HR: 10:20h
AN: V11I-01 INVITED     [PDF]
TI: Application of K-Ar Dating to the Chronology of Young Volcanic Centers
AU: * Lanphere, M A
EM: alder@usgs.gov
AF: Marvin A. Lanphere, 345 Middledfield Road MS-910, Menlo Park, CA 94025 United States
AB: K-Ar dating and a derivative technique, $^{40}$Ar/$^{39}$Ar dating, are methods of high-precision chronology applicable to young volcanic centers. Cascade volcanoes studied in detail by several USGS volcanologists, Duane Champion paleomagetist, and me include Mt. Baker, WA; Mt. Rainier, WA; Mt. Adams, WA; Mt. Hood, OR; Crater Lake, OR; and Medicine Lake, CA. For Mt. Adams using detailed geologic mapping by Hildreth and Fierstein and 74 K-Ar ages for 63 mapped units, Hildreth and Lanphere established a detailed chronology for the stratovolcano. Good agreement has been achieved for K-Ar ages and $^{40}$Ar/$^{39}$Ar ages of rocks from Mt. Adams as young as 36 ka. A similar detailed chronology has been established for other Cascade volcanoes using andesites, in particular. These chronologies often take 10 years or more to develop. Major advantages of the $^{40}$Ar/$^{39}$Ar technique are the ability to work with small sample sizes and the possibility to push the technique to very young ages. The Campanian Ignimbrite erupted from the Campi Flegrei crater near Naples, Italy is an example of the use of small samples. Nine incremental-heating ages were determined on samples of sanidine ranging in size from 47 mg to 67 mg. These samples yielded ages for the Campanian Ignimbrite ranging from 37.1 $\pm$ 0.75 ka to 39.5 $\pm$ 0.62 ka and averaging 38.1 $\pm$ 0.8 ka. Other workers have proposed $^{40}$Ar/$^{39}$Ar ages for the Campanian Ignimbrite of 37.1 $\pm$ 0.4 ka and 39.3 $\pm$ 0.1 ka. An example of the use of $^{40}$Ar/$^{39}$Ar dating of very young samples is the Christian Era (CE) age of the Vesuvius eruption of year 79. Eight packets of sanidine weighing 213-296 mg from two localities, Casti Amanti in Pompeii and Villa Poppea in nearby Oplontis, yielded a weighted-mean incremental-heating age of 1924 $\pm$ 66 years. The known age for the CE 79 eruption of Vesuvius is 1924 years. Earlier studies of Vesuvius by other workers yielded an $^{40}$Ar/$^{39}$Ar age for the Villa Poppea locality of 1922 $\pm$ 72 years.
DE: 1035 Geochronology
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting