40Ar/(39Ar) geochronology of rhyolites erupted following collapse of the Yellowstone caldera, Yellowstone Plateau volcanic field: implications for crustal contamination

Details 40Ar/(39Ar) geochronology of rhyolites erupted following collapse of the Yellowstone caldera, Yellowstone Plateau volcanic field: implications for crustal contamination 40Ar/(39Ar) geochronology of rhyolites erupted following collapse of the Yellowstone caldera, Yellowstone Plateau volcanic field: implications for crustal contamination

Jul 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996e%26psl.142...91g&link_type=abstract

Earth and Planetary Science Letters, Volume 142, Issue 1-2, p. 91-107.

Computer Science

7

Scientific paper

Single-crystal laser-probe 40Ar/(39Ar) dating of 133 grains of sanidine and plagioclase has enabled us to resolve the eruption ages of the Upper Basin Member rhyolites - the lava flows and related tuffs that erupted within the Yellowstone Caldera shortly after its collapse 630 ky ago on eruption of the Lava Creek Tuff. Two lavas and a tuff that erupted from the eastern ring-fracture zone yield an eruptive age of 481 +/- 8 ka, whereas two flows from the western ring-fracture zone yield eruptive ages of 516 +/- 7 and 198 +/- 8 ka. Most of the units contain old xenocrysts, explaining why previous attempts at dating these earliest post-caldera units by the conventional K-Ar method yielded poorly resolved and, in some cases, anomalous ages. The tuff shows the most severe contamination. Grains from a single pumice lapilli in the tuff show as large an age range as those from bulk vitrophyre, indicating that the xenocrysts were incorporated in the magma prior to its near-surface explosive fragmentation. Diffusion calculations indicate that the xenocrysts could not have remained in the magma for more than a few years without degassing and giving ages indistinguishable from the phenocrysts. Thus, the contamination represented by the xenocrysts probably occurred during fracturing and conduit propagation, rather than during caldera collapse, which took place more than 100 ky earlier. The apparent ages of xenocrysts and their compositions as determined by electron microprobe suggest that the Eocene Absaroka volcanics are the main contaminant, with a single xenocryst probably coming from Precambrian basement rocks. Most of the xenocrysts are difficult to distinguish optically or chemically from feldspar phenocrysts, illustrating the necessity of single-crystal analysis to date many young volcanic rocks accurately.

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