Subduction-Driven Volatile Recycling: A Global Mass Balance

The model adopted here follows the box-model approach proposed by Barry & Hilton (2016), which considers two main mantle reservoirs represented by the primitive, deep mantle (PLM) sampled by primitive (high 3He/4He) mantle plumes (e.g., OIBs, Yellowstone, Iceland) and the depleted, convecting upper (DMM), notably sampled at mid-ocean ridges. In line with the mass-balance presented in this contribution, the PLM and DMM are taken to represent 10wt.% and 90wt.%, respectively, of the total mass of the mantle. The model is then built on the following assumptions: (1) the N ingassing flux commences at the onset of subduction and is constant through time; (2) a constant proportion of the total N ingassing flux (F) is subducted into each respective mantle reservoir (defined here as FPLM and FDMM) and instantaneously mixes with each reservoir during subduction; (3) the mantle outgassing fluxes from the PLM and DMM reservoirs are constant through time, but may be varied from one simulation to another; (4) the model must produce N contents ([N]) and isotope compositions (δ15N) of the PLM and DMM reservoirs that correspond to present-day observations; and (5) the initial mantle δ15N prior to the onset of subduction is common to the PLM and DMM reservoirs, with a value of either -6‰ (Labidi et al. 2020; Main Text) or -40 ‰ (Barry & Hilton 2016; Supplemental Figure 3). The DMM and PLM reservoirs are considered to have evolved to their current δ15N (-5‰ and +3‰, respectively) and [N] (Tables 1) compositions as a consequence of additions of different amounts of subducted sedimentary N (δ15N = +5 ‰).