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.
Since the 1950s, scientists have been aware of an underwater mountain chain snaking through the world’s oceans and noted its similarities to volcanic chains on land. By the 1970s, evidence – both direct and indirect – had piled up to conclusively demonstrate the volcanic nature of this "great global rift." We now know that this approximately 60,000 kilometer-long volcanic ridge system plays a key role in plate tectonics, and that volcanic activity along the rift created the crust that covers approximately 70% of Earth’s surface. Unlike easily accessible volcanoes on land, the submarine volcanic system is much more challenging to study. Steady progress has been made over the decades, but fundamental questions remain regarding this global ridge system. Where, how, and how often do eruptions occur? What does the magma plumbing system look like and how frequently is the magma reservoir replenished? Advances in seafloor mapping and observation over the past 10-15 years now allow us to map at unprecedented detail (up to 1 meter resolution). This allows us to begin to map seafloor volcanic systems in much the same way that we have been able to map volcanoes on land, and it allows us to begin to answer some of the above questions in the context of those detailed geologic maps. This collaborative project was designed to combine recent technological developments in seafloor exploration with recent advancements in geochemistry and paleomagnetism to quantify the variability in volcanic eruptions on three segments of the southern Juan de Fuca Ridge, located in the Pacific Ocean off the coasts of Washington and Oregon. (Paleomagnetism uses records of Earth’s magnetic field recorded in geological materials like lava flows to either interpret past variations in Earth’s magnetic field or to interpret a variety of geological processes.) In order to address questions related to frequency of eruptions, it is necessary to develop a chronology for the region. The paleomagnetic work funded by this grant is meant to constrain eruption ages for relatively young (less than ~2.5 thousand years) flows where radiometric dating methods are ineffective. This is done by comparing geomagnetic "paleo"intensity recorded in volcanic glass with models of field behavior over the past 3,000 years. Significant results can be broken into two regions, the first focusing on a large volcano that represents focused volcanic activity, Axial Seamount. Recent dedicated efforts have resulted in detailed geologic mapping of the Axial summit caldera, and the paleointensity data here can be used most directly to place additional age constraints on the flows. Based on the relative superposition of the flows, it is possible to assign relative flow ages within different regions of the caldera, but the paleointensity data allow us to refine the relative timing of flows between regions. Some additional age constraints are provided by carbon (14C) dating of the sediments overlying the lava flows. The paleointensity data are in all cases consistent with these 14C ages and in a number of instances allow us to further refine ages or place ages on flows with no sediment cover. Overall, the paleomagnetic ages are consistent with a model of episodic eruptions with an average recurrence interval of approximately 30 years over the past 250 years. Away from Axial Seamount, detailed flow maps are not available, but the paleointensity data allow us to place age constraints on flows thought to be less than a few hundred years old, based on lack of sediment cover. Older samples are associated with 14C ages taken from the overlying sediments and are up to about 6500 years old. Because these samples have independent age estimates, they actually allow us to refine our understanding of magnetic field behavior in this region, which is less complete as we go further back in time. Based on these data, we can say that current models of field behavior likely underestimate the magnitude of intensity fluctuations in the field. Additionally, samples with similar ages but very different paleointensity values suggest that extremely rapid variations in the field are possible, as previously observed in data from Israel. This project contributed to the training and development of three undergraduate students, who gained a basic background in the field of rock magnetism and paleomagnetism, and were involved in sample preparation and generation of data, as well as data processing and interpretation. In addition to the answering fundamental questions regarding submarine eruptive timing and geomagnetic field behavior, the increased understanding of volcanic eruption frequency may help to estimate volcanic hazards in some parts of the world.
