Multidisciplinary assessment of volatile release beneath North Island, New Zealand: geologic, geochemical, and thermodynamic forward modeling approach

Abstract:

Volatiles released during subduction zone metamorphism control the locus and intensity of arc magma production and related degassing. The systematics of volatile flux are a function of the subduction zone geometry and thermal parameters, the lithologies being subducted, and the volatile contribution from mantle wedge and/or assimilated crustal lithologies. We present a forward model for H₂O and CO₂ production and release from the subducting slab beneath the North Island of New Zealand for 250km along trench (from Whakaari in the N to Taranaki in the S). The model inputs are obtained from a steady-state 3D thermal model of the Hikurangi subduction zone, chemical and C, O, N isotope analyses of samples obtained from IODP 375 (Site U1520), and petrologic and geophysical studies of Hikurangi plateau structure. Crustal assimilant is represented by accreted metasediments of the Torlesse and Waipapa Terranes for which we measured δ¹³C_red and δ¹⁵N.

We model subduction of a normal oceanic crust + serpentinized mantle peridotite as well as three idealized plateau structures containing variably altered, thickened oceanic crust. Seismic studies indicate significant sediment accretion along the Hikurangi margin and we include only 500m of pelagic carbonate in the model input. Devolatilization reactions and volatile loss are calculated using the thermodynamic software Perple_X assuming instantaneous fluid loss (i.e. approximating channelized fluid flow). The model results show a broad depth range of CO₂ loss in the North extending from subarc depths to >220km depth. Towards the South, there is an increase in subarc CO₂ flux but a significant decrease in backarc release. Total whole-margin flux estimates of slab-derived CO₂ for the four models range from 3.9–6.2 Tg/yr, or 1.3–2.1% of the global flux.

Analyzed arc gases have δ¹³C_VPDB from -11.2 to -1.4‰, and CO₂/³He from 2x10⁹ to 2.6x10¹¹, indicating 20-87% C contribution from subducted carbonate (median=64.8%). Overlap in δ¹³C_red and δ¹⁵N of the incoming sediments and Torlesse wall rocks complicates differentiation of C sources, but seismic evidence and geographic distributions of gas chemistry are consistent with the reduced C component being derived largely from assimilation. Additional gas sampling and analyses in 2021 will aid in further refinement of volatile sources.