Cycling of CO2 and N2 Along the Hikurangi Subduction Margin, New Zealand: An Integrated Geological, Theoretical, and Isotopic Approach

Abstract

We present a quantitative assessment of the input and output of CO2 and N2 along the Hikurangi margin based on the chemical and stable isotope composition of sediments and basalts (from IODP 375), previously accreted metasedimentary rocks, and volcanic/hydrothermal gases (together with noble gas data for the latter). We compare these results with 3-D thermo-petrologic models for four lithologic structures, representing different plateau inputs. The model results indicate that 59%–85% of initially subducted C and 5%–12% of N is lost from the slab during metamorphism, with both volatiles being dominantly sourced from altered oceanic crust with some contribution from subducted sediment at the forearc-arc transition (75–90 km depth). The δ13CVPDB and CO2/3He values for the arc gases range from −8.3 to −1.4‰ and 2 × 109 to 2.7 × 1011, indicating contributions from slab carbonate, organic C, and mantle C of 67%, 30%, and 3%, respectively. The δ15Nair and N2/36Ar values of arc gases are −1.0 to +2.3‰ and 1.54 × 104 to 1.9 × 105, indicating slab and mantle contributions of 74% and 26%. The δ13C signature of gases requires addition of organic C by tectonic erosion and/or shallow crustal assimilation. These calculations yield whole-margin fluxes of 5.4–7.0 Tg/yr for CO2 and 0.0022–0.0057 Tg/yr for N2, corresponding to ∼2.2% and 1%–30% of the global CO2 and N2 flux from subaerial volcanoes worldwide (assuming no loss during transit). This unique assessment of volatile cycling could prove useful in refining regional and global estimates of volatile recycling efficiency.