Taupo Volcanic Zone Research Articles

3D Anatomy of a 60-year-old siliceous hot spring deposit at Hipaua-Waihi-Tokaanu geothermal field, Taupo Volcanic Zone, New Zealand

In 1941, a 72-m-deep bore (Healy's Bore 2; HB2) at the Hipaua-Waihi-Tokaanu geothermal field, Taupo Volcanic Zone, New Zealand, tapped alkali chloride water of 145 °C at the bottom of the well. Since then, thermal fluid (<100 °C) has intermittently flowed from the HB2 wellhead, resulting in formation of a 20 m wide by 50 m long sinter terrace. The discharging fluid at the vent was previously recorded to have a measured pH range of 7.5–8 and silica concentration of 263–287 mg/kg. The sinter terrace is dominated by low rimstone dams forming terracettes, where colorful brown, orange and pink microbial mats occur on surfaces bathed by the discharging fluid. In 2002, 11 shallow cores (up to 0.5 m deep, 3 cm diameter) were collected from the HB2 sinter terrace. Petrography, X-ray Powder Diffraction, X-Ray Fluorescence and Scanning Electron Microscopy were used to evaluate the stratigraphic, sedimentological and mineralogical character of the sinter. Surface and core samples allowed identification of six mid-temperature (<60 °C) sinter facies (conical peak, bubble mat, finely laminated, tufted peak, network, oncoidal), one low-temperature (<40 °C) sinter facies (palisade), and four tepid to dried (once wet) sinter facies (plant-rich, peloidal, clotted marsh). Sinter breccia also was common in the cores. The sinter is interbedded with sand, mud, detrital material and andesite fragments. Presence of kaolinite clay indicates localized secondary overprinting via acidic steam condensate. All sinter cores comprise opal-A silica. Based on the 1941 onset of spring flow with intermittent discharge from HB2 to 2002, and measured core thicknesses, we estimate a maximum sinter accumulation rate of 10 mm/yr close to the vent (core H9, 5 m S directly downslope from vent) and 3.5 mm/yr farthest from the discharge point (core H4, 22 m S directly downslope from vent). Yellow and pink stained horizons in the core indicate localized acidic steam overprinting of detrital sediments and sinter. Our sedimentological facies model illuminates the history of development of an immature, 60-year-old sinter terrace, providing insights into the complexity of sinter formation processes, depositional conditions, varying fluid flow regimes, channel migration pathways, biotic preservation and the effects of hydrothermal alteration in a rare 3D example of siliceous hot-spring build up over a known period of time. Such features are useful analogs in studies of hydrothermal settings associated with early life on Earth and possibly Mars.

Stratigraphy, provenance and localisation of the titanomagnetite placer at Waikato North Head, South Auckland, New Zealand

The sequence of coastal and river sands of Pleistocene to Holocene age at the north head of the Waikato River is ~80-m thick; the sands are located in a fault-angle depression at the southern end of the Awhitu Peninsula, a 40-km-long coastal sand barrier. The sequence at Waikato North Head (WNH) consists of three main formations: Awhitu Sands, Hood Sands and Mitiwai Sands. The Waiuku Black Sand member of the Hood Sands and the Entrican Sand member of the Mitiwai Sands contain titanomagnetite-rich dune sands, which are currently mined by New Zealand Steel and constitute a giant placer deposit that has a total resource of ~90 Mt Fe. A tephra at the top of the Awhitu Sands is correlated with the ~1000 ka Waiuku (Potaka) tephra. The Mitiwai Sands overlie the 1.85 ka Taupo Pumice. Comparison of electron probe microanalyses (EPMA) of titanomagnetites in heavy mineral separates of surface and drill-hole samples indicated that the bulk of the titanomagnetite in the Waiuku Black sand and the Entrican Sand is derived from andesitic rocks of the Taranaki Volcanoes 220 km to the south. The Waiuku Black Sand may correlate with an influx of andesitic clasts and mafic minerals in the cover beds of the ~400 ka Ararata Terrace in south Taranaki. There is also a contribution of titanomagnetite and minor ilmenite from the ignimbrites of the Taupo Volcanic Zone (TVZ), particularly in the Lower Hood and Entrican sands. Automated mineralogy data collected on selected sand samples contain silicate and resistate minerals that indicate other potential sources in addition to the Taranaki andesites and TVZ ignimbrites. Minor olivine was likely derived from the late Pleistocene basalts of the South Auckland volcanic field, whereas almandine and epidote were likely sourced from metasedimentary rocks of the Murihiku and Waipapa terranes. The formation of the giant placer at WNH was a multistage process involving (1) a supply of titanomagnetite-bearing sand eroded from the late Pleistocene–Holocene andesites of the Taranaki volcanoes that was transported northwards along the coast by the prevailing longshore drift; (2) concentration of titanomagnetite by wave action on beach faces and collection in a coastal embayment on the northern side of a headland of basement rocks at Port Waikato; (3) further concentration by wind action into dune sands during interglacial low sea levels; and (4) preservation by coeval subsidence in a fault-angle depression on the down-thrown north side of the Waikato Fault.

