Single-crystal sup 40 Ar/ sup 39 Ar dating of the Olorgesailie Formation, southern Kenya rift

Journal Article · Sun Jun 10 00:00:00 EDT 1990 · Journal of Geophysical Research; (United States)

DOI:https://doi.org/10.1029/JB095iB06p08453· OSTI ID:5009640

Deino, A ^[1]; Potts, R ^[2]

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1. Geochronology Center of the Inst. of Human Origins, Berkeley, CA (United States)
2. Smithsonian Institution, Washington, DC (United States)

Single-crystal laser fusion {sup 40}Ar/{sup 39}Ar analyses and several conventional bulk fusion {sup 40}K- {sup 40}Ar dates have been used to determine the age of volcaniclastic strata within the Olorgesailie Formation and of associated volcanic and sedimentary units of the southern Kenya rift. In the principal exposures along the southern edge of the Legemunge Plain, the formation spans the interval from approximately 500 to 1,000 ka. Deposition continued to the east along the Ol Keju Nyiro river where a tuff near the top of the formation has been dated at 215 ka. In these exposures, the formation is unconformably overlain by sediments dated at 49 ka. A possible source for the Olorgesailie tephra, the Ol Doinyo Nyokie volcanic complex, contains as ash flow dated at {approximately} 1 Ma, extending the known age range of this complex to encompass that of virtually the entire Olorgesailie Formation in the Legemunge Plain. These geologic examples illustrate the importance of the single-crystal {sup 40}Ar/{sup 39}Ar dating technique whereby contaminant, altered, or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits. The authors have presented a computer-modeling procedure based on an inverse-isochron analysis that promotes a more objective approach to trimming {sup 40}Ar/{sup 39}Ar isotope data sets of this type.

• OSTI ID: 5009640
• Journal Information: Journal of Geophysical Research; (United States), Vol. 95:B6; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
GEOLOGIC FORMATIONS
AGE ESTIMATION
KENYA
ARGON 39
ARGON 40
COMPUTER CALCULATIONS
DEPOSITION
GEOLOGIC HISTORY
GEOLOGIC STRATA
ISOTOPE DATING
ISOTOPE RATIO
POTASSIUM 40
RIFT ZONES
SEDIMENTARY ROCKS
TUFF
VOLCANIC ROCKS
AFRICA
ALKA
ARGON ISOTOPES
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
BETA-PLUS DECAY RADIOISOTOPES
DEVELOPING COUNTRIES
ELECTRON CAPTURE RADIOISOTOPES
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOLOGIC STRUCTURES
IGNEOUS ROCKS
ISOMERIC TRANSITION ISOTOPES
ISOTOPES
LIGHT NUCLEI
NANOSEC LIVING RADIOISOTOPES
NUCLEI
ODD-ODD NUCLEI
POTASSIUM ISOTOPES
RADIOISOTOPES
ROCKS
STABLE ISOTOPES
YEARS LIVING RADIOISOTOPES
580000* - Geosciences