Fission-track dating of volcanic sediments contaminated with older grains
Depositional ages of sediments can be determined using fission-track single grain ages on zircons from layers of volcanic ash or bentonite that have been contaminated by older grains during eruption or minor reworking of the ash. This is done by compiling an age probability distribution or age spectrum for a sample from individual grain ages. The youngest peak in the age spectrum approximates the time of deposition. In most contaminated samples 30 or more grains need to be counted to produce a reliable spectrum. However, useful, reproducible ages can be obtained by counting less than 10 grains in samples where most of the older, contaminating grains can be removed by hand. The few older grains that remain after removing the obviously abraded ones may then be eliminated by examination of the age spectrum. Although ages determined in this way are probably not accurate enough for use in precisely defining stratigraphic boundaries, they still provide a means of obtaining an absolute age in sediments that could not be meaningfully dated by other methods. Samples of bentonitic mudstone and tuffaceous sandstone from central Utah and Mexico that are contaminated with older grains have been dated in this manner. The youngest age mode for each sample correlates closely with the known ages of the deposits, while the ages calculated from indiscriminately using all the grains in a sample by conventional fission-track methods are frequently older and geologically meaningless.
- Research Organization:
- Brigham Young Univ., Provo, UT (USA)
- OSTI ID:
- 6199734
- Report Number(s):
- CONF-8510489-
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
SEDIMENTS
AGE ESTIMATION
STRATIGRAPHY
ZIRCON
FISSION TRACKS
BENTONITE
MEXICO
PROBABILISTIC ESTIMATION
UTAH
VOLCANIC ROCKS
VOLCANISM
CLAYS
DEVELOPING COUNTRIES
FEDERAL REGION VIII
GEOLOGY
IGNEOUS ROCKS
LATIN AMERICA
MINERALS
NORTH AMERICA
OXYGEN COMPOUNDS
PARTICLE TRACKS
ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
USA
ZIRCONIUM COMPOUNDS
ZIRCONIUM SILICATES
580100* - Geology & Hydrology- (-1989)