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A re-assessment of the nitrogen geochemical behavior in upper oceanic crust from Hole 504B: implications for subduction budget in Central America

A re-assessment of the nitrogen geochemical behavior in upper oceanic crust from Hole 504B: implications for subduction budget in Central America
A re-assessment of the nitrogen geochemical behavior in upper oceanic crust from Hole 504B: implications for subduction budget in Central America

The geochemical behavior of N during seawater-oceanic crust alteration remains poorly constrained. Yet, it is a central parameter to assess the flux of N to subduction zones. Most studies proposed that hydrothermally altered basaltic rocks are enriched in N relative to fresh basalts. However, published data from DSDP/ODP Hole 504B, a reference site for the composition of the oceanic crust, suggest that seawater alteration leads to the N depletion of the upper ocean crust. To better address this issue, we analyzed N concentration and isotope composition of 21 altered basalts from the lavas and sheeted dikes sampled by Hole 504B. These new analyses show significant N enrichment (up to 14.1 ppm) relative to fresh degassed MORB (∼1 ppm). The differences observed between earlier and modern data are interpreted as resulting from analytical artifact due to the earlier use of a molybdenum crucible for N extraction. Furthermore, our new data show a progressive decrease of N concentration with depth, from 14.1 to 1.4 ppm. Nitrogen isotope compositions display a large range, with δ15N values from −0.9 to +7.3‰, and most likely reflect multiple stages of alteration with fluids of various compositions. In contrast to N concentration, δ15N values do not show a global depth trend but oscillate around a mean value of 3.0 ± 2.2‰ (1SD). The N concentration shows a positive correlation with bulk rock δ18O values, suggesting that N behavior during alteration process is mainly controlled by temperature. We propose that N speciation in the hydrothermal fluid is dominated by NH3/NH4 at low temperature (<200 °C) but is transformed to N2, associated with H2, at higher temperature (>200 °C). These new data are used to re-evaluate the global flux of N input into Central American subduction zone, showing that the upper basaltic crust represent about 20% of the total N buried in subduction zone. A comparison with previous results obtained on N degassed in volcanic arc illustrates that, in “warm” subduction zone like Central America, up to 50% of the subducted N may be transferred to the deep mantle. This contrasts with “cold” subduction environments, where >80% of the N inputs escape sub-arc slab devolatilization and supports that the geothermal gradient plays a major role in determining the N fate in subduction zones.

hydrothermal, isotopes, nitrogen, oceanic crust, subduction
0012-821X
1-10
Busigny, Vincent
573b915a-a86b-4f04-8818-5b98e636c081
Cartigny, Pierre
32e2a76e-4069-4543-b0ee-237894c25518
Laverne, Christine
f40796b6-e03a-4fea-b149-89130118129f
Teagle, Damon
396539c5-acbe-4dfa-bb9b-94af878fe286
Bonifacie, Magali
bf072d26-3f2e-42d7-aa3f-a9c04c3e2962
Agrinier, Pierre
cdf79a29-60ea-49e3-8ebd-3106cb4241d0
Busigny, Vincent
573b915a-a86b-4f04-8818-5b98e636c081
Cartigny, Pierre
32e2a76e-4069-4543-b0ee-237894c25518
Laverne, Christine
f40796b6-e03a-4fea-b149-89130118129f
Teagle, Damon
396539c5-acbe-4dfa-bb9b-94af878fe286
Bonifacie, Magali
bf072d26-3f2e-42d7-aa3f-a9c04c3e2962
Agrinier, Pierre
cdf79a29-60ea-49e3-8ebd-3106cb4241d0

Busigny, Vincent, Cartigny, Pierre, Laverne, Christine, Teagle, Damon, Bonifacie, Magali and Agrinier, Pierre (2019) A re-assessment of the nitrogen geochemical behavior in upper oceanic crust from Hole 504B: implications for subduction budget in Central America. Earth and Planetary Science Letters, 525, 1-10, [115735]. (doi:10.1016/j.epsl.2019.115735).

Record type: Article

Abstract

The geochemical behavior of N during seawater-oceanic crust alteration remains poorly constrained. Yet, it is a central parameter to assess the flux of N to subduction zones. Most studies proposed that hydrothermally altered basaltic rocks are enriched in N relative to fresh basalts. However, published data from DSDP/ODP Hole 504B, a reference site for the composition of the oceanic crust, suggest that seawater alteration leads to the N depletion of the upper ocean crust. To better address this issue, we analyzed N concentration and isotope composition of 21 altered basalts from the lavas and sheeted dikes sampled by Hole 504B. These new analyses show significant N enrichment (up to 14.1 ppm) relative to fresh degassed MORB (∼1 ppm). The differences observed between earlier and modern data are interpreted as resulting from analytical artifact due to the earlier use of a molybdenum crucible for N extraction. Furthermore, our new data show a progressive decrease of N concentration with depth, from 14.1 to 1.4 ppm. Nitrogen isotope compositions display a large range, with δ15N values from −0.9 to +7.3‰, and most likely reflect multiple stages of alteration with fluids of various compositions. In contrast to N concentration, δ15N values do not show a global depth trend but oscillate around a mean value of 3.0 ± 2.2‰ (1SD). The N concentration shows a positive correlation with bulk rock δ18O values, suggesting that N behavior during alteration process is mainly controlled by temperature. We propose that N speciation in the hydrothermal fluid is dominated by NH3/NH4 at low temperature (<200 °C) but is transformed to N2, associated with H2, at higher temperature (>200 °C). These new data are used to re-evaluate the global flux of N input into Central American subduction zone, showing that the upper basaltic crust represent about 20% of the total N buried in subduction zone. A comparison with previous results obtained on N degassed in volcanic arc illustrates that, in “warm” subduction zone like Central America, up to 50% of the subducted N may be transferred to the deep mantle. This contrasts with “cold” subduction environments, where >80% of the N inputs escape sub-arc slab devolatilization and supports that the geothermal gradient plays a major role in determining the N fate in subduction zones.

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Busigny_et_al_2019_EPSL_Final - Accepted Manuscript
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Accepted/In Press date: 26 July 2019
e-pub ahead of print date: 9 August 2019
Published date: 1 November 2019
Keywords: hydrothermal, isotopes, nitrogen, oceanic crust, subduction

Identifiers

Local EPrints ID: 434437
URI: http://eprints.soton.ac.uk/id/eprint/434437
ISSN: 0012-821X
PURE UUID: 5f26e37b-283e-445c-94c3-091664a842df
ORCID for Damon Teagle: ORCID iD orcid.org/0000-0002-4416-8409

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Date deposited: 23 Sep 2019 16:31
Last modified: 07 Oct 2020 06:39

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Contributors

Author: Vincent Busigny
Author: Pierre Cartigny
Author: Christine Laverne
Author: Damon Teagle ORCID iD
Author: Magali Bonifacie
Author: Pierre Agrinier

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