Morphological and geochemical variations along the eastern Galápagos Spreading Center

Abstract

As the eastern Galápagos Spreading Center (GSC) shallows westward toward the Galápagos Archipelago, axial morphology evolves from a low‐relief, valley‐and‐ridge terrain to an increasingly prominent axial ridge, closely mirroring the western GSC. Between the Inca Transform (∼85.5°W) and its western termination near 91°W, the eastern GSC comprises seven morphological segments, separated by five morphological discontinuities and the eastward propagating 87°W overlapping spreading center. Combined morphologic and geochemical data divide the eastern GSC into two domains independent of the fine‐scale morphologic segmentation. The western domain is defined by its axial ridge morphology and highly variable lava population. Elemental data define steep along‐axis gradients, reflecting a complex source that includes one or more hot spot–related components in addition to a highly depleted component. The eastern domain is defined by transitional, valley‐and‐ridge morphologies and a surprisingly invariant lava population. This population is dominated by shallow crystal fractionation processes and displays significantly less variability attributable to multiple source components. The Galápagos hot spot has long been known to have a symmetrical, long‐wavelength influence on crustal accretion along the GSC. Existing isotopic and new elemental data define twin “geochemical peaks” that we interpret as loci for transfer of distinct source components from the Galápagos plume to the GSC. Although Na8 and Fe8 values lie within the negatively correlated global array, Na8 increases with decreasing axial depth, contrary to global trends and consistent with emerging deep, hydrous melting models that predict decreasing overall extent of melting despite increasing melt production. Support for hydrous melting comes from decreasing heavy REE, increasing La/Sm and La/Yb, and the systematics of decreasing FeO and increasing CaO and Al2O3 with decreasing distance to the hot spot. Overall, an enriched, deep melt component appears to coexist in the shallow mantle with a ubiquitous, depleted primitive melt component, consistent with new models for channelized melt flow connecting a deep hydrous melt regime with the dry shallow mantle. Nevertheless, an absence of low‐Fe lavas suggests that hydrous melting is strictly limited beneath the eastern GSC, becoming dominant only near the western geochemical peak where input from a hydrous “Northern” or “Wolf‐Darwin” plume component is inferred.