JpGU-AGU Joint Meeting 2020

Presentation information

[E] Oral

S (Solid Earth Sciences ) » S-GC Geochemistry

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

convener:Takeshi Hanyu(Japan Agency for Marine-Earth Science and Technology, Research Institute for Marine Geodynamics), Gray E Bebout(Lehigh University), Hirochika Sumino(Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo), Yuji Sano(Division of Ocean and Earth Systems, Atmosphere and Ocean Research Institute, University of Tokyo)

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

Gabe S Epstein1, *Gray E Bebout1,2, 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)

Keywords: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), δ13Ccarb = +0.9 ± 1.1‰ (VPDB), and δ13Cred = -25.9 ± 1.2‰ (VPDB). Wall-rock metasediments have δ15N = +2.4 to +6.4‰, δ13Cred = -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 CH413C = -35 to -53‰) and that within the TVZ gases is CO2 13C = -2 to -10‰). Forearc noble gas ratios have crustal to atmospheric values (± minor mantle contribution; 3He/4He = 0.2-1.7 RA 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 RA, δ15N = +1.3 ± 0.9‰, and [N2/36Ar]/AIR = 1–10). Overlap in δ13Cred 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 Corg 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.