Biomolecules from Fossilized Hot Spring Sinters: Implications for the Search for Life on Mars.

Hot spring environments are commonly dominated by silica sinters that precipitate by the rapid cooling of silica-saturated fluids and the activity of microbial communities. However, the potential for preservation of organic traces of life in silica sinters back through time is not well understood. This is important for the exploration of early life on Earth and possibly Mars. Most previous studies have focused on physical preservation in samples <900 years old, with only a few focused on organic biomarkers. In this study, we investigate the organic geochemistry of hot spring samples from El Tatio, Chile and the Taupo Volcanic Zone, with ages varying from modern to ∼9.4 ka. Results show that all samples contain opaline silica and contain hydrocarbons that are indicative of a cyanobacterial origin. A ∼3 ka recrystallized, quartz-bearing sample also contains traces of cyanobacterial biomarkers. No aromatic compounds were detected in a ∼9.4 ka opal-A sample or in a modern sinter breccia sample. All other samples contain naphthalene, with one sample also containing other polyaromatic hydrocarbons. These aromatic hydrocarbons have a thermally mature distribution that is perhaps reflective of geothermal fluids migrating from deep, rather than surface, reservoirs. These data show that hot spring sinters can preserve biomolecules from the local microbial community, and that crystallinity rather than age may be the determining factor in their preservation. This research provides support for the exploration for biomolecules in opaline silica deposits on Mars.

Genesis of Recent Mafic Magmatism in the Taupo Volcanic Zone, New Zealand: Insights into the Birth and Death of Very Large Volume Rhyolitic Systems?

Abstract Mafic magmatism of the rifting Taupo Volcanic Zone (TVZ) of the North Island, New Zealand, is volumetrically minor, but is thought to tap the material that provides the heat source for voluminous rhyolite production through partial melting of the crust, which ultimately results in very large volume explosive eruptions. We have studied the major and trace element chemistry of 14 mafic samples from across the entire TVZ, and the U isotopic composition of whole-rocks, groundmasses and separates of mafic mineral phases from a selection of nine samples (with the remaining five too sparsely phyric for mineral separation). Some minerals yield significant 234U enrichments despite groundmass and whole-rock close to 238U–234U secular equilibrium, pointing to uptake of variably hydrothermally altered antecrystic minerals prior to the eruption of originally sparsely phyric to aphyric mafic magmas. However, incompatible trace element patterns indicate that there are three chemically distinct groups of samples, and that samples may be used to derive primary melt compositions. We employ the latest version of the Arc Basalt Simulator (ABS5) to forward model these compositions, deriving mantle source parameters including mantle fertility, slab liquid flux, mantle volatile content, degree of melting, and P–T conditions of melt segregation. We show that mafic rocks erupted in areas of old, now inactive calderas constitute low-degree, deep melts, whereas those in areas of active caldera-volcanism are high-degree partial melts segregated from a less depleted source at an intermediate depth. Finally, high-Mg basaltic andesites erupted in the SW and NE of the TVZ point to a fertile, shallow mantle source. Our data are consistent with a petrogenetic model in which mantle melting is dominated by decompression, rather than fluid fluxing, and progresses from shallow to deeper levels with time. Melt volumes initially increase to a tipping point, at which large-scale crustal melting and caldera volcanism become prominent, and then decrease owing to progressive depletion of the mantle wedge by melting, resulting in the dearth of heat provided and eventual cessation of very large volume rhyolitic volcanism. ABS5 modelling therefore supports the notion of a direct link between the chemistry of recently erupted mafic magmas and the long-term activity and evolution of rhyolitic volcanism in the TVZ.

