Detrital uraninite and the early Earth's atmosphere: SIMS analyses of uraninite in the Elliot Lake District and the dissolution kinetics of natural uraninite (Ontario)

This thesis focuses on the “detrital” uraninite and pyrite in quartz-pebble conglomerate deposits of the pre-2.2 Ga age, which holds one of the most critical evidence for the model of the low-O₂ atmosphere. The questions to be addressed here are two-fold: (1) the origin(s) of uraninite grains in the quartz-pebble conglomerate deposits, and (2) the kinetic stability of uraninite and pyrite in surface environments.; Isotopic (Pb and O) and chemical compositions were determined by combining two in-situ techniques, SIMS (CAMECA ims 1270) and EPMA for over 50 sub-grain spots (∼20 μm) of uraninite from the Stanleigh Mine in the Elliot Lake district, Canada. Uraninite shows much lower δ 18O values of −10 to −25‰ when compared to the expected value for their pegmatite/granite derivation. 207Pb/ 206b ages of uraninite show a peak of ∼1.8 Ga, which is younger than depositional age of the host Huronian Supergroup (2.2 to 2.45 Ga).; A series of laboratory experiments was conducted to determine the dissolution kinetics of uraninite as a function of pH (4 to 7), pCO₂ (10 −3.5 to 10−0.5 atm), pO₂ (0.2 and <0.02 atm), and the composition (0.1 to 8 wt% ThO₂) of uraninite. The high surface roughness of natural uraninite explains the orders of magnitude of discrepancies in dissolution rates between natural uraninite and synthetic UO₂.; The empirical dissolution rate laws are obtained as: R=&cubl0;10^-7.8&sqbl0;H⁺&sqbr0; ^0.39+10^-7.8&sqbl0;HCO3 ^-&sqbr0;^0.75&cubr0;&sqbl0;O2&sqbr0; ^0.27&parl0;molm ^-2s^-1&parr0; for low Th uraninite (sample FR: ThO₂ < 0.4%), and: R=10^-9.2&sqbl0;H⁺&sqbr0;^0.39 +10^-10.2&sqbl0;HCO3 ^-&sqbr0;^0.43 &parl0;molm^-2 s^-1&parr0; for high Th uraninite (sample CN: ThO₂ = 8%), where [H+] and [HCO ₃−] are in mol/l. When the surface area is normalized by BET surface area, the dissolution rates of natural uraninite are found to be similar to that of synthetic UO₂.; A simple kinetic model was made based on the new experimental data on dissolution kinetics of uraninite together with the recent literature data on the dissolution kinetics for pyrite and feldspars. The model presented here imposed severe constraints on the survival of pyrite rather than uraninite. Uraninite becomes kinetically more stable than plagioclase at a pO₂ of about 10% PAL (Present Atmospheric Level), but pyrite is only stable below 1% PAL pO₂ under a pCO₂ of 1 to 1,000 PAL. (Abstract shortened by UMI.)...