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The Pennsylvania State University the Graduate School College Of The Pennsylvania State University The Graduate School College of Earth and Mineral Sciences DETRITAL URANINITE AND THE EARLY EARTH’S ATMOSPHERE: SIMS ANALYSES OF URANINITE IN THE ELLIOT LAKE DISTRICT AND THE DISSOLUTION KINETICS OF NATURAL URANINITE A Thesis in Geosciences by Shuhei Ono © 2001 Shuhei Ono Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 2001 We approve the thesis of Shuhei Ono. Date of Signature Hiroshi Ohmoto Professor of Geochemistry Thesis Advisor Chair of Committee Lee R. Kump Professor of Geosciences Peter J. Heaney Associate Professor of Geology Kwadwo Osseo-Asare Professor of Metallurgy Peter Deines Professor of Geosciences Associate Head of the Graduate Research Program in Geosciences iii ABSTRACT 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-O2 atmosphere until about 2.2 Ga. In this thesis, the relationship between detrital uraninite and paleo-pO2 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. Isotopic (Pb and O) and chemical compositions were determined by combining two in-situ techniques, SIMS and EPMA, for ~20 µm-diameter areas of over 50 individual grains of uraninite from the Stanleigh Mine in the Elliot Lake district, Canada. Uraninite has much lower δ18O values of –10 to –22 ‰ than expected for their pegmatite/granite derivation. 207Pb-206Pb 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. % ThO2) and the previously published data on Pb isotopic compositions of galena, the uraninite is most likely to be detrital in origin. A series of laboratory experiments was conducted to determine the -3.5 -0.5 dissolution kinetics of uraninite as a function of pH (4 to 7), pCO2 (10 to 10 atm), pO2 (0.2 and <0.02 atm), and Th contents (0.1 to 8 wt. % ThO2). The resulting empirical dissolution rate laws are -7.8 + 0.39 -7.8 - 0.75 0.27 -2 -1 R = {10 [H ] + 10 [HCO3 ] }[O2] (mol m s ) for low Th uraninite (sample FR: ThO2 = 0.4 wt. %), and iv -9.2 + 0.39 -10.2 - 0.43 -2 -1 R = 10 [H ] + 10 [HCO3 ] (mol m s ) + - for high Th uraninite (sample CN: ThO2 = 8 wt. %), where [H ] and [HCO3 ] 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 UO2. 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) pO2, but pyrite is only stable below 1 % PAL pO2 under a pCO2 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 pO2 was less than 1 % PAL. Occurrences of pyrite and uraninite in pre-2.2 Ga quartz-pebble conglomerate deposits are the expected consequence of a low-pO2 (<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 pO2 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. v TABLE OF CONTENTS LIST OF FIGURES..........................................................................................viii LIST OF TABLES ............................................................................................ xii ACKNOWLEDGEMENTS..............................................................................xiii CHAPTER 1 INTRODUCTION............................................................................................... 1 ORIGIN(S) OF “DETRITAL” URANINITE: IS IT EVIDENCE FOR A LOW PO2 ATMOSPHERE? ................................................................................................ 2 QUANTITATIVE ESTIMATES OF PO2: HOW LOW IS LOW?...................................... 4 SIGNIFICANCE................................................................................................... 6 CHAPTER 2 IN-SITU SIMS ANALYSES OF OXYGEN AND LEAD ISOTOPIC COMPOSITION OF URANIUM-BEARING MINERALS FROM THE STANLEIGH MINE, THE ELLIOT LAKE DISTRICT, ONTARIO, CANADA .................................................................................................... 9 ABSTRACT........................................................................................................ 9 INTRODUCTION............................................................................................... 10 Age distribution of uranium deposits and the evolution of atmospheric oxygen....................................................................................................... 10 The Huronian Supergroup and “the rise of oxygen”................................... 12 Objective of this study ............................................................................... 15 METHOD ........................................................................................................ 17 Samples..................................................................................................... 17 SIMS isotopic and EPMA chemical analyses............................................. 17 Oxygen isotopic composition of sericite .................................................... 20 RESULTS AND DISCUSSION.............................................................................. 21 Textures and compositions of uranium minerals and associated minerals... 21 vi Uranothorite and uraninite ..................................................................... 21 Brannerite.............................................................................................. 25 Organics ................................................................................................ 28 Pyrite..................................................................................................... 29 Sericite-Microcline ................................................................................ 29 Uranium - lead ages of uranothorite, uraninite and brannerite .................... 30 Discordia ages of uranium minerals ....................................................... 31 207Pb-206Pb ages of uranium minerals ...................................................... 35 Oxygen isotope systematics of uraninite .................................................... 35 AN ALTERNATIVE MODEL ............................................................................... 41 SUMMARY...................................................................................................... 43 CHAPTER 3 THE DISSOLUTION KINETICS OF NATURAL URANINITE AS A FUNCTION OF PH, PCO2, AND PO2 DETERMINED IN A THIN-LAYER FLOW-THROUGH REACTOR................................................................. 74 ABSTRACT...................................................................................................... 74 INTRODUCTION............................................................................................... 76 EXPERIMENTS................................................................................................. 79 Samples..................................................................................................... 79 Flow through system.................................................................................. 81 pH change ................................................................................................. 84 SEM observation of surface textures.......................................................... 85 EXPERIMENTAL SERIES ................................................................................... 86 Experiment I (EXI).................................................................................... 86 Experiment II (EXII) ................................................................................. 88 DISCUSSION.................................................................................................... 89 Effect of flow rates .................................................................................... 89 Empirical rate law and comparison with previous research ........................ 91 Surface area of natural uraninite and UO2 glass.......................................... 95 Dependence on oxygen concentration and the dissolution model ............... 98 CONCLUSIONS .............................................................................................
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