Temporal-volume probabilistic hazard model for a supervolcano: Taupo, New Zealand

Supervolcano eruptions are very low probability, but extremely high impact, geohazards. Taupo volcano hosted the youngest known supereruption (VEI8), the c. 1100 km3 Oruanui (Kawakawa) eruption at 25.4 ka BP. Eruptions from Taupo have had regional to global environmental effects, and a supereruption is acknowledged as one of the greatest risks to the sustainability of our modern civilization. However, unlike less extreme volcanic systems, there has been little quantification of the hazard from individual supervolcanoes. Since the Oruanui eruption, we have a record of 28 smaller (between 0.01 and 45 km3) eruptions, of which the largest was the Taupo eruption of CE 232. Hence eruptions from a supervolcano can be of a size that can be mitigated against, and forecasting the magnitude of future events is vital. Most probabilistic supereruption forecasts aggregate many volcanoes in order to use a time-invariant (Poisson process) probability of eruption occurrence. The few studies of individual supervolcanoes have assumed that eruption volumes are independent of the volcano history. Here we apply more sophisticated temporal-volume models to identify and capture possible long-term process controls on when and how large an eruption will be. We find that the volume history of the volcano has a strong modulating effect on the likelihood of eruption, and that the magnitude is positively correlated with the likelihood of an eruption. The volumes of the largest eruption(s) from Taupo may not be consistent with the model based on the output of the single volcano. Widening the spatial limits to the central Taupo Volcanic Zone, and considering caldera-forming eruptions from the last 1.6 My, we find that the same model can satisfactorily explain the eruption record. However, the most significant influence on these large volumes is the elapsed time since the previous caldera-forming event rather than the likelihood of eruption, implying that the magma bodies required to support such eruptions can take significant time to assemble. The probability of an eruption at least as large as the Taupo eruption within the next 500 years is estimated at between 0.5% and 1.3%.

InSAR observations over the Taupō Volcanic Zone's cone volcanoes: insights and challenges from the New Zealand volcano supersite

ABSTRACT Launched in 2015, the New Zealand volcano supersite provides regular radar acquisitions covering all the major calderas and cone volcanoes which form the Taupō Volcanic Zone (TVZ). This paper documents new InSAR (Interferometric Synthetic Aperture Radar) observation of deformation over the cone volcanoes within the TVZ including the Tongariro Volcanic complex (TVC) and White Island. Observations over White Island support earlier observations indicating that the stability of the crater wall is controlled, at least in part, by the water level in the crater lake with downslope motion of ∼50 mm/yr observed during the latest period of lake drop in early 2019. Deformation over the TVC is dominated by the continued subsidence above the 2012 Te Maari eruption site at rates of ∼30 mm/yr with no evidence of deformation elsewhere. Synthetic models, used to test the potential detectability of magma movement beneath Ruapehu and White Island, suggest that for small volumes (<0.005–0.02 km3), intruded over short periods of time, it would be difficult for InSAR data alone to identify their accumulation. These challenges are not unique to New Zealand's volcanoes and are likely to be faced at other volcanic systems globally.

