Past and present generation of natural hydrogen at a convergent plate boundary – the case of Aotearoa New Zealand

Abstract

Convergent margins provide an array of geological settings where serpentinization of ultramafic rocks may generate natural hydrogen. Aotearoa New Zealand (A-NZ) is an archetypal example of a convergent margin where tectonic assembly, spatiotemporal variability of serpentinization, fault-controlled permeability, and basin evolution are first-order controls on hydrogen systems. Two dominant hydrogen generation modes occur along the A-NZ plate boundary, where >125 sites of ≥1% H 2 emission are reported. Firstly, ophiolitic Dun Mountain-Maitai and volcanic arc Brook Street terranes are emplaced at or near-surface. Both terranes are variably serpentinized, including pristine un-serpentinized peridotite massifs, reflecting highly heterogeneous, multi-scaled, fault and fracture-controlled fluid access. Natural hydrogen prospectivity is driven by spatial and temporal configuration of serpentinizing ultramafics with overlying sedimentary sequences. This coupling is largely restricted to western North Island. Secondly, active Hikurangi subduction underplates and serpentinizes oceanic plateau crust (and capping sediments), with upward fault-controlled fluid migration and extensive fluid expulsion. Methane is the dominant emitted gas, with hydrogen largely consumed by microbial methanogenesis apart from localised migration along seaward-verging thrust faults. We postulate a “hydrogen accumulation” zone below ∼4 km beneath the inner third of the margin. Both generation modes are enhanced by Neogene crustal-scale faults that increase meteoric water access to near-surface ultramafic rocks. Globally notable hydrogen outgassing at Poison Bay, associated with the Alpine Fault, has ∼76% H 2 and is geochemically akin to Zambales emissions. The Taranaki thrust juxtaposes Dun Mountain-Maitai basement beneath >10 km thick sedimentary sequences that have proven reservoirs and lithological seals.