Mechanisms of geochemical and geophysical variations along the western Galápagos Spreading Center

Improved insights into the processes of hot spot–ridge interaction along the Galápagos Spreading Center (GSC) are revealed by geochemical data between ∼91°W and 98°W. Principal components analysis reveals that >87% of the total isotopic variability can be explained with only two mantle source components. The “depleted” component has lower ratios of highly to moderately incompatible elements, higher Nd isotopic ratios, and lower Sr and Pb isotopic ratios. The second component is relatively enriched in incompatible elements, has more radiogenic Pb and Sr and less radiogenic Nd, and is comparable to the C or “common” mantle component observed at many locations in the Pacific. The enriched component's signature is strongest nearest the hot spot at ∼92°W and diminishes with distance from the hot spot to 95.5°W. Near 95.5°W, lava compositions change sharply, becoming dominated by the depleted component and remaining so farther west, to 98°W. Thus, the Galápagos hot spot most clearly influences the composition of the GSC between 91°W and 95.5°W. The depleted component between 91°W and 98°W differs from that evident at the Galápagos Archipelago, along the GSC east of 91°W, and along the East Pacific Rise. This suggests some form of compositional zoning in the regional mantle. If the depleted materials are intrinsic to the Galápagos mantle plume, then the plume is bilaterally zoned and feeds a depleted component to the GSC at 91°W–98°W that is distinct from the depleted material elsewhere in the region. This possibility is supported by melting models in which the Galápagos plume is a uniform mixture of a depleted matrix and fine‐scale enriched veins. The expression of the more fusible veins is predicted to be enhanced nearest the hot spot (∼92°W), where plume‐like upwelling drives rapid flow and melting deeper in the melting zone (where the veins are melting). With increasing westward distance from the hot spot, the deep, plume‐driven flow is predicted to decrease, as does the expression of the enriched veins in lava compositions. The model therefore adequately explains the compositional and crustal variations from 92°W to 95.5°W. The average model composition of the plume in this region does not differ significantly from that of the ambient mantle beneath other ridges not influenced by hot spots.