Cycling of CO<sub>2</sub> and N<sub>2</sub> Along the Hikurangi Subduction Margin, New Zealand: An Integrated Geological, Theoretical, and Isotopic Approach
Gabe S. Epstein, Gray E. Bebout, Bruce Christenson, Hirochika Sumino, Ikuko Wada, C. A. Werner, D. R. Hilton
Abstract
Abstract We present a quantitative assessment of the input and output of CO 2 and N 2 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 δ 13 C VPDB and CO 2 / 3 He values for the arc gases range from −8.3 to −1.4‰ and 2 × 10 9 to 2.7 × 10 11 , indicating contributions from slab carbonate, organic C, and mantle C of 67%, 30%, and 3%, respectively. The δ 15 N air and N 2 / 36 Ar values of arc gases are −1.0 to +2.3‰ and 1.54 × 10 4 to 1.9 × 10 5 , indicating slab and mantle contributions of 74% and 26%. The δ 13 C 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 CO 2 and 0.0022–0.0057 Tg/yr for N 2, corresponding to ∼2.2% and 1%–30% of the global CO 2 and N 2 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.