Petrogenetic Processes near OSCs and Transforms Inferred from the Chemistry of Seamount Lavas: Constraints on the Deep Thermal Structure and Processes beneath EPR Offsets

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Hot mantle material upwelling in a broad zone beneath active ridge crests like the East Pacific Rise (EPR) melts to provide magmas feeding the narrow axial neovolcanic zone plus active near-axis seamounts. Knowledge of the thermal structure, mantle flow patterns, melt-generation sites, and conduit and storage geometry within the broad subaxial zone is important for understanding magma supply and tectonic processes at active ridge crests. This study will complement on-going studies of EPR axial processes by integrating the results of new seamount petrologic study in order to construct a fuller, more comprehensive picture of the thermal structure and petrogenesis beneath the broad subaxial zone. Active seamounts near the EPR axis occur at along-axis depth minima and offsets (OSCs and transforms). The former type are well-studied and these studies provide several testable hypotheses for the interaction of axial and seamount petrogenetic processes. In contrast, seamounts at offsets have received no systematic study. Proposed in this study is a 30-day seabeam/dredging program to survey and sample 25 seamounts (87 dredges) in various tectonic settings at OSCs and transforms and to complete a petrologic/geochemical study of the samples. The goals of this study are to chemically characterize the lavas of seamounts at offsets and to use that data to test half a dozen discrete hypotheses for the origin of seamounts at offsets, mantle flow and melting processes, and magma supply geometry at depths beneath the EPR and its surroundings. The petrogenetic interpretation of the seamount lavas will be combined with interpretations for the origin and evolution of coeval, EPR axial magmas to gain a broader understanding of petrogenetic-tectonic processes occurring beneath the EPR. The study has been designed to minimize potential problems of spatial/temporal correlation.