Nitrogen recycling in subduction zones

Halama, Ralf (2012) Nitrogen recycling in subduction zones [Talk] In: The Lübeck Retreat, Collaborative Research SFB 574 Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters, 23.-25.05.2012, Lübeck.

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Nitrogen (N) is the seventh most abundant element in the universe and accounts for ~78 vol.% of the
Earth’s atmosphere. Knowledge of the amount of N being subducted and the extent to which N is
released from subducting rocks during devolatilization is of fundamental importance in understanding
Earth’s nitrogen cycle. Estimates of the amounts and isotopic compositions of subducted N are critical
in evaluating whether or not the N isotope compositions of certain mantle-derived magmas reflect
retention of N in deeply subducted oceanic lithosphere and sediments [1, 2]. It is also critical in
attempts to balance subduction zone N inputs from the subducting plate with N outputs in arc volcanic
gases [3, 4]. For N, the presence of a significant imbalance between a large, isotopically heavy subducted flux compared to an isotopically light, relatively small outgassed flux suggests that
significant amounts of N were trapped in the mantle during Earth’s history [5].
Nitrogen concentrations and isotopic compositions of high- and ultrahigh-pressure mafic eclogites,
aimed at characterizing the subduction input flux of N in deeply subducting oceanic crust, show
positive !15N values and elevated N contents, which are distinct from fresh MORB but overlap with
altered oceanic crust [6]. The N systematics indicate negligible effects of metamorphic
devolatilization, but some eclogite suites show evidence for fluid-mediated addition of a sedimentary
N component.
Concentrations and isotopic compositions of N in hydrated mantle rocks were determined in samples
reflecting different stages of the subduction zone cycle, from oceanic alteration to high-pressure
metamorphism, to assess redistribution and isotope fractionation of N by ultramafic dehydration and
to determine the magnitude of N subduction in hydrated slab mantle. Low-grade serpentinized
peridotites that formed during early stages of subduction have variable !15N values, ranging from
values close to the composition of the depleted mantle (-5‰) to isotopically heavier values
overlapping those of modern marine sediments and metamorphosed sedimentary rocks. This
suggests an addition of organic-sedimentary N to the peridotites, incorporated via serpentinization
during bending–related faulting of the slab and/or via metasomatic additions during hydration in the
forearc mantle wedge. Nitrogen is retained in HP peridotites down to depths of at least 60-70 km, and
there is apparently no significant loss of N due to dehydration. Hence, the sedimentary N isotopic
signature, derived from interaction with serpentinizing fluids, is largely preserved during prograde
dehydration of the slab mantle.
Calculated global input fluxes for a range of representative subducting sections of unmetamorphosed
and HP-metamorphosed slabs, all incorporating serpentinized slab mantle, range from 1.1 x 1010 to
3.9 x 1010 mol N2/year. The best estimate for the !15Nair of the subducting slab is +4±1‰, supporting
models that invoke recycling of subducted N in mantle plumes and consistent with general models for
the volatile evolution on Earth.
References: [1] Marty and Dauphas, 2003, EPSL 206:397-410; [2] Jia et al., 2003, EPSL 215:43-56; [3] Li et al., 2007, GCA
71:2344-2360; [4] Mitchell et al., 2010, G3 11:Q02X11, doi:10.1029/2009GC002783; [5] Javoy, 1998, Chem. Geol. 147:11-25;
[6] Halama et al., 2010, GCA 74:1636-1652

Document Type: Conference or Workshop Item (Talk)
Keywords: Geodynamics
Research affiliation: OceanRep > SFB 574 > C1
OceanRep > SFB 574
Kiel University
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Date Deposited: 21 Sep 2012 10:24
Last Modified: 29 May 2013 09:10

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