Computer Science
Scientific paper
Mar 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002e%26psl.197...51s&link_type=abstract
Earth and Planetary Science Letters, Volume 197, Issue 1-2, p. 51-64.
Computer Science
12
Scientific paper
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 -63° 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+/-8 ka (+/-2σ) 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+/-10 ka and by 37 kyr from basaltic lavas at Haleakala and Tahiti, which record the Kamikatsura Event 899+/-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.
Brown Laurie L.
Singer Brad
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