Detrital uraninite and the early Earth's atmosphere: SIMS analyses of uraninite in the Elliot Lake district and the dissolution kinetics of natural uraninite
Open Access
- Author:
- Ono, Shuhei
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 10, 2001
- Committee Members:
- Hiroshi Ohmoto, Committee Chair/Co-Chair
Kwadwo Asare Osseo Asare, Committee Member
Peter J Heaney, Committee Member
Lee Kump, Committee Member - Keywords:
- Proterozoic
atmosphere
oxygen
dissolution kinetics
quartz-pebble conglomerate
SIMS
Archean
detrital
uraninite - Abstract:
- <html> <body lang=EN-US style='tab-interval:.5in'> The occurrence of rounded grains of uraninite and pyrite in pre-2.2 Ga quartz-pebble conglomerate deposits represents key evidence for the model of the low-O<sub>2</sub> atmosphere until about 2.2 Ga. In this thesis, the relationship between detrital uraninite and paleo-pO<sub>2</sub> level was critically evaluated by addressing the following two questions: 1) what is(are) the origin(s) of uraninite in the quartz-pebble conglomerate deposits, and 2) what is the kinetic stability of uraninite and pyrite in surface environments.</p> Isotopic (Pb and O) and chemical compositions were determined by combining two <i style='mso-bidi-font-style:normal'>in-situ</i> techniques, SIMS and EPMA, for ~20 <span style='font-family:Symbol'>m</span>m-diameter areas of over 50 individual grains of uraninite from the Stanleigh Mine in the Elliot Lake district, Canada. Uraninite has much lower <span style='font-family:Symbol'>d</span><sup>18</sup>O values of -10 to -22 ‰ than expected for their pegmatite/granite derivation. <sup>207</sup>Pb-<sup>206</sup>Pb ages of uraninite centered near 1.8 Ga, which is younger than the depositional age of the host Huronian Supergroup (2.2 to 2.45 Ga). Although these isotopic characteristics point to a non-detrital origin of uraninite, they are likely to have been overprinted at ~1.8 Ga or later. Because of high Th concentrations in uraninite (4 to 12 wt. % ThO<sub>2</sub>) and the previously published data on Pb isotopic compositions of galena, the uraninite is most likely to be detrital in origin. </p> A series of laboratory experiments was conducted to determine the dissolution kinetics of uraninite as a function of pH (4 to 7), pCO<sub>2 </sub>(10<sup>-3.5</sup> to 10<sup>-0.5</sup> atm), pO<sub>2</sub> (0.2 and <0.02 atm), and Th contents (0.1 to 8 wt. % ThO<sub>2</sub>). The resulting empirical dissolution rate laws are</p> <p class=equation><i style='mso-bidi-font-style:normal'>R</i> = {10<sup>-7.8</sup> [H<sup>+</sup>]<sup>0.39</sup> + 10<sup>-7.8</sup>[HCO<sub>3</sub><sup>-</sup>]<sup>0.75</sup>}[O<sub>2</sub>]<sup>0.27</sup><span style='mso-tab-count:1'> </span>(mol m<sup>-2</sup> s<sup>-1</sup>)</p> <p class=MsoBodyText2>for low Th uraninite (sample FR: ThO<sub>2</sub> = 0.4 wt. %), and </p> <p class=equation><i style='mso-bidi-font-style:normal'>R</i> = 10<sup>-9.2</sup> [H<sup>+</sup>]<sup>0.39</sup> + 10<sup>-10.2</sup>[HCO<sub>3</sub><sup>-</sup>]<sup>0.43</sup><span style='mso-tab-count:1'> </span>(mol m<sup>-2</sup> s<sup>-1</sup>)</p> <p class=MsoBodyText2>for high Th uraninite (sample CN: ThO<sub>2</sub> = 8 wt. %), where [H<sup>+</sup>] and [HCO<sub>3</sub><sup>-</sup>] are in mol/l. When the surface area is normalized by BET method, the dissolution rates of natural uraninite are found to be slower than that previously determined rates by several orders of magnitude but similar to those of synthetic UO<sub>2</sub>. </p> A simple kinetic model was constructed based on new experimental data on dissolution kinetics of uraninite together with data from the recent literature on the dissolution kinetics for pyrite and feldspars. The model suggests that uraninite becomes kinetically more stable than plagioclase below 10 % of present atmospheric level (PAL) pO<sub>2</sub>, but pyrite is only stable below 1 % PAL pO<sub>2</sub> under a pCO<sub>2</sub> of 1 to 1,000 PAL. Thus the presence of uraninite and pyrite and the absence of plagioclase in the quartz-pebble conglomerate indicates that atmospheric pO<sub>2</sub> was less than 1 % PAL. </p> Occurrences of pyrite and uraninite in pre-2.2 Ga quartz-pebble conglomerate deposits are the expected consequence of a low-pO<sub>2</sub> (<1% PAL) atmosphere at the time of deposition. However, this expectation is based on a number of assumptions concerning the source rocks for the pyrite, uraninite, and feldspar and the nature of the weathering and depositional environment, in particular whether it acted as a closed or open system with short or long exposure times to the atmosphere. Moreover, the dearth of pyrite in Archean and Paleoproterozoic paleosols argues for atmospheric pO<sub>2</sub> values in excess of 0.001 % PAL. Paleosols may be more representative of the weathering environment than conglomerate deposits, whose sediments may have been derived from a variety of sources, and thus may ultimately be better indicators of the oxygen level of the ancient atmosphere.</p> </body> </html>