Quantification of nitrogen in subducting slabs and its recycling in the forearc and sub-arc regions
Creators: Li, Kan
Publisher: name: University of Alberta Library
Year: 2023

<p>Nitrogen, a life-essential element, is exchanged between Earth’s surface and interior through subduction, which has been acting as a dominant driving force regulating the long-term evolution of Earth’s major N reservoirs. However, the nitrogen input fluxes into global subduction zones and the recycling efficiency of subducted slab N inside subduction zones remain loosely constrained. In this thesis, I present six separate studies in two chapters to characterize the nitrogen in the poorly studied mafic-ultramafic components of subducting slab and the recycling of N in the forearc to sub-arc regions.<br>
Chapter 2 is focused on the nitrogen characterization of the entire subducting slab, from top to bottom, including altered basalts, altered sheeted dikes and gabbroic rocks and oceanic serpentinites.<br>
    In Chapter 2.1, from the large variation of nitrogen concentrations in global altered basalts (1.3 – 48.4 ppm), I found that the magnitude of nitrogen enrichment in the upper oceanic crust was primarily controlled by the nitrogen availability in the surrounding environment, which is ultimately related to the nitrogen abundance of basal sediments and/or seawater. With the consideration of this primary factor, we estimated a global nitrogen input flux of 3.7±0.3 ×109 mol·yr˗1 for the 300 – 600 m upper oceanic crust.<br>
    In Chapter 2.2, I examined nitrogen features in the sheeted dikes and gabbroic rocks, which were dominantly affected by moderate- to high-temperature hydrothermal alteration. The examined sheeted-dike and gabbroic samples from global ocean floor ubiquitously contain elevated nitrogen concentrations (2.7 – 33.5 ppm). Mixing modeling revealed that the secondary nitrogen was predominantly sourced from seawater with d15N values of -1‰ – +7‰. Based on the sheeted dike and gabbroic data, I estimated a global nitrogen input flux of 21.8 (-1.3)/(+0.9) – 30.6(±2.2)×109 mol·yr˗1 for the total oceanic crust.<br>
    In Chapter 2.3, I examined oceanic serpentinites with variable serpentinization temperatures, from high temperatures (up to &gt; 350 ºC) at depths to low temperatures (&lt; 150 ºC) near seafloor. The results show that these serpentinites are also enriched in secondary nitrogen (3.2 – 18.6 ppm) predominantly sourced from seawater with d15N value of -1‰ – +6‰. Based on these data, the serpentinized sub-oceanic mantle is estimated to contribute 3.3×109 mol·yr˗1 nitrogen to global subduction zones.<br>
Chapter 3 is devoted to the characterization of nitrogen recycling inside subduction zones as the slab is subducted into the forearc to sub-arc regions.<br>
    In Chapter 3.1, I investigated the nitrogen characteristics of serpentinites exhumed from the mantle wedge in the Izu-Bonin-Mariana (IBM) mud volcanoes, corresponding to a slab depth of 9 ‒ 29 km. The results show that these serpentinites contain variable amounts of recycled nitrogen (3.6 – 16.4 ppm), which is likely sourced mainly from pore fluid with minor contribution from the subducting sediments. Estimation suggests that the contribution of recycled slab nitrogen (2.6±1.3 × 106 mol·yr-1) to the serpentinized mantle wedge only accounts for up to 0.7% of the nitrogen input flux of subducting sediments solely at IBM, suggesting that recycling of slab N at the early stage of subduction is minor, at least in the IBM subduction zone.<br><br>
    In Chapter 3.2, as a complimentary study to Chapter 3.1, I investigated the nitrogen characteristics of blueschist-facies meta-basalts (peak metamorphic conditions: 320 – 450 ºC and 0.9 – 1.1 GPa) from the Heilongjiang Complex in Northeast China. The results show significant incorporation of sedimentary nitrogen (13.9 to 122.6 ppm) into the meta-basalts inside the subduction channel. This indicates that the labile nitrogen in sediments released during early subduction within the forearc region can be at least partially re-fixed into the more refractory meta-igneous part of the subducting slab, facilitating the retention of nitrogen for deep subduction and recycling.<br>
    In Chapter 3.3, through the mass balance between the nitrogen input fluxes estimated from the reference sites offboard the Central America (Holes 1256D and 504B) and IBM (Holes 801C and 1149D) trenches and nitrogen output fluxes through arcs, I showed that, despite of the contrasting thermal regimes between the warm Central America and cold IBM subduction zones, a large fraction (&gt; 50%) of the slab nitrogen is transferred beyond sub-arc depths.<br>
In Chapter 4, I summarized our current state of knowledge about the distribution of nitrogen in subducting slabs and its recycling in the forearc and sub-arc regions, as slabs sink.</p>