Dating transitionally magnetized lavas of the late Matuyama Chron: Toward a new Ar-40/Ar-39 timescale of reversals and events

The K-Ar based geomagnetic polarity timescale was constructed using data fr om lavas and tuffs that bracketed, but rarely dated, the transitions betwee n polarity intervals. Subsequent Ar-40/Ar-39 dating indicated that the ages of some polarity transitions had been underestimated by about 6%. Although the accepted ages of the polarity chron boundaries have increased, their p recise temporal definition remained uncertain. We have taken a different ap proach and used incremental-heating techniques to obtain 18 new Ar-40/Ar-39 ages from basaltic lavas within flow sequences at Punaruu Valley, Tahiti, and Haleakala volcano, Hawaii. These lavas record transitional paleomagneti c directions corresponding to four mid-Pleistocene polarity reversals or ev ents. Three lavas from Punaruu Valley previously thought to record the Cobb Mountain Normal Polarity Subchron (CMNS) gave a mean age of 1.105 +/- 0.00 5 Ma, indicating that they were erupted about 76 kyr after the CMNS; this p eriod of transitional field behavior is designated the Punaruu event. In ad dition, seven new Ar-40/Ar-39 ages from the Punaruu Valley indicate that th e Jaramillo Normal Polarity Subchron (JNS) lasted about 67 kyr, starting at 1.053 +/- 0.006 Ma and ending 0.986 +/- 0.005 Ma. This agrees with astrono mical estimates but conflicts with JNS ages proposed by Spell ann McDougall [1992] and Izett and Obradovich [1994] on the basis of Ar-40/Ar-39 dating of rhyolite domes in the Valles Caldera. Indistinguishable Ar-40/Ar-39 ages of seven lavas, including one from Punaruu Valley and six from Haleakala t hat record broadly similar intermediate paleodirections, suggest that the K amikatsura event occurred at 0.886 +/- 0.003 Ma. Moreover, these data indic ate that the Kamikatsura event occurred 20-40 kyr after another geomagnetic event, most probably taking place at 0.92 Ma. We designate this earlier fi eld behavior the Santa Rosa event, adopting its name from that of a transit ionally magnetized rhyolite dome which happened to figure prominently in th e original definition of the end of the JNS in the 1968 study of Doell et a l. [1968]. The discovery of these new short-lived polarity events during th e Matuyama reversed chron suggests that the 400 kyr period between 1.18 and 0.78 Ma experienced no less than 7 and perhaps more than 11 attempts by th e geodynamo to reverse. This newly determined higher frequency of geomagnet ic activity illustrates vividly the importance of obtaining precise age con trol directly from transitionally magnetized rocks.