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

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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