Clay Mineralogy, Oxygen Isotope Geochemistry, and Water/Rock Ratio Estimates, Te Mihi Area, Wairakei Geothermal Field, New Zealand

Abstract

Dioctahedral clays from an active continental geothermal system have been studied to assess their usefulness as proxies of paleo-hydrological and thermal conditions in the subsurface. Drill cuttings from Well WK244 in the Te Mihi area of the Wairakei Geothermal Field, New Zealand, were analyzed to determine the mineralogical, morphological, and isotopic characteristics of hydrothermal clays in these samples. Mixed-layer illite-dioctahedral smectite (I-S) and R0 chlorite-trioctahedral smectite are the main clay minerals, with I-S clays varying downward from R1 to R3 ordering and 50 to >90% illite over 160 m. The proportion of illite in I-S correlates positively with downhole temperature (r = 0.98) and I-S morphology changes from high aspect ratio ribbons, laths, and hairy fibers to pseudo-hexagonal plates with depth. Swelling clay percentages determined using the methylene blue method show a strong positive correlation with %S in I-S (r = 0.91), validating use of methylene blue as a rapid field tool for characterizing the smectite to illite transition in this active geothermal environment. The oxygen isotopic composition of I-S (dd18OI-S) decreases systematically with depth, and mostly reflects a progressive increase in subsurface temperature during clay formation. Estimates of water/rock ratios calculated using δ18OI-S values display stratigraphic variability that corresponds to variations in permeability. Oxygen isotopic measurements of I-S are a useful tool for understanding reservoir and permeability evolution in such geothermal systems and their related fossil analogs.

Clay Minerals in an Active Hydrothermal Field at Iheya‐North‐Knoll, Okinawa Trough

“…Dominance of di‐octahedral clay minerals in island‐arc and back‐arc basins is probably due to felsic original‐rock or ‐sediments with low Mg and Fe contents. Di‐octahedral clay minerals are also formed abundantly in onland geothermal fields where the original‐rock is felsic (Inoue et al, ; Libbey et al, ).…”

How Clay Mineral Assemblages Affect Instability on the Upper Slope of the Hikurangi Subduction Zone, New Zealand

“…The Taupo Volcanic Zone (TVZ) fits the prerequisites for a regionally extensive source of volcanic and volcaniclastic rocks (Figure 1a) (Cole et al., 2014; Cronin et al., 1999; Downs et al., 2014; Gravley et al., 2016; Hidgson & Manville, 1999; Kohn & Topping, 1978; Lowe et al., 2013; Procter et al., 2014; Wilson & Rowland, 2016). Their chemical weathering products are demonstrably smectite‐dominant, and smectite is ubiquitous in TVZ geothermal and hydrothermal systems (Heap et al., 2020; Hocking et al., 2010; Libbey et al., 2013; Simpson et al., 2019). Heavily weathered tephra is also widespread in coastal exposures around the Bay of Plenty (Iso et al., 1982).…”

Oxygen Isotopes in Authigenic Clay Minerals: Toward Building a Reliable Salinity Proxy

“…Oxygen‐isotope analyses were performed at the University of Western Ontario following Libbey et al () and references therein. Reproducibility was better than ±0.2‰.…”