Evolution of Intraplate Volcanism in a Diffuse Igneous Province: A Case Study of the Pribilof Islands, Alaska

Montana State University, Bozeman MT

Investigators

Abstract

This proposal is for an integrated field, 40Ar/39Ar dating, geochemical, isotopic, and petrologic study of volcanism in the Pribilof Islands, Alaska. The two principal Pribilof Islands, St. Paul and St. George, are key volcanic fields in the little studied Bering Sea basalt province (BSBP). The BSBP is an example of a 'diffuse igneous province,' a newly recognized and poorly understood type of intraplate volcanism. The Pribilof Islands have been selected for this study because they allow investigation of two major BSBP volcanic fields in a setting where factors such as lithospheric thickness and composition, distance from the Aleutian arc front, and other tectonic factors are essentially constant. This in turn allows assessment of whether differences exist in the evolution of the centers due to factors such as structural evolution, mantle and crustal mass balance involved in magmatism, and magma reservoirs and plumbing systems. The specific objectives of the project are to 1) document the volcanic and geologic evolution of the Pribilof Islands through detailed geologic mapping, sampling, and 40Ar/39Ar dating; 2) identify any temporal trends in composition through geochemical analyses of samples whose ages are well constrained; 3) determine the geometry and timing of faulting on the islands and its influence on eruptive activity and localization of distinct vent types; and 4) obtain suitably complete chemical (XRF and ICP-MS), isotopic (Sr, Nd, Pb, and O), and petrologic data for the volcanic rocks and included xenoliths so that the nature of the mantle and crustal source rocks and parent magmas can be constrained, the degrees of partial melting estimated, and the petrogenesis of the magmas determined. A significant aspect of this work is that there are several broad impacts that extend well beyond understanding the petrology of the volcanic rocks. These impacts draw from the important human and wildlife communities on the Pribilof islands. Notable aspects are that the islands host the largest Aleut communities on earth, are the seasonal breeding grounds for over half of the world's northern fur seals, and are the nesting sites for nearly three million seabirds. It is likely that differences in the distributions of these populations among the islands reflect the timing and migration of Pleistocene to Holocene volcanism and faulting in the region. Better understanding of the geologic evolution of the Pribilof Islands will therefore permit greater insight into the dominant controls on the contrasting geomorphology of each island and their contrasting wildlife and human populations.