See more from this Division: General Discipline Sessions
See more from this Session: Geochemistry; Geochemistry, Organic
Abstract:
Prior to analysis, we tested the effect of acid strength and temperature on monazite dissolution rates. Our data demonstrate that 7090% of un-annealed monazite dissolves in a single dissolution step using 6N HCl for 12 hours at 120°C. Annealed monazite dissolves at slower rates3050%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 510 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 samplethe 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.
See more from this Division: General Discipline Sessions
See more from this Session: Geochemistry; Geochemistry, Organic