Collaborative Research: Developing New Models of Oceanic Magmatism and Source Heterogeneity Using the 8 degree 20' N Seamounts as Windows into the Sub-Ridge Mantle

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Eruptions of lava form submarine volcanoes, known as seamounts, near mid-ocean ridge spreading centers. These lavas can provide important information about what Earth’s uppermost (~ 120 miles) mantle is composed of and how it melts to form the majority of Earth’s crust. At present no studies have examined long chains of seamounts adjacent to mid-ocean ridges to address these fundamental questions. In this study, we plan to analyze over 400 well located lava samples from the 8°20'N seamount chain. The chain was previously sampled and mapped in 2016 and 2018 using the submersible Alvin and autonomous vehicle Sentry. The geochemical, mineralogical, and age data to be obtained under this project will allow us to determine the origin of the magmas that feed the seamounts and nearby ridge. The data will also tell us how those magmas evolve to form oceanic lavas, and how the volcanism is related to regional tectonic processes. The study will promote advances in the field of geochemistry, marine geology, and geodynamics. This project will provide significant mentorship and training for students at a variety of levels. It will fully or partially support three graduate students at Florida and Boise State and will provide training for several undergraduates in geochemical techniques and interpretation, and support senior research projects. The project will also support education and diversity by funding two female scientists, one a first-generation college graduate, toward obtaining their PhDs in geochemistry. Both principal investigators are heavily involved in outreach and education at the K-12 levels in Florida and Idaho.Sparse sampling and geochemical analyses of seamounts scattered throughout the northeastern Pacific suggest the sub-ridge mantle associated with the East Pacific Rise (EPR) has far greater compositional variability then is preserved in lavas erupted on-axis. While the relative homogeneity of axial lavas has resulted in intriguing generalizations about magma differentiation and mixing processes at mid-ocean ridges (MOR), seamount studies have had a significant impact on our understanding of the source compositions for oceanic basalts, the scales of mantle heterogeneity, and mantle melting systematics at MOR. However, many of these studies have focused on dredged lavas from individual seamounts with very few combining in situ sampling and high-resolution mapping from a single chain oriented perpendicular to the ridge axis. To fill this gap in knowledge about the sub-ridge mantle, two research cruises in 2016 and 2018 investigated the 8°20’N seamount chain that extends ~200km west of the EPR. The 8°20’N seamount chain was unexplored and virtually unsampled before our highly successful OASIS cruises. Geophysical, observational, and preliminary geochemical data from this nearly continuous chain of volcanoes led us to unexpected discoveries about their construction and revealed extreme compositional variability in the near MOR mantle that was previously only surmised from regional seamount studies in the northeast Pacific. The location, orientation, and preliminary data from the 8°20'N seamount chain make it an ideal a natural laboratory to investigate source heterogeneity and melting systematics in the near-ridge mantle. We propose to study the petrologic processes and mantle sources feeding MOR and near-axis volcanoes though comprehensive geochemical analyses of lavas and melt inclusions from the 8°20’N seamounts. Site-specific sampling using HOV Alvin and extensive, fine-scale mapping with AUV Sentry along the seamount chain, coupled with our previous comprehensive investigations of the 8°- 10°N EPR provide an exceptional opportunity to constrain geodynamic and geochemical models of mantle melting near MOR and determine the extent and length scales of mantle heterogeneity. We propose to collect a wide spectrum of petrologic and geochemical data including Sr, Nd, Pb, and Os isotopes, melt inclusion compositions, volatile contents, and 40Ar/39Ar age dates of lavas from the extensive collection of samples we have from the 8°20’N seamount chain. These results will be combined with numerical models to produce a petrologic model of mantle melting and magmatic processes in the near-ridge mantle. This extensive off-axis seamount chain provides a rare opportunity to probe the compositions of the mantle near MOR and to understand how magmas are spatially and temporally distributed in the shallow mantle and crust. Results from this research have implications for the generation of the majority of Earth’s crust.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.