Journal
CHEMICAL GEOLOGY
Volume 583, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.chemgeo.2021.120474
Keywords
Nitrogen remobilization; Subduction zone; Blueschist; melange; Heilongjiang complex; Deep nitrogen recycling
Categories
Funding
- National Natural Science Foundation of China [41730210]
- China Institute of University of Alberta
- NSERC
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This study demonstrates that labile nitrogen in sediments released during early subduction can be at least partially (if not all) re-fixed into the more refractory metaigneous part of the subducting slab, which facilitates the retention of nitrogen for deep subduction and recycling.
Nitrogen (N) in subducting slab resides in both sediments and altered oceanic crust (AOC). The extent of subduction-zone metamorphic devolatilization of N in these reservoirs determines the amount of remaining N for deep recycling. However, N behavior inside the subduction zone, particularly below the forearc region, remains poorly understood so far. Here, we studied the pillow-shaped epidote-blueschist facies meta-basalts (hereafter refer to as blueschists) in the Heilongjiang Complex in Northeast China, aiming to understand the N behavior during the early subduction stage. Results show that these blueschists contain N (13.9 to 122.6 ppm; average: 51.0 ppm) up to seven times higher than the AOC (including pillow lavas) entering global trenches (N < 19 ppm; average 7 ppm). The N concentrations of the blueschists correlate with the Ba and Th concentrations, indicating that significant N enrichment of the blueschists took place inside the subduction zone in addition to small N enrichment on seafloor. The N concentrations and delta N-15 values of the blueschists show two distinct mixing trends, indicating that two types of ammonium (NH4+) were added into the blueschists. One is characterized by relatively high abundance and positive delta N-15 value (similar to +3 parts per thousand), which can be attributed to the direct assimilation of NH4+ in metamorphic fluids derived from the sedimentary components in the melange. The other is characterized by relatively low abundance and extremely low delta N-15 values (similar to -16 parts per thousand). This NH4+ is consistent with a product of abiotic reduction of sediment-derived N-2. This study demonstrates that the labile N in sediments released during early subduction can be at least partially (if not all) re-fixed into the more refractory metaigneous part of the subducting slab, which facilitates the retention of N for deep subduction and recycling.
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