THE ROLE OF LITHOSPHERIC GABBROS ON THE COMPOSITION OF GALAPAGOS LAVAS

Abstract

New geochemical data combined with a compilation of all published data for the Galapagos basalts indicate that the Galapagos Archipelago can be divided into two very different volcanic areas, east and west of the 91° W fracture zone. In both regions, the interaction between plume melts and depleted upper mantle lavas most likely controls the composition of the erupted basalts. However, we find important differences in the trace element compositions between lavas from the two regions. First, Ti/Gd and Nb/La ratios correlate with 3He/4He, and all three ratios decrease with increasing distance from Fernandina. These correlations suggest that the high Ti/Gd and Nb/La ratios are typical of the plume mantle source that carries the high 3He/4He signature. Second, there is an increase in the Ba/Th, K/La, Sr/Nd, Eu/Eu*, Pb/Nd ratios, and a higher 87Sr/86Sr at similar 143Nd/144Nd (high ΔSr) from west to east across the fracture zone, and from north to south within the eastern region. These geochemical characteristics resemble those of plagioclase-rich cumulates from the oceanic crust and ophiolite complexes. The compositions of the lavas from the eastern region can be explained by interaction of basalts with plagioclase-rich cumulate during melt percolation through the oceanic lithosphere. We argue that kinetic interaction of magmas with plagioclase-rich cumulates, previously formed either in the Galapagos Spreading Center or beneath the leading edge of the plume on the western region, are responsible for the observed composition of basalts erupted in the eastern region. © 2007 Elsevier B.V. All rights reserved.