Investigation of Biotite/Sanidine Age Discordance

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AbstractEAR-0126144Heizler and McIntoshNew Mexico Institute of Mining and TechnologyCogenetic biotite and sanidine from volcanic rocks often yield discordant 40Ar/39Ar ages. The typical age discordance is between 0.2 and 1.0 Ma. The discordance has important implications for detailed, high precision volcanic history studies, for intercalibration of K/Ar with U/Pb and other geochronological methods, and the age of flux monitor standards. This proposal will investigate the causes of the biotite/sanidine discordance through detailed geochronological analyses of two well-studied Mid-Tertiary volcanic sequences from the Mogollon-Datil volcanic field of New Mexico. The expected outcome of this study could be far reaching, if it is shown that biotite ages are plagued by inaccuracy that is not evident in individual analyses. Because biotite is the most commonly dated volcanic and plutonic mineral phase and it often yields highly precise apparent ages, determination of accuracy of biotite ages is essential to a wide variety of tectonic, volcanic, and geological studies.