Complex crater fields formed by steam-driven eruptions: Lake Okaro, New Zealand

Abstract Steam-driven eruptions are caused by explosive vaporization of water within the pores and cracks of a host rock, mainly within geothermal or volcanic terrains. Ground or surface water can be heated and pressurized rapidly from below (phreatic explosions), or already hot and pressurized fluids in hydrothermal systems may decompress when host rocks or seals fail (hydrothermal eruptions). Deposit characteristics and crater morphology can be used in combination with knowledge of host-rock lithology to reconstruct the locus, dynamics, and possible triggers of these events. We investigated a complex field of &amp;gt;30 craters formed over three separate episodes of steam-driven eruptions at Lake Okaro within the Taupo volcanic zone, New Zealand. Fresh unaltered rock excavated from initially &amp;gt;70 m depths in the base of phase I breccia deposits showed that eruptions were deep, “bottom-up” explosions formed in the absence of a preexisting hydrothermal system. These phreatic explosions were likely triggered by sudden rise of magmatic fluids/gas to heat groundwater within an ignimbrite 70 m below the surface. Excavation of a linear set of craters and associated fracture development, along with continued heat input, caused posteruptive establishment of a large hydrothermal system within shallow, weakly compacted, and unconsolidated deposits, including the phase I breccia. After enough time for extensive hydrothermal alteration, erosion, and external sediment influx into the area, phase II occurred, possibly triggered by an earthquake or hydrological disruption to a geothermal system. Phase II produced a second network of craters into weakly compacted, altered, and pumice-rich tuff, as well as within deposits from phase I. Phase II breccias display vertical variation in lithology that reflects top-down excavation from shallow levels (10–20 m) to &amp;gt;70 m. After another hiatus, lake levels rose. Phase III hydrothermal explosions were later triggered by a sudden lake-level drop, excavating into deposits from previous eruptions. This case shows that once a hydrothermal system is established, repeated highly hazardous hydrothermal eruptions may follow that are as large as initial phreatic events.

Ground motion models for Arias intensity, cumulative absolute velocity, peak incremental ground velocity, and significant duration in New Zealand

This study proposes empirical ground motion models for a variety of non-spectral intensity measures and significant durations in New Zealand. Equations are presented for the prediction of the median and maximum rotated components of Arias intensity, cumulative absolute velocity, cumulative absolute velocity above a 5 cm/s2 acceleration threshold, peak incremental ground velocity, and the 5% to 75% and 5% to 95% significant durations. Recent research has highlighted the usefulness of these parameters in both structural and geotechnical engineering. The New Zealand Strong Motion Database provides the database for regression and includes many earthquakes from all regions of New Zealand with the exceptions of Auckland and Northland, Otago and Southland, and Taranaki. The functional forms for the proposed models are selected using cross validation. The possible influence of effects not typically included in ground motion models for these intensity measures is considered, such as hanging wall effects and basin depth effects, as well as altered attenuation in the Taupo Volcanic Zone. The selected functional forms include magnitude and rupture depth scaling, attenuation with distance, and shallow site effects. Finally, the spatial autocorrelation of the models’ within-event residuals is considered and recommendations are made for developing correlated maps of intensity predictions stochastically.

Seismic Evidence of Magmatic Rifting in the Offshore Taupo Volcanic Zone, New Zealand

AbstractRelationships between extensional tectonics and magmatism are ubiquitous in continental rifts and oceanic spreading centers. Yet few studies document interactions between extensional faults and mantle melts in volcanic arcs. We constrain the crustal structure of the extensional offshore Taupo Volcanic Zone (TVZ) from a marine multichannel and wide‐angle seismic experiment. The TVZ crust thins from &gt;26 km to ∼18–19 km across ∼50 km in the Bay of Plenty. Elevated P wave velocities in the lower crust indicate mafic additions. Magmatic sills between 4‐ and 15‐km depth lie beneath listric normal faults in a ∼40‐km‐wide active rift zone. P wave velocities in the middle and upper crust along the arc front are ∼0.3–0.5 km/s slower than in the adjacent crust, indicating a possible thermal anomaly imparted by heat from magmatic intrusions. We propose that rifting in the offshore TVZ is partially compensated by intrusions and assisted by thermal weakening of the lithosphere.

A zircon U-Pb geochronology for the Rotokawa geothermal system, New Zealand, with implications for Taupō Volcanic Zone evolution

