Multiplicity of Magma Source Characteristics and Melting Processes for Late Cenozoic Basalts of the Bering Sea Volcanic Province (BSVP), Alaska

The Bering Sea Volcanic Province consists of a number of large basaltic late Cenozoic volcanic fields in a broad region inboard from the Aleutian arc front to the Arctic Circle, and from Chukchi Peninsula in Russia to Western Alaska, which we have dated by 40Ar/39Ar at ~6.0-0.1 Ma, but with some flows being too young to date by this method. We estimate that >1000 km3 of magma was erupted through the eastern part of the BSVP, all within the past 6 Ma. Combining age information with volume estimates reveals that the intensity of volcanic activity in the region has increased through time, with only about 15% of lava erupted before 3 Ma, and about 45% of all late Cenozoic magma being erupted within the last 300 k.y. Eruption rates also increase toward more recent times: 6-Ma basalts having erupted at the rate of ~70 m3/km2/yr, while the youngest basalts (? 0.7 Ma), are estimated to have erupted at a rate of ~225 m3/km2/yr. All late Cenozoic volcanic rocks are enriched in highly incompatible trace elements with strongly fractionated REE patterns (LaN/YbN = 4.7-23.7) suggesting Grt-bearing source. Nd-Sr-Hf isotopic compositions of the rocks (143Nd/144Nd = 0.512939-0.513139; 87Sr/86Sr = 0.702653-0.704342; 176Hf/177Hf = 0.283098- 0.283257) are similar to those of MORB and the depleted varieties of OIB. Lead isotopic ratios fall mostly below the Northern Hemisphere Reference Line (NHRL) in the Pacific MORB field, but the youngest rocks display a distinct trend toward the EM-II values (206Pb/204Pb = 18.21-19.10; 207Pb/204Pb = 15.42-15.62; 208Pb/204Pb = 37.72-38.88). Regional differences in both trace element and isotopic compositions are apparent, indicating that BSVP magmatism does not have a single origin. Lavas with the arc signature are prevalent near the southern margin of the province while those with an OIB signature dominate the central and northern sectors. Also, selected regions in the province exhibit a systematic increase in 87Sr/86Sr with time (from 0.7027 to up to 0.7043 within the last 300 k.y.), marking change in the nature of the melts from dominantly asthenospheric to mainly lithospheric.