Uranium irradiation history of carbonado diamond; implications for Paleoarchean oxidation in the Sao Francisco Craton

Carbonado is a porous polycrystalline diamond rock found in central African and Brazilian placer deposits. It contains unsupported radiogenic isotopes of He, Ne, Kr, Xe, and Pb. Here we show that these, and the radiation-related defects introduced to the diamond structure, are a result of uranium pr...

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Bibliographic Details
Published in:Geology (Boulder) 2016-07, Vol.44 (7), p.527-530
Main Authors: Magee, Jr, Teles, Guilherme, Vicenzi, Edward P, Taylor, Wayne, Heaney, Peter J
Format: Article
Language:English
Subjects:
actinides
Archean
atmosphere
Brazil
carbonado
Chapada Diamantina Group
diamond
Diamonds
enrichment
Espinhaco Supergroup
geochemistry
Geological time
isotope ratios
isotopes
lead
Lithosphere
metals
native elements
nesosilicates
orthosilicates
Oxidation
oxygen
Paleoarchean
paleoenvironment
Pb-206/Pb-204
Pb-207/Pb-206
polycrystalline materials
Polycrystals
Precambrian
Proterozoic
radiation damage
radioactive isotopes
radioactivity
Radioisotopes
Rocks
Sao Francisco Craton
silicates
South America
stable isotopes
Stratigraphy
textures
Tombador Formation
upper Precambrian
uranium
zircon
zircon group
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Summary:Carbonado is a porous polycrystalline diamond rock found in central African and Brazilian placer deposits. It contains unsupported radiogenic isotopes of He, Ne, Kr, Xe, and Pb. Here we show that these, and the radiation-related defects introduced to the diamond structure, are a result of uranium precipitation, with no isotopic or spectroscopic evidence of Th enrichment. The daughter products are unsupported due to Proterozoic U remobilization. Combining existing carbonado Pb isotope data with recent studies of the geochronology of the tectonic evolution of the Sao Francisco craton (eastern South America) reveals that the most likely scenario is Paleoarchean uranium enrichment of carbonado, followed by Mesoproterozoic uranium dissolution. Under all possible scenarios, the carbonado radiation damage history requires U mobilization in the Mesoarchean or late Paleoarchean. This is consistent with recent studies of South Africa and India Mesoarchean paleosols, which also show evidence for local oxygen activity greater than that of the Archean atmosphere and ocean. While those studies rely on whole-rock trace element and transition metal stable isotope measurements, this combination of crystallographic defects, sedimentary geochronology, and radiogenic isotopes supports the same conclusions of nonmarine, near-surface Archean oxygen enhancement.
ISSN:0091-7613
1943-2682
DOI:10.1130/G37749.1