NITROGEN IN ULTRAMAFIC AND SEDIMENTARY MÉLANGE MATRICES AND ITS IMPLICATIONS FOR SUBDUCTION ZONE NITROGEN BUDGETS

Fluid and volatile mobilization is enhanced in subduction interface mélanges consisting of coherent blocks of metamorphosed oceanic crust, metasediments, and lithospheric mantle entrained in highly deformed matrices. Mélange matrices can be sedimentary or ultramafic in composition, with sediment-rich mélange more common in the shallow fore-arc and accretionary wedge and ultramafic-rich mélange more characteristic of the deeper fore-arc. At present, nitrogen (N) concentrations and δ¹⁵N in subduction-related metasedimentary and metabasaltic rocks and their protoliths are quantified for a number of localities; however, we do not have as strong a grasp on these variables in mélange matrix rocks despite their importance for fluid transport and geochemical evolution of subduction interfaces.

In this study, we report N concentrations and δ¹⁵N for well-characterized ultramafic (Syros, Greece, and New Caledonia) and sedimentary (San Simeon, California, US) mélange matrices; bulk rock geochemistry of all samples; and petrography of ultramafic samples. Syros samples (n=7) are massive and deformed serpentinites, and New Caledonia samples are chlorite and talc schists (n=2). In Syros samples, N concentrations range from 28-69 ppm with a median of 36 ppm, and δ¹⁵N_air ranges from -0.2 to +6.9‰ with a median of +6.0‰. In New Caledonia samples, N concentrations range from 25-102 ppm and δ¹⁵N ranges from -0.2 to +0.5‰. San Simeon samples are deformed metagraywackes (n=8). Preliminary data of these sedimentary matrix samples show N concentrations ranging from 226 to 574 ppm with a median of 412 ppm and δ¹⁵N ranges of +0.4 to +1.34‰ with a median of +1.1‰. These sedimentary matrix data have a more constrained δ¹⁵N range and higher N concentrations than the ultramafic matrices in this study. Samples of ultramafic matrix from Syros and New Caledonia show ranges in N concentration and δ¹⁵N similar to those previously reported for coherent oceanic crust and meta-serpentinite. San Simeon sedimentary matrix samples show N concentrations similar to and δ¹⁵N values lower than those previously reported for seafloor sediment and metasedimentary rocks. We use these data to understand the effect of fluid alteration or devolatilization on N isotopic values in shallow versus deeper fore-arc settings and its implications for the N budget in sediment-rich versus sediment-starved subduction zones.