Collaborative Research: Tracking nitrogen in mélange matrix from fore-arc to sub-arc depths with implications for deep nitrogen cycling: A combined field and experimental approach

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The nitrogen (N)-cycle as it relates to the bio-, atmo-, and hydro-spheres have been well studied due to N’s abundance in Earth’s atmosphere and importance for life. In the less understood solid Earth N cycle, plate tectonics has regulated N fluxes between surface and deep Earth reservoirs over much of Earth’s history, affecting the bulk Earth N distribution over millennial timescales. Thus, the mass balance of nitrogen (N) delivered to the deep Earth during subduction and then returned to the surface during volcanism and degassing is critically important, yet efficiency estimates are highly variable. Current estimates suggest that 45-74% of subducted N does not return to the surface through arc volcanism. This implies that N is being sequestered in the deep Earth in variable amounts, which could reflect factors such as subducting plate composition or subduction conditions. Candidates for deep, poorly characterized N reservoirs include mid-lower continental crust, subcontinental mantle, fore-arc to sub-arc mantle, or deeper mantle (i.e., deeper upper mantle, transition zone or lower mantle). This highlights a need for N measurements on appropriate samples as well as thorough constraints on N behavior in these reservoirs. New key constraints on nitrogen (N) distribution and processing within the fore-arc to sub-arc regions of subduction zones will be provided. First, some of the first N composition measurements of sediment-rich and serpentinite-rich mélange matrix rocks and minerals to characterize the distribution of N during fore-arc processing will be made. Second, phase equilibria experiments to assess the stability of key N hosting minerals and measure N melt/fluid-mineral partition coefficients on mélange-matrix materials as a function of several factors (pressure, temperature, oxygen fugacity, chlorine content, and partial melt composition) to track N behavior during dehydration and partial melting in the slab at sub-arc depths will be performed. Data from the proposed study along with those from previous studies will be used to quantify the amount of N that is delivered from the fore-arc to the sub-arc processing zone, in which minerals it is hosted, and how it varies by dominant lithology (sediment or serpentinite). How much N is released from the slab during sub-arc processing versus how much is sequestered in the sub-arc mantle in subduction zones of different thermal states will then be quantified. These will constitute novel constraints on N behavior that can be applied to subduction regimes throughout Earth’s history. Hence, it will also be used to address the feedback and evolution of N across the coupled solid Earth-atmosphere systems. This proposal supports two early career female PIs, two graduate students, and two+ undergraduates from University of Arizona (UA) and University of Southern California (USC). To enhance collaboration and broaden participation, the PIs will offer a joint virtual graduate seminar on deep volatile cycling including students at UA and USC. The team at UA will develop a museum display at UA’s Alfie Norville Gem & Mineral Museum on high pressure-high temperature geoscience research (including laboratory equipment and research applications).As the most abundant constituent of the Earth’s atmosphere and as an essential ingredient of life, the behavior of nitrogen (N) in the present-day atmosphere, oceans, crust and biosphere (collectively known as the surficial reservoirs) have been relatively well-studied. However, the N composition of the Earth’s surficial reservoirs may not have remained the same throughout Earth’s history and this may have implications for early Earth climate and evolution of life. Nitrogen is exchanged between the Earth’s surficial reservoirs and the deep interior via plate tectonics, especially subduction zones. In subduction zones, N in the Earth’s crust (along with components from the atmosphere, ocean and biosphere) is pulled into the mantle or the interior of the Earth. Some proportion of the N from the mantle escapes back into the atmosphere and ocean by volcanic degassing. This N exchange between the surface and interior is not well-constrained and this proposed study aims to fulfill a key component of this knowledge gap. The N composition of typical subduction zone rocks will be measured to determine where N is hosted as pressure and temperature increase. Laboratory experiments at conditions in the Earth’s mantle will be performed to understand the behavior of N once the crust enters the mantle and melts. The objective is to eventually use these results to estimate how the N composition of the Earth’s mantle and atmosphere have changed through Earth’s history. This proposal supports two early career female PIs, two graduate students, and two+ undergraduates from University of Arizona (UA) and University of Southern California (USC). To enhance collaboration and broaden participation, the PIs will offer a joint virtual graduate seminar on the proposed theme including students from both institutions. The team at UA will develop a museum display at UA’s Alfie Norville Gem & Mineral Museum on plate tectonics connecting the surface and interior of the Earth, which would be an excellent medium to educate the public on state-of-the-art research.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.