A U-Pb zircon geochronology for the Rotokawa geothermal system, central Taupō Volcanic Zone (TVZ), New Zealand, provides new constraints on the chronostratigraphy and volcanic and structural evolution of this area and the broader TVZ. The 3-km-thick volcanic sequence at Rotokawa is mainly composed of rhyolitic ignimbrites linked to large caldera-forming events from sources outside the field area, but locally sourced andesite and rhyolite lavas and intrusions are also present. Crystallisation age spectra (and consequent estimates of eruption age) have been obtained on zircons from hydrothermally altered magmatic rocks by Secondary Ion Mass Spectrometry techniques using a SHRIMP-RG instrument. The oldest rock dated is a Tahorakuri Formation ignimbrite (eruption age estimate of 1.87 ± 0.03 Ma) which, along with comparable-age units at other nearby TVZ geothermal systems (Ngatamariki, Ohaaki), is among the oldest silicic volcanic deposits known in the TVZ. These ignimbrites collectively onlap a basal andesite lava pile, up to 1.2 km thick at Rotokawa, that in turn rests on the Mesozoic basement greywacke. The base of the lava pile is more faulted than its top surface, implying that rifting and graben formation had started along the line of the modern TVZ arc prior to 1.84 Ma and is not a younger feature. Between ~1.8 Ma and 700 ka, there are no rocks represented at Rotokawa, with the next oldest lithology being a 720 ± 90 ka rhyolite lava. At 350 ka, the Rotokawa area was buried by regionally extensive ignimbrites of the Whakamaru Group, which have since subsided by ~700 m but not been greatly faulted. Ignimbrites and sediments of the Waiora Formation were then emplaced, coevally with widespread and volumetrically greater volcanism in the Maroa and Ngatamariki areas 13 km northwest and 8 km north of Rotokawa, respectively. Local rhyolites of the Oruahineawe Formation, dated at ~100 ka, were emplaced both as extrusive domes and shallow intrusions below the area. Sedimentary rocks of the Huka Falls Formation and deposits of the 14C-dated 25.4 ± 0.2 ka Oruanui eruption capped and sealed the system, which has since been disrupted by hydrothermal eruption events. The largest of these occurred at ~6.8 ka (14C date) broadly coincident with a resumption of eruptive activity at Taupō volcano, 20 km to the south-southwest. Notable aspects of the evolution of the Rotokawa area are the early onset of rifting and subsidence along the line of the modern arc, the lack of volcanic activity for >1 Myr from 1.84 Ma to 720 ka, the lack of faulting and only modest subsidence since 350 ka, and the contrasts in volcanic and subsidence histories with other, nearby geothermal systems.

Structure and evolution of the Wairakei–Tauhara geothermal system (Taupo Volcanic Zone, New Zealand) revisited with a new zircon geochronology

The central part of the Taupo Volcanic Zone (TVZ), New Zealand, is notable for the number (23) and size of its high-temperature geothermal systems. Of these, the Wairakei-Tauhara geothermal system collectively represents the largest commercially developed example. Natural thermal output prior to utilisation, i.e. before 1950 was ∼530MW. Since 1950, drilling of over 300 closely spaced wells for power generation has provided geological data for a comprehensive stratigraphic framework. Within this framework, the strata are dominantly rhyolitic pyroclastic rocks and lava bodies, with interbedded fluvial and lacustrine deposits. Until now, no subsurface units had been directly dated and their ages were constrained only by their positions relative to the surficial 25.4 ka Oruanui ignimbrite, or to buried ignimbrite that is correlated with the 349 ka Whakamaru Group. This paper presents 33 suites of U-Pb age data and a limited number of reconnaissance U-Th disequilibrium age data from magmatic zircon crystals (zircons) to date the rocks of the Wairakei-Tauhara system. The oldest units dated are intermediate lavas from ca. 1.7 Ma, while the youngest locally vented eruptives of Tauhara dacite are tentatively dated to ca. 60 ka. Placing the age data within the established drillhole record allows for the recognition of major volcano-tectonic events and spatially clustered local volcanism that represents magmatic heat sources for geothermal activity within the Wairakei-Tauhara area. In contrast to the Rotokawa and Ngatamariki geothermal systems 10–20km to the northeast, the Wairakei-Tauhara area was affected by two episodes of caldera collapse associated with deposits dated at 349 ka and 310 ka, leading to major faulting and abrupt variations in thicknesses of units. The faulting and deposit thickness variations established in these two events remain major controls on present-day permeability of the geothermal system. The new age data and stratigraphy demonstrate development of distinct magma systems through time which may have stimulated previous periods of hydrothermal activity. However, from zircon age data from the rhyolite-andesite mixed Tauhara dacite, we propose that the current geothermal system was initiated at ca. 60 ka and is thermally maintained by continuing contributions from the same magmatic system that has sporadically vented distinctive rhyolite since.

