Under the volcano: A new dimension in Ar-Ar dating of volcanic ash

Abstract

A significant part of the history of the Greek island of Kos has been decoded from a handful of volcanic ash by argon-argon laser probe dating of single crystals of the minerals sanidine, plagioclase and quartz. On a 3-D argon isotope plot, the data define six straight lines, corresponding to four distinct volcanic events from 161 to 1728 ka. All four events produced plagioclase crystals, whose ages and Ca/K ratios are consistent with the known evolution of the rest of the Aegean arc. Sanidine appeared only in the final eruption at 161 ka. The joint occurrence of quartz and plagioclase at a 925 ka event implies that quartz did not reside for millennia in the magma, as has been suggested for quartz from the Bishop Tuff, California. The step-heated cooling age of a schist fragment from the ash provides the first radiometric evidence for ~250 Ma crust under the volcano.