New ages for the climactic eruptions at Yellowstone; single-crystal (super 40) Ar/ (super 39) Ar dating identifies contamination
Gansecki, C A; Mahood, G A; McWilliams, M (1998) New ages for the climactic eruptions at Yellowstone; single-crystal (super 40) Ar/ (super 39) Ar dating identifies contamination. Geology (Boulder), vol.26, no.4, pp.343-346, Apr 1998
| Authors | Gansecki, C A; Mahood, G A; McWilliams, M |
|---|---|
| Year | 1998 |
| Title | New ages for the climactic eruptions at Yellowstone; single-crystal (super 40) Ar/ (super 39) Ar dating identifies contamination |
| Source | Geology (Boulder), vol.26, no.4, pp.343-346, Apr 1998 |
| ISSN | 0091-7613 |
| GeoRef | 1998-027836 |
| Keywords | absolute age; alkali feldspar; Ar/Ar; ash-flow tuff; Cenozoic;chronostratigraphy; dates; eastern Idaho; electron probe data;eruptions; feldspar group; framework silicates; Fremont County Idaho;Huckleberry Ridge Tuff; Idaho; igneous rocks; Lava Creek Tuff; magmacontamination; Mesa Falls Tuff; national parks; northwestern Wyoming;Park County Wyoming; phenocrysts; public lands; pyroclastics;Quaternary; rhyolite tuff; sanidine; silicates; single-crystal method;statistical analysis; tephrochronology; Teton County Wyoming; tuff;United States; volcanic rocks; volcanism; welded tuff; Wyoming;xenocrysts; Yellowstone National Park |
| Notes | With GSA Data Repository Item 9839 |
Abstract
Ash beds associated with the three climactic Yellowstone ignimbritesform important Quaternary chronostratigraphic markers over much of thecontinental United States. Previous K-Ar ages determined on crystalconcentrates from these ashes varied by as much as 60-100 k.y.Laser-fusion super 40 Ar/ super 39 Ar dating of single sanidinegrains from these units reveals a small number of grains withanomalously old ages. Eliminating these from the weighted averagesresults in highly precise refined ages of 2.003+ or -0.014, 1.293+ or-0.012, and 0.602+ or -0.004 Ma 2sigma errors for the HuckleberryRidge Tuff, Mesa Falls Tuff, and member B of the Lava Creek Tuff,respectively. Individual single-grain ages that are slightly too oldcould result from incomplete degassing of xenocrysts in the magma.Electron-microprobe analyses of sanidine splits reveal no obviousxenocrystic compositions, suggesting another possibility--thatphenocrysts from the crystallized rind of the magma chamber werere-entrained into the magma prior to eruption. Contamination andnatural variation in phenocryst age may create larger uncertainty inbulk-crystal dating of young silicic volcanic rocks than incompleteextraction of Ar from sanidine.