The Santa Rosa Event: 40Ar/39Ar and paleomagnetic results from the Valles rhyolite near Jaramillo Creek, Jemez Mountains, New Mexico

The Jaramillo Event was originally defined by Doell and Dalrymple in 1966 on the basis of K^Ar ages from sanidine in the normally, transitionally and reversely magnetized rhyolite domes named Cerro del Abrigo, Cerro Santa Rosa I and Cerro Santa Rosa II, respectively, that erupted following collapse of the archetypal Valles Caldera, New Mexico. We have collected new paleomagnetic data from the three domes and new 40Ar/39Ar laser fusion and furnace incremental heating experiments on sanidine crystals from the Cerro Santa Rosa I rhyolite. Step-wise alternating field and thermal demagnetization techniques applied to 52 samples from seven sites indicate that the original paleomagnetic results of Doell and Dalrymple [Science 152 (1966) 1060^1061] are valid. Cerro del Abrigo is normally magnetized, whereas the Santa Rosa I dome is transitional with an inclination of 363‡ toward the east and the Santa Rosa II dome is of reversed polarity. Twenty-five laser fusion experiments on sanidine crystals from the the Cerro Santa Rosa I dome, together with the saddle-shaped spectrum obtained by incrementally heating the sanidine in a furnace reveal that this rhyolite contained a small but significant component of excess argon prior to eruption. Our preferred age of 936 þ 8 ka ( þ 2c) for the Santa Rosa I rhyolite is based on the concordant laser fusion isochron and incremental heating plateau ages. This age is significantly older than was inferred on the basis of earlier laser fusion 40Ar/39Ar results that suggested a trapped component characterized by a 40Ar/36Ar ratio lower than the atmospheric value of 295.5. Our new age distinguishes the Santa Rosa I dome by 65 kyr from the termination of the Jaramillo normal subchron 1001 þ 10 ka and by 37 kyr from basaltic lavas at Haleakala and Tahiti, which record the Kamikatsura Event 899 þ 6 ka. Moreover, this determination is a factor of three more precise than the best previous estimate for a proposed geomagnetic event in this time period. The Cerro Santa Rosa I rhyolite dome, once intimately linked with the end of the Jaramillo Event and the acceptance of plate tectonic theory, now defines a highly resolved feature, the Santa Rosa Event, in a terrestrial geomagnetic reversal time scale that is consistent with the global record of magnetic field intensity from marine sediments. ß 2002 Elsevier Science B.V. All rights reserved.