ESR DATING OF THE ROTOITI BRECCIA IN THE TAUPO VOLCANIC ZONE, NEW-ZEALAND

Electron spin resonance (ESR) can be used to determine the time of formation of quartz in volcanic rocks, based on the trapping of charges in the quartz resulting from environmental radiation. The method has been used on quartz from a rhyolitic pyroclastic flow, the Rotoiti Breccia, deposited NE of Rotorua, New Zealand. It yields a date of 45.2 +/- 8.2 ka BP, the mean of ages of 48.6 and 41.8 ka BP obtained from the Al and Ti centers, respectively. This result is in good agreement with previous C-13 ages of > 41.6 ka BP and 44.2 +/- 4.3 ka BP, but does not agree with a date of 71 +/- 6 ka BP, based on uranium disequilibrium. We suggest that this discrepancy reflects the time difference between attainment of equilibrium at depth, and the emergence time of the pyroclastic flow on the surface where radiation bombardment commenced.