We also present a whole set of new U-series data on historical, recent, and older samples from Merapi (Indonesia), and show that the (C ages available on three samples [R. Keller, Temporal variations in magma composition at Merapi volcano (Central Java, Indonesia): magmatic cycles during the past 2000 years of explosive activity, J. It also opens the possibility to extend the geomagnetic field variation curve back into the past few millennia.
Th disequilibria can be used to date Holocene volcanic rocks from some well selected volcanoes.
A systematic study of these disequilibria on historical or well-dated volcanic samples is indeed first required to test the applicability of this method.
Two examples are described here to illustrate its potential. Etna, the good correlation observed between ( activities at the time of eruption and Th contents in lava flows from the last two millennia [M.
Etna: constraints from U–Th–Ra–Pb radioactive disequilibria and Sr isotopes in historical lavas, Earth Planet. The calculated ages are in good agreement with those deduced from the archaeomagnetic curve describing the variation of the geomagnetic field direction in southern Italy [J.
Critical to these dating methods is the assumption that there was no radiogenic argon ( Hualalai basalt, Hawaii (AD 1800-1801) 1.6±0.16 Ma; 1.41±0.08 Ma Mt. Stromboli, Italy, volcanic bomb (September 23, 1963) 2.4±2 Ma Mt.
Etna basalt, Sicily (May 1964) 0.7±0.01 Ma Medicine Lake Highlands obsidian, Glass Mountains, California ( in the 1800-1801 Hualalai flow, Hawaii, resided in fluid and gaseous inclusions in olivine, plagioclase, and pyroxene in ultramafic xenoliths in the basalt, and was sufficient to yield "ages" of 2.6 Ma to 2960 Ma.
Further confirmation comes from diamonds, which form in the mantle and are carried by explosive volcanism into the upper crust and to the surface. obtained a K-Ar isochron "age" of 6.0±0.3 Ga for 10 Zaire diamonds, it was obvious excess was inherited from the mantle source areas of the magmas. Snelling, "Discordant Potassium-Argon Model and Isochron ' Ages' for Cardenas Basalt (Middle Proterozoic) and Associated Diabase of Eastern Grand Canyon, Arizona," in R.
132 (1995) 25–41] is used to infer the ages of several newly analysed lava flows.
Thompson, Geomagnetic secular variation in Sicily and revised ages of historic lavas from Mt.
Patane, Variation séculaire de la direction du champ géomagnétique enregistrée par les laves de l'Etna et du Vésuve pendant les deux derniers millénaires, C.
Patane, Archaeomagnetic dating of Mediterranean volcanics of the last 2100 years: validity and limits.