[S-GC48] Volatiles in the Earth - from Surface to Deep Mantle

[SGC48-03] C-N-He-Ar Cycling at the Hikurangi Subduction Margin, New Zealand

Gabe S Epstein¹、*Gray E Bebout^1,2、Bruce W Christenson³、Hirochika Sumino⁴、Ikuko Wada⁵、Cynthia A Werner⁶、David R Hilton⁷ (1.Dept. Earth and Environmental Sciences, Lehigh University, USA、2.Okayama University, Institute for Planetary Materials、3.National Isotope Center, GNS Science, New Zealand、4.Dept. of Basic Science, University of Tokyo、5.Dept. of Earth Sciences, University of Minnesota, USA、6.USGS-Contractor, USA、7.Scripps Institution of Oceanography, USA)

キーワード：subduction zone, noble gas, carbon, nitrogen, New Zealand, Hikurangi

We are evaluating the cycling of C, N, and noble gases at the Hikurangi margin, with ~35 analyses of gases from across the forearc-arc-backarc and further analyses planned for April-May 2020. We present C-N concentrations and isotope compositions of sediments outboard of the trench and wall-rock metasediment in the Taupo Volcanic Zone (TVZ). We compare these data with noble gas and C-N data for gases from fumaroles and thermal springs. Ongoing work includes thermal modeling, thermodynamic calculations of prograde devolatilization, and estimation of TVZ CO₂ output flux.

The incoming sediment section at IODP Site 1520 consists of uppermost terrigenous trench-fill (7 ± 3 wt.% carbonate, 0.39 ± 0.17 wt.% organic C), pelagic sediment (61 ± 21 wt.% carbonate, 0.24 ± 0.15 wt.% organic C), and lowermost volcaniclastics (13 ± 14 wt.% carbonate). Isotope compositions are relatively uniform, with δ¹⁵N = +4.4 ± 0.9‰ (AIR), δ¹³C_carb = +0.9 ± 1.1‰ (VPDB), and δ¹³C_red = -25.9 ± 1.2‰ (VPDB). Wall-rock metasediments have δ¹⁵N = +2.4 to +6.4‰, δ¹³C_red = -25.0 ± 1.9‰. Trench-fill sediments are largely removed by accretion, thus the carbonate-rich section likely contributes more to the gas emissions.

The dominant C-bearing gas phase in the forearc is CH₄ (δ¹³C = -35 to -53‰) and that within the TVZ gases is CO₂ (δ¹³C = -2 to -10‰). Forearc noble gas ratios have crustal to atmospheric values (± minor mantle contribution; ³He/⁴He = 0.2-1.7 R_A and ⁴⁰Ar/³⁶Ar ≧ 296), while He-Ar and C-N isotope values of gases from the TVZ are consistent with mantle and recycled sedimentary contributions (³He/⁴He = 4–7 R_A, δ¹⁵N = +1.3 ± 0.9‰, and [N₂/³⁶Ar]/AIR = 1–10). Overlap in δ¹³C_red and δ¹⁵N of incoming sediments and wall rocks complicates differentiation of C sources but, given the accretion of the trench-fill sequence, the apparent sediment-derived C_org component (about 30%, after [1]) and N in the gases could reflect contamination by Torlesse/Waipapa wall rocks.

[1] Sano & Marty (1995) Chem. Geol. 119, 265-274.