PROGRESSIVE CHANGES IN THE COMPOSITION OF COBB HOTSPOT LAVAS DUE TO THINNING LITHOSPHERE

The Cobb-Eickelberg seamounts in the northeast Pacific form an age-progressive chain from Axial Seamount, the current locus of the Cobb hotspot, to Marchand Seamount near the Aleutian Trench. The seamount chain records the composition of Cobb hotspot lavas for the past 33 Myr. In this first comprehensive geochemical study of the Cobb chain, major and trace element compositions were measured for 21 whole-rock samples from 13 Cobb seamounts, and Sr, Nd, Pb, and Hf isotopic ratios were measured for 16 of the samples. ³He/⁴He was measured for olivine phenocrysts from Gluttony Seamount. In general, the Cobb Seamounts become progressively more depleted and MORB-like with decreasing age and distance from the Juan de Fuca Ridge. The lithosphere has become thinner over the Cobb hotspot as the Juan de Fuca Ridge has migrated to the hotspot from the southeast, converging with it at about 0.5 Ma. Changing lithospheric thickness appears to have controlled the composition by determining the extent and depth of mantle melting and the proportions of hotspot and MORB mixing components. The data are consistent with increased melting as the lithosphere thinned and variable mixing between asthenosphere with a Juan de Fuca Ridge N-MORB composition and a smaller proportion (≤15%) of an isotopically distinct plume composition.