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

Abstract

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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar results that suggested a trapped component characterized by a 40 Ar/ 36 Ar 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

Astronomically calibrated ages for geomagnetic reversals within the Matuyama chron

“…In addition to the Cobb Mountain subchron and Kamikatsura polarity interval, they identified two short normal polarity events, called the Santa Rosa and Punaruu polarity intervals. Recently, Singer and Brown (2002) gave an age of 899 ± 6 ka for the Kamikatsura polarity interval and re-dated the Santa Rosa event recorded in rhyolite domes of Valles Caldera, New Mexico, at 936 ± 8 ka (Table 1), which were originally dated by the K-Ar and 40 Ar/ 39 Ar methods (Doell et al, 1968;Spell and McDougall, 1992;Izett and Obradovich, 1994). This age interpretation is consistent with independent evidence.…”

“…Based on this nomenclature, our two normal polarity episodes, which have astronomical ages of 920-925 ka and 1173-1185 ka (Table 1), are most likely to represent the Santa Rosa polarity interval and Cobb Mountain subchron, respectively. It should be noted that our astronomically tuned ages for these two normal polarity episodes are slightly younger than those of Singer and Brown (2002). Although additional excursions/events could also exist near the Jaramillo subchron (Takatsugi and Hyodo, 1995;Singer et al, 1999), their durations are apparently short (<1000 kyr) and are unlikely to correlate to the two episodes observed in core MD972143, which have durations of thousands of years.…”

“…In addition, there are four short normal polarity intervals in the Matuyama chron. The youngest one, which lies between the MBB and the Jaramillo subchron, probably corresponds to the Kamikatsura (Maenaka, 1983;Champion et al, 1988) or Santa Rosa polarity interval (Singer et al, 1999;Singer and Brown, 2002), and the normal polarity episode below the Jaramillo subchron may represent the Cobb Mountain subchron (Mankinen et al, 1978). The two normal polarity episodes below the Olduvai subchron are considered to represent the Réunion subchron (McDougall and Watkins, 1973), although the earlier one, which falls within the reworked foraminiferal sandy layer, cannot be recognized with confidence.…”

Where to find 1.5 million yr old ice for the IPICS &quot;Oldest-Ice&quot; ice core

“…Knowing mean Antarctic temperature and environmental conditions in the Southern Ocean region from isotopic and chemical ice core records over this time period would help us to understand potential changes in ice dynamics and the relationship between surface temperature and the marine isotope record. A 1.5 Myr record might also contain evidence of the Jaramillo normal polarity event at about 1.0 Myr as well as shorter term excursions known from the paleomagnetic timescale (Singer and Brown, 2002), providing an ice core view of two magnetic reversals, which would be recorded in a cosmogenic isotope ice core record covering that time interval (Raisbeck et al, 2006b). Moreover, this may provide chronological tie points to marine and terrestrial climate records via paleomagnetism.…”

Low-latitude paleosecular variation and the time-averaged field during the late Pliocene and Quaternary—Paleomagnetic study of the Michoacan-Guanajuato volcanic field, Central Mexico

“…Age uncertainties make difficult to affirm whether these findings represent geomagnetic phenomena. It may be speculated however that site Yahuarato records the 40 Ar-39 Ar dated Big Lost event (incremental heating age of 580.2±7.8 ka after Singer and Brown, 2002). Sites El Fresno and Cerro Grande both yielded fully reversed paleodirections.…”