A great deal has been learned about ridge morphology, volcanism, and magmatic processes since the advent of seafloor observation, yet fundamental finer-scale crustal accretion processes and their rates remain largely unknown. This proposal leverages recent technological and methodological developments to quantify the spatial, morphological, and geochemical variance within the neovolcanic zone of three segments of the intermediate-spreading southern Juan de Fuca Ridge (JdFR) to evaluate five hypotheses about crustal accretion and melt supply: Hypothesis 1: Crustal accretion at intermediate-spreading ridges is episodic on timescales of decades to hundreds of years. Hypothesis 2: Crustal accretion is distributed across the <8km width of the JdFR neovolcanic zone. Hypothesis 3: Crustal accretion at intermediate-spreading ridges is mainly accomplished during infrequent large-volume, large-effusion-rate eruptions rather than the more common small-volume small-effusion-rate eruptions. Hypothesis 4: Magma reservoir replenishment occurs on ~decadal time scales at normal ridge segments, but ~millennial timescales at inflated segments.Hypothesis 5: Source compositions are more variable than melting processes at intermediate rate ridges.To evaluate these hypotheses, we propose a synthesis of our time-constrained geologic and volcanologic data for each segment to assess how their structural and magmatic evolution has fluctuated over thousands of years. We propose high-precision trace element and radiogenic isotope analyses to integrate geochemical and morphological variability with seismological observables to assess crustal-level magma replenishment rates and differentiation. We propose uranium-series analyses for the dual purpose of supplementing eruption age constraints and evaluating how mantle-melting processes compare between adjacent ?inflated? and ?normal? ridge segments. We also propose to use targeted magnetic paleointensity measurements to measure ages when neither radiocarbon nor U-series will work. This proposal brings two early career researchers Brian Dreyer and Julie Bowles further into mid- ocean ridge science with the guidance of two senior scientists, Clague and Gill. Undergraduate student will be involved in laboratory training, data analysis and interpretation, and presentation of results. Results and public outreach items will be posted on the Submarine Volcanism Project webpage at www.mbari.org/volcanism. Our results are particularly significant for Axial Seamount as it is a terminus for the regional cabled ocean observatory of the Ocean Observatories Initiative (OOI) designed to address fundamental questions key to understanding the evolution of oceans and submarine volcanoes over the next 25-30 years. Our proposal complements that objective because it develops the volcanological and geological understanding of how the magmatic system arrived at its present configuration over the last several decades to millennia. This proposal supports the major objectives of both the Marine Geology and Geophysics and Ridge2000 Programs. Dissemination of high-resolution (1-m) bathymetric and geologic maps, volcanologic histories, and geochemical and geomagnetic data for CoAxial segment, Axial Seamount, and northern Cleft segments will offer an unparalleled view of the processes that shape the mid-ocean ridge system, providing context and developing synergic interaction among the ridge science community.
