Ananya Mallik

Ananya Mallik

Volatile exchange between the Earth’s surficial and deeper reservoirs due to subduction determines the dynamics and evolution of the reservoirs through geological time. In spite of being a ubiquitous volatile element in the Earth and abundantly present in the Earth’s mantle, the behavior of nitrogen (N) in the mantle and exchange of N between the surficial and deeper terrestrial reservoirs have only gained attention recently. Previous studies [e.g. 1, 2] have estimated the recycling efficiency of N (the percentage of incoming N in subduction zones surviving the arc filter and entering the deep mantle) by difference of incoming and volcanic outgassing fluxes; these have obtained disparate estimates ranging between 0% to 80-92%. Also, such an approach ignores the possibility that a fraction of the N released from the subducted slab may not be outgassed but rather sequestered in the overriding plate. In this study [3], we empirically constrained the recycling efficiency of N by focusing on the mechanism of N transfer from slabs by aqueous fluids and/or hydrous partial melts, addressing a broad range of thermal regimes applicable in modern-day subduction zones. We estimate the amount of N carried by aqueous fluids as a function of P, T and fO₂ along slabs using the experimentally constrained model from a previous study [4]. We constrain the amount of N carried by hydrous partial melts in intermediate to hot subduction zones by bracketing between the most efficient transfer (experimentally determined solubility limit) and conservative transfer (modeled partitioning of N between key N hosting minerals and slab-melt). We find that globally 45-74% of incoming N in subduction zones survives past the arc magmatism filter and enters the deep mantle. We will briefly discuss the possible implications of such recycling efficiency on the evolution of the atmosphere and the mantle through time.

[1] Fischer et al. (2002) Science 297

[2] Barry & Hilton (2016) GPL 2

[3] Mallik et al. (2018) EPSL 482

[4] Li & Keppler (2014) GCA 129