ESR dating of quartz phenocrysts in some rhyolitic extrusive rocks using Al and Ti impurity centres

ESR has been employed to determine the eruption date of rhyolitic rocks from three different tectonic settings. Samples from the Olkaria complex, Kenya rift valley, yielded ages of 31 +/- 3 and 38 +/- 5 ka (148a) and 23 +/- 4 and 34 +/- 8 ka (570) from the Al and Ti centres, respectively. These compare with an inferred formation age of 65 +/- 12 ka (148a), obtained from U-series isochrons, and an eruption age of 5.7-9.7 ka (570), based on C-14 and lake stands. Glass (melt) inclusions in sample 570 may have distorted the ESR age. The Youngest Toba Tuff (YTT), Sumatra, gave an average ESR eruption age of 81 +/- 17 ka which compares well with K/Ar and 40Ar/39Ar ages in the range 73-75 ka. However, there is evidence of thermal annealing in the Oldest (OTT) and Middle (MTT) Toba Tuffs by the more recent YTT eruption, with mean ages of ca. 349 ka and 326 ka, respectively, significantly below the K/Ar age of 840 +/- 30 ka (OTT) and the 40Ar/39Ar age 501 +/- 5 ka (MTT). A sample from the Battleship Rock Tuff, Jemez Volcanic Field, New Mexico, gave a mean ESR age of ca. 247 ka which contrasts markedly with a previously determined ESR age of 59 +/- 6 ka, but compares more favourably with previous K/Ar ages of 278 +/- 52, 130 +/- 70 and 180 +/- 70 ka. Subsequent geothermal activity and high ambient temperatures in this area may be responsible for these apparent discrepancies. (C) 1999 Elsevier Science Ltd. All rights reserved.