NSFGEO-NERC: Data Mining the Deep: Combining Geochemistry and Imaging Spectroscopy to Quantify the Impact on Ocean Chemistry of Deep Hydrothermal Circulation at Mid-Ocean Ridges

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Ocean crust is formed at mid-ocean spreading ridges and covers more than 60% of the surface of Earth. Because of the continuous plate tectonic cycle and subduction, the ocean crust is geologically young (<200 million years), much younger than the rocks that make up continents. Heated seawater circulates throughout the ocean crust from its creation until it gets consumed in subduction zones, reacting with the rock to form new minerals. Key elements and molecules such as water, CO2, K, and Mg are exchanged between seawater and the ocean crust. These interactions between fluids and the rock are a major – but not yet quantified – contributor to seawater chemistry and, through subduction, the composition of the interior of Earth. A key challenge is that ocean drilling has not sampled the deepest ocean crust many kilometers below the seafloor. However, in recent years, the International Continental Scientific Drilling Program’s Oman Drilling Project has drilled a few kilometers of oceanic crust and the uppermost mantle in Oman, where tectonics have pushed this critical portion of Earth’s crust onto the continent. In this project, scientists at the California Institute of Technology will work with colleagues at the universities of Southampton and Plymouth in the UK to investigate these drill cores to work out how much exchange there has been between seawater and the deep ocean crust. The project takes an innovative approach that combines detailed but traditional laboratory analyses of select samples of the core with a novel technique, micro-imaging infrared spectroscopy, which uses the reflectance of infrared light to determine the mineralogy of the entire drill core at a spatial resolution of less than one millimeter. This research will achieve major advances in understanding of deep circulation of fluids within the ocean crust by objective characterization at a scale and sampling completeness previously impossible. It will yield insights into the contribution of fluid-rock reactions to global geochemical cycling and seawater chemistry as well as constrain the role of deep fluid circulation in cooling and alteration of the lower oceanic crust. A website will be built with a core viewer to disseminate mineral maps of the drill cores to the broader scientific community and for educational use in classes. The project supports the training of undergraduate students. Recent estimates suggest that hydrothermal fluid fluxes through upper crustal lavas and dikes are insufficient to solidify and cool new crust close to the mid-ocean ridge axes, requiring major, but hitherto unquantified, deep hydrothermal circulation in the lower crust. To date, neither the pathways nor the geochemical consequences of this deep circulation have been considered in detail, but preliminary estimates indicate that these systems are necessary to export 40-60% of the available heat from the lower crust to the oceans and may influence ocean chemistry for key elements. In this joint project between NSF-supported scientists at Caltech and NERC-supported scientists at the Universities of Southampton and Plymouth in the UK, these cores from the Oman Drilling Project will be investigated to test two hypotheses that connect the formation of the ocean crust to the wider Earth system: (A) that fault zones in the lower ocean crust exhibit mineralization and hydrothermal alteration that record their role as major conduits for thermal and chemical exchange between the crust and ocean waters; and (B) that deep hydrothermal circulation in the lower crust has significant impacts on global geochemical cycles of specific elements including C, O, S, Li, B, Sr, Mg, Ca and base metals. The project has four objectives: (1) Determine the mineral assemblages of altered rocks and their geochemistry with depth in the oceanic crust and the requisite temperatures and chemistries of the alteration fluids. (2) Quantify elemental and isotopic exchanges between seawater and the lower oceanic crust and uppermost mantle and their contribution to global chemical cycles of the composition of seawater; (3) Determine the importance of deep hydrothermal circulation, including in fault zones, in cooling the lower oceanic crust; and (4) Build a website with core viewer hosting mineral maps and imaging spectroscopy datasets to enable use by future researchers and students. The objectives will be achieved through four tasks that map minerals in the core with imaging spectroscopy and petrologic and geochemical tie-ins, additional geochemical characterization, modeling and synthesis of results to determine chemical and thermal budgets, and development of a web viewer for mineral maps of the core. 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.