A NEW CALIBRATION POINT FOR THE LATE MIOCENE SECTION OF THE GEOMAGNETIC POLARITY TIME-SCALE - 40AR/39AR DATING OF LAVA FLOWS FROM AKAROA VOLCANO, NEW-ZEALAND

Recent dating of geomagnetic field reversals over the past 2 Ma, by both the radiometric and as astrochronologic techniques, indicates that the K-Ar derived ages for these reversals are too young. We report herein on 40Ar/39Ar incremental heating study of two geomagnetic field reversals trapped in lavas on Akaroa Volcano, New Zealand, that demonstrate that the K-Ar derived ages for field reversals at 10-9 Ma are too young by about 7%. The new age for the termination of chron 5, a critical tie point on the geomagnetic polarity time-scale for late Miocene time, is 9.67 +/- 0.11 Ma.