CAREER: Evaluating the Continental Crust as a Key Reservoir of Earth's Nitrogen and Constraining the Nitrogen Abundance of Primitive Arc Magmas

About This Grant

Nitrogen (N) is an important part of cells in living beings, and it moves around different parts of the Earth like the air, land, oceans, and the deep interior. This movement helps shape the air we breathe and how suitable our planet is for living things over time. Places where the Earth's plates push against each other, called subduction zones, are excellent for studying how nitrogen moves between the surface and the interior. Recent studies show that these subduction zones send more nitrogen deep inside the Earth than volcanoes belch out. The continental crust might be a good place to store some of the nitrogen that is transferred to the Earth's interior in subduction zones. But scientists need to find out how much nitrogen is really in that crust to see if this idea is correct. In this project, the team will find out how much nitrogen is in the continental crust by taking samples from two places: one in Sierra Nevada, California, and another in the Transverse Ranges, California, and its equivalent lower crust in Chino Valley, Arizona. They will do experiments to see how nitrogen is shared between the lower crust materials and the melting rock. Some of their research will also be part of a summer camp for high school students from the Tucson Unified School District (TUSD). This camp will focus on the geology of Tucson and Arizona to help students connect with their community. The goal is to inspire students to learn more about geosciences and encourage them to study it at the University of Arizona. The uptake and exchange of the life-essential element nitrogen (N) between the Earth’s surface reservoirs (atmosphere, crust, oceans, biosphere) and the interior (lithosphere and deeper) via plate tectonics has regulated the atmospheric composition and habitability through geological history. The geologically recent subduction zones serve as an excellent natural laboratory to understand these uptakes and exchanges. Current estimates show that the output of nitrogen from subducted slabs is higher than the volcanically outgassed nitrogen. The continental arc crust could be an excellent contender for uptake of the ‘excess nitrogen’ but to test the hypothesis, the nitrogen abundance of continental arc crust needs to be determined. Also, the nitrogen abundance of primitive arc magmas is a key piece in understanding the fluxes of nitrogen between different subduction zone reservoirs and it is poorly constrained. Lower arc crust comprised of cumulates from differentiation of primitive arc magmas may serve as an excellent proxy to determine the nitrogen abundance of the primitive magmas. In this proposal, researchers will determine the nitrogen abundance of the arc continental crust. They will analyze samples from two sections of lower and mid to upper arc crust – one from Sierra Nevada, California and the other from Transverse Ranges, California (mid to upper crust) and its tectonically bulldozed lower crust in Chino Valley, Central Arizona. The team will perform nitrogen partitioning experiments between lower crustal cumulate minerals and arc melt. Finally, they will use those partition coefficients along with modeling the fractional crystallization of arc magmas to determine the nitrogen contents of primitive arc magmas that best explain the measured nitrogen abundances in the lower crust. Some aspects of the research plan will be integrated (along with other methodologies in geosciences) in a summer camp for high school students from Tucson Unified School District (TUSD). The summer camp will be themed around the geological history of Tucson and Arizona to provide the students a ‘sense of place’. The camp will serve as a feeder into a longer-term program to incorporate high school students in research-based education in geosciences. The objective of the summer camp is to motivate local high school students in geosciences, with the expectation that this would eventually result in enhanced enrollment of local high schoolers in geosciences at the University of Arizona. 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.