Nitrogen content and isotopic composition of oceanic crust at a superfast spreading ridge: A profile in altered basalts from ODP Site 1256, Leg 206

The present paper provides the first measurements of both nitrogen content and isotopic composition of altered oceanic basalts. Samples were collected from Ocean Drilling Program Site 1256 located at the eastern flank of the East Pacific Rise. Twenty‐five samples affected by low temperature alteration were analyzed. They include moderately altered basalts together with veins and related alteration halos and host rocks, as well as unique local intensely altered basalts showing green (celadonite‐rich) and red (iron oxyhydroxide‐rich) facies. Nitrogen contents of moderately altered basalts range from 1.4 to 4.3 ppm and are higher than in fresh MORB. Their δ15N values vary in a large range from +1.6 to +5.8‰. Veins, halos, and host rocks are all enriched in N relative to moderately altered basalts. Notably, veins show particularly high N contents (354 and 491 ppm) associated with slightly low δ15N values (+0.4 and −2.1‰). The intensely altered red and green facies samples display high N contents of 8.6 and 9.7 ppm, respectively, associated with negative δ15N values of −3.8 and −2.7‰. Detailed petrological examination coupled with N content suggests that N of altered basalts occurs as ammonium ion (NH4+) fixed in various secondary minerals (celadonite, K‐ and Na‐feldspars, smectite). A body of evidence indicates that N is enriched during alteration of oceanic basalts from ODP Site 1256, contrasting with previous results obtained on basalts from DSDP/ODP Hole 504B (Erzinger and Bach, 1996). Nitrogen isotope data support the interpretation that N in metasomatizing fluid occurred as N2, derived from deep seawater and likely mixed with magmatic N2 contained in basalt vesicles.