Cycling Of Co2 And N-2 Along The Hikurangi Subduction Margin, New Zealand: An Integrated Geological, Theoretical, And Isotopic Approach

We present a quantitative assessment of the input and output of CO2 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 delta C-13(VPDB) and CO2/He-3 values for the arc gases range from -8.3 to -1.4 parts per thousand and 2 x 10(9) to 2.7 x 10(11), indicating contributions from slab carbonate, organic C, and mantle C of 67%, 30%, and 3%, respectively. The delta N-15(air) and N-2/Ar-36 values of arc gases are -1.0 to +2.3 parts per thousand and 1.54 x 10(4) to 1.9 x 10(5), indicating slab and mantle contributions of 74% and 26%. The delta C-13 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 N-2,N- corresponding to similar to 2.2% and 1%-30% of the global CO2 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.