samples.

To allow for ten or more dissolution steps, we used 3.1N HCl at 80°C for 12 hours for all 9 samples. Preliminary data reveal the following: 1) Un-annealed samples dissolve 5–10 times faster than their annealed counterparts. 2) Annealed fractions of 5 out of 6 samples yielded “too young” U-Pb ages, suggesting that outer portions of the grains either experienced some Pb loss, or may represent later-stage growth. 3) U-Pb ages for the second step are equivalent to the “known age” of the samples, suggesting that the first step removed most or all of the monazite that experienced Pb loss or was a younger overgrowth. The exception is the Proterozoic sample—the first two dissolution steps yield “too old” U-Pb ages, evidently reflecting preferential leaching of Pb. We attribute this to the sample being more metamict than younger samples, such that annealing produced only limited repair of the radiation damage.