Phase Equilibria of Ultramafic-Rich and Sediment-Rich Mélange Lithologies in Cold to Hot Subduction Zones - Implications for Deep Nitrogen Cycling

Nitrogen (N) is a life-essential element dominant in the present atmosphere and found in the hydrosphere and lithosphere. Knowledge of the distribution of N in various reservoirs of Earth requires an in-depth understanding of deep N cycling. Specifically, gaps exist in our understanding of N behavior in mélanges during subduction to sub-arc depths. Exhumed subduction-related mélanges consist of diverse lithologies, including highly deformed sediment-rich (mud-shale) and ultramafic-matrices (serpentinite and chlorite schists), rich in water (>7wt.%). The potential of such a wide range of mélange matrices to store or deliver N into the sub-arc mantle depends on the phase equilibria, which are not well-constrained as there are limited studies. Here, we performed piston-cylinder experiments to investigate the phase equilibria of two synthetic analogs of end-member mélange compositions at sub-arc conditions (2GPa and 800-1225°C). We used the composition of natural samples from San Simeon (Franciscan Complex, USA) and Syros (Greece) and mixed them at two different ratios that represents sediment-rich (90%Simeon-10%Syros) and ultramafic-rich mélanges (90%Syros-10%Simeon). We also explore the effect of water (5-10wt.%).

Partial melting of hydrous ultramafic matrix produces orthopyroxene, olivine, mica, and amphibole coexisting with hydrous basaltic melts (50wt.% SiO2, Mg# 75-60). The solidus at 10 wt.%H2O is < 1000°C, whereas at 5wt.%H2O is < 1100°C. Mica and amphibole are potential key N-hosting minerals in ultramafic mélanges stable up to 1000°C. On the other hand, partial melting of sediment-rich mélanges produces olivine-free residues with abundant orthopyroxene, amphibole, mica, garnet, and quartz coexisting with hydrous rhyolitic melts (65-75wt.% SiO2, Mg# 41-54). The solidus at both 10 wt.% and 5wt.%H2O is < 800°C. At these T ranges, amphibole is the dominant N-hosting mineral stable up to 1000°C.

Our study indicates that ultramafic and sedimentary mélanges have the potential to held back N by key N-hosting minerals in cold to hot modern subduction zones and can help transport N to depths below the arc magma factory. If these mélanges rise diapirically into the core of the mantle-wedge owing to positive buoyancy, N may be retained in the diapir up to 1000°C.