Chemistry of sands from the modern Indus River and the Archean Witwatersrand basin: Implications for the composition of the Archean atmosphere

Journal Article · Fri Mar 01 00:00:00 EST 1991 · Geology; (USA)

DOI:https://doi.org/10.1130/0091-7613(1991)019<0265:COSFTM>2.3.CO;2· OSTI ID:5598116

Maynard, J B; Ritger, S D ^[1]; Sutton, S J ^[2]

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1. Univ. of Cincinnati, OH (USA)
2. Univ. of Texas, Austin (USA)

Both the Indus River and the Witwatersrand basin contain sand with grains of detrital uraninite. Because this mineral is easily oxidized, its presence in Archean strata as a detrital particle has been used as evidence for a low-oxygen atmosphere before 2.5 Ga. However, its presence in modern sand from the Indus River system has been used to argue that detrital uraninite does not provide information about the oxygen concentration of Earth's early atmosphere. Petrographic and chemical study of sand from these two sources reveals differences that suggest the modern Indus sand cannot be used as an analog for the Archean Witwatersrand occurrences. The Witwatersrand quartzites are depleted in Ca, Mg, and Na, indicating that the original sand from which they formed had been subjected to intense weathering. The chemical index of alteration (CIA), a commonly used indicator of degree of weathering, yields an average value of about 0.80 for Witwatersrand quartzites, comparable to modern tropical streams such as the Orinoco that drain deeply weathered terrains under tropical conditions (CIA=0.75). In contrast, the CIA for Indus sand is 0.45, indicating virtually no chemical weathering. The significance of Archean quartz-pebble conglomerates is not just that they contain unstable detrital phases like uraninite and pyrite, but that these particles are associated with rocks whose compositions suggest intense weathering. These conglomerates must have been subjected to intense weathering under tropical conditions, either in their source area or at the site of deposition, and the preservation of minerals like uraninite such conditions is indeed strong evidence for a low-oxygen atmosphere.

• OSTI ID: 5598116
• Journal Information: Geology; (USA), Vol. 19:3; ISSN 0091-7613
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
PAKISTAN
SEDIMENTARY BASINS
SAND
GEOCHEMISTRY
SOUTH AFRICA
URANIUM DEPOSITS
ATMOSPHERIC CHEMISTRY
CALCIUM
CONGLOMERATES
MAGNESIUM
MINERALIZATION
OXYGEN
PALEOCLIMATOLOGY
PETROGRAPHY
PRECAMBRIAN ERA
PYRITE
RIVERS
SODIUM
TIME DEPENDENCE
URANIUM ORES
WEATHERING
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ALKALI METALS
ALKALINE EARTH METALS
ASIA
CHALCOGENIDES
CHEMISTRY
DEVELOPED COUNTRIES
DEVELOPING COUNTRIES
ELEMENTS
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GEOLOGIC STRUCTURES
IRON COMPOUNDS
IRON SULFIDES
METALS
MINERAL RESOURCES
MINERALS
NONMETALS
ORES
PALEONTOLOGY
RESOURCES
ROCKS
SEDIMENTARY ROCKS
STREAMS
SULFIDE MINERALS
SULFIDES
SULFUR COMPOUNDS
SURFACE WATERS
TRANSITION ELEMENT COMPOUNDS
580000* - Geosciences
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