HR: 1340h
AN: U33A-0016    [Abstracts]
TI: Eruption rates at Fernandina volcano, Galapagos archipelago, from cosmogenic helium in surficial lava flows
AU: * Kurz, M D
EM: mkurz@whoi.edu
AF: Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, MS #25, Woods Hole, MA 02543 United States
AU: Rowland, S
EM: scott@soest.hawaii.edu
AF: University of Hawaii, SOEST 2525 Correa Rd, Honolulu, HI 96822 United States
AU: Curtice, J
EM: jcurtice@whoi.edu
AF: Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, MS #25, Woods Hole, MA 02543 United States
AU: Saal, A
EM: Alberto_Saal@Brown.edu
AF: Brown University, Dept. Geological Sciences, Providence, RI 02912 United States
AU: Naumann, T
EM: aftrn@uaa.alask.edu
AF: University of Alaska, Geological Sciences, Anchorage, AK 99508 United States
AB: Fernandina volcano is considered the center of the Galapagos hotspot because it lies at the leading (western) edge of the archipelago with respect to plate motion, has been the most active in historical times, and is characterized by extremely high 3He/4He. In an effort to determine the growth rate of the volcano, we have measured exposure ages using cosmogenic 3He measurements in olivine phenocrysts from surficially exposed lavas. Using satellite data, Rowland (1996) subdivided these flows into young, intermediate, and old, and calculated their respective surface areas, but had no method for determining ages, except for the few witnessed historical flows. The lava flows studied here span the entire exposed age range, based on both the Rowland (1996) map and field observations of weathering characteristics, vegetation, and stratigraphy. The surface exposure dates are all close to the limit of the method due to their young ages, low olivine contents in the lavas, low equatorial 3He production rates, and exposures near sea level. The oldest surfaces, found as small outcrops on the northern coast of Fernandina are therefore the most reliably dated using cosmogenic 3He, and demonstrate that the subaerial part of the volcano has been entirely resurfaced within the last 5 Ka. These data alone confirm that Fernandina has had relatively high eruption rates during that interval. The ages of the youngest flows cannot be accurately determined because cosmogenic 3He levels are below detection limits (by melting of olivine in vacuo), but they do yield useful age limits, of less than 0.8 Ka. The young flows have a surface area of 358 km2 and an approximate volume of 3.3 km3; using this volume, the minimum eruption rate of Fernandina over the last ~1 Ka has been ~0.005 km3 yr-1. This is a lower limit because the ages are upper limits, and many young flows entered the ocean. There are few eruption rates to compare for ocean island volcanoes, but this lower limit eruption rate is similar in magnitude to the 100 year average (1850-1950) eruption rate for Kiluaea of 0.009±.001 km3 yr-1 (Dvorak and Dzurisin, 1993). These data demonstrate the utility of exposure ages for extremely young flows, which will be of interest for volcanic hazard assessment and studies of species distribution. The data can also be used to evaluate temporal geochemical evolution; magmatic 3He/4He ratios (determined by crushing in vacuo) and other isotope tracers do not vary systematically with time. The relative geochemical uniformity of Fernandina (e.g. Allan and Simkin, 2000) may be due to the extremely short time scale represented by exposed lava flows.
DE: 1033 Intra-plate processes (3615, 8415)
DE: 1040 Radiogenic isotope geochemistry
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 8425 Effusive volcanism
DE: 8488 Volcanic hazards and risks
SC: Union [U]
MN: Fall Meeting 2005