HR: 1340h
AN: V23B-1433    [Abstracts]
TI: High-Precision 40Ar/39Ar Geochronology and Geology of St. George Island, Pribilof Islands, Alaska: Implications for Eruption Rates in the Bering Sea Basalt Province
AU: * Feeley, T C
EM: tfeeley@montana.edu
AF: Department of Earth Sciences, Montana State University, Bozeman, MT 59717, United States
AU: Cosca, M A
EM: Michael.Cosca@unil.ch
AF: Institute of Mineralogy and Geochemistry, University of Lausanne, Lausanne, CH-1015, Switzerland
AU: Hamblock, J M
EM: hamblock@montana.edu
AF: Department of Earth Sciences, Montana State University, Bozeman, MT 59717, United States
AU: Underwood, S J
EM: sunderwood@mymail.msu.montana.edu
AF: Department of Earth Sciences, Montana State University, Bozeman, MT 59717, United States
AB: New high-precision ⁴⁰Ar/³⁹Ar ages and geologic mapping establish an eruptive chronology for St. George Island, Pribilof Islands, Alaska. St. George is part of the Bering Sea basalt province (BSBP), a group of 15 late Cenozoic (mostly < 6 Ma) alkalic to tholeiitic basaltic volcanic fields widely distributed on islands in the Bering Sea, along the west coast of Alaska, and along the coast of northeast Russia. Twelve samples of washed, but otherwise untreated, whole-rock basalts from St. George were cut with a micro-wire saw into chips ~3 mm³ in size and irradiated for ⁴⁰Ar/³⁹Ar analysis. The chips were incrementally heated with a CO₂ laser equipped with an integrator lens, and analyzed using a NU Instruments Noblesse mass spectrometer equipped with a Faraday cup and two ion counting electron multipliers. Detector intercalibration was done using automated air pipettes. A minimum of 20 heating steps were measured per sample, with the data often defining age plateaux. Isochron plots of the data yield ages ranging from 1.57 ± 0.04 to 2.89 ± 0.11 Ma, with trapped ⁴⁰Ar/³⁶Ar ratios ranging from 312 to 330. The stratigraphic positions of the dated rocks are known directly from field relations and there are no discrepancies between the ⁴⁰Ar/³⁹Ar ages and this sequence. Geochemical data combined with the age data indicate no progressive petrologic trends during evolution of the magmatic system, except for intermittent eruption of distinctive plagioclase-phyric basalts with low to moderate MgO contents (7 - 5 wt%) beginning at ~2.0 Ma. The new age data combined with volume estimates indicate an average subaerial eruption rate of ~107 m³km^-2yr^-1, which is adjusted for 3% sedimentary and ultramafic basement rocks beneath the volcanic pile, an average vesicularity of 5%, and an assumed surficial erosion value of 20%. This rate is identical to the estimate (110 m³km^-2yr^-1) by Mukasa et al. (JGR 112, 2007) for St. George Island. Both estimates, however, do not account for significant coastal erosion and faulting that has modified St. George since the end of volcanism, as evidenced by numerous E\-W to NE\-SW striking fault ridges and nearly continuous marine terraces and wave-eroded sea cliffs that surround the island, the highest of which rise over 300 m above narrow rocky shorelines. In this regard, geologic, structural, and geomorphic relations suggest that as much as 25% of the island (by area) may have been removed due to faulting and coastal erosion, which significantly impacts estimated eruption rates and comparison to other intraplate basaltic volcanic fields in the BSBP and elsewhere.
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
DE: 3615 Intra-plate processes (1033, 8415)
DE: 8415 Intra-plate processes (1033, 3615)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting