AGE AND DURATION OF THE MATUYAMA-BRUNHES GEOMAGNETIC POLARITY REVERSAL FROM AR-40 AR-39 INCREMENTAL HEATING ANALYSES OF LAVAS/

Constraints on the timing of geomagnetic polarity reversals have come mainly from K-AT, or more recently Ar-40/Ar-39, age determinations of lavas or their K-rich phenocrysts that erupted prior or subsequent to particular geomagnetic events. We have obtained Ar-40/Ar-39 isochron a ges using incremental heating techniques on groundmass separates, phen ocryst-poor whole rock samples, or plagioclase, from eight basaltic to andesitic lavas that erupted during the Matuyama-Brunhes (M-B) polari ty transition at four geographically dispersed sites. These eight lava s range from 784.6 +/- 7.1 ka to 770.8 +/- 5.2 ka (1 sigma errors); th e weighted mean, 778.7 +/- 1.9 ka, gives a high-precision age that is remarkably consistent with revised astronomical age estimates for the M-B polarity transition 16,12,13]. Despite uncertainties in absolute c alibration of Ar-40/Ar-39 ages relative to mineral standards used as n eutron fluence monitors, our age determinations are consistent with fi ve other Ar-40/Ar-39 studies focused on the M-B transition. These resu lts confirm that the earlier K-Ar based geomagnetic polarity time scal e underestimated the age of the M-B reversal by about 6%. None of the eight isochron ages are distinguishable from one another at the 95% co nfidence level. However, we are tantalizingly close to testing for the duration of this reversal. One lava at the base of a sequence of tran sitionally magnetized flows in Chile and the uppermost lava in a simil ar sequence on Maui are only just indistinguishable in age at the 95% confidence level and preserve different magnetic orientations. We sugg est that the similar to 12 kyr difference in age represents an upper l imit for the duration of the reversal and is similar to the period of low magnetic field intensity associated with records of the M-B revers al from deep sea sediment cores. Together with the short duration (sim ilar to 2 kyr) of the directional reversal observed in several differe nt marine sediment sections, our data suggest that reversal of the fie ld's direction could have occurred at slightly different times dependi ng on the position of the recording site.