Attenuation in the mantle wedge beneath super-volcanoes of the Taupo Volcanic Zone, New Zealand

SUMMARYThe Taupo Volcanic Zone has a 120-km-long section of rhyolitic volcanism, within which is a 60-km-long area of supervolcanoes. The underlying subducted slab has along-strike heterogeneity due to the Hikurangi Plateau's prior subduction history. We studied 3-D Qs (1/attenuation) using t* spectral decay from local earthquakes to 370-km depth. Selection emphasized those events with data quality to sample the low Qs mantle wedge, and Qs inversion used varied linking of nodes to obtain resolution in regions of sparse stations, and 3-D initial model. The imaged mantle wedge has a 250-km-long 150-km-wide zone of low Qs (&lt;300) at 65–85 km depth which includes two areas of very low Qs (&lt;120). The most pronounced low Qs feature underlies the Mangakino and Whakamaru super-eruptive calderas, with inferred melt ascending under the central rift structure. The slab is characterized by high Qs (1200–2000), with a relatively small area of reduction in Qs (&lt;800) underlying Taupo at 65-km depth, and adjacent to the mantle wedge low Qs. This suggests abundant dehydration fluids coming off the slab at specific locations and migrating near-vertically upward to the volcanic zone. The seismicity in the subducted slab has a patch of dense seismicity underlying the rhyolitic volcanic zone, consistent with locally abundant fractures and fluid flux. The relationship between the along-arc and downdip slab heterogeneity and dehydration implies that patterns of volcanism may be strongly influenced by large initial outer rise hydration which occurred while the edge of the Hikurangi Plateau hindered subduction. A second very low Qs feature is 50-km west above the 140-km-depth slab. The distinction suggests involvement of a second dehydration peak at that depth, consistent with some numerical models.

Anisotropy as an indicator for reservoir changes: example from the Rotokawa and Ngatamariki geothermal fields, New Zealand

SUMMARYWe investigate the relation between geothermal field production and fracture density and orientation in the Ngatamariki and Rotokawa geothermal fields, located in the Taupo Volcanic Zone, New Zealand using shear wave splitting (SWS). We determine the SWS parameters for 17 702 microseismic events across 38 stations spanning close to 4 yr from 2012 to 2015. We compare the strength of anisotropy to changes in field production and injection. We also compare the orientation of the anisotropy to in situ and regional measurements of maximum horizontal stress orientation. ($S_{\mathrm{ H}_{\mathrm{ max}}}$). Due to the volume of unique events (approximately 160 000), shear wave phases are picked automatically. We carry out automatic SWS measurements using the Multiple Filter Automatic Splitting Technique (MFAST). The SWS measurements are interpreted in the context of stress aligned microcracks. Outside both fields and within Ngatamariki, fast polarizations align with the NE–SW regional orientation of $S_{\mathrm{ H}_{\max}}$. Within Rotokawa a greater complexity is observed, with polarizations tending toward N–S. We observe increases in per cent anisotropy coinciding with the start of production/injection in Ngatamariki and then a later correlated drop in per cent anisotropy and vP/vS ratios in southern Ngatamariki as injection is shifted to the north. This relationship is consistent with pore fluid pressure within the reservoir being affected by local changes in production and injection of geothermal fluids causing cracks to open and close in response.

Tree ferns and tea trees in biogeochemical exploration for epithermal Au and Ag in New Zealand

Biogeochemical orientation surveys were undertaken at low sulphidation epithermal Au–Ag occurrences in the Hauraki Goldfield–Coromandel Volcanic Zone and the Taupo Volcanic Zone, and at the Waiotapu active geothermal area in the Taupo Volcanic Zone. Several plant species were sampled, including the foliage of tree ferns and tea trees. The ferns – silver fern ( ponga ), rough tree fern ( wheki ) and black tree fern ( mamaku ) – were ubiquitous and were the easiest species to sample, although tea tree was the dominant genus at Waiotapu. At the Waiotapu geothermal area, significantly higher concentrations of Ag, Au, Sb, As, Cs and Rb were present in samples close to Champagne Pool than elsewhere, confirming its location as the main outflow source of Au, Ag and their pathfinder elements. The fern survey areas at Luck at Last mine, Pine Sinter and Ohui in the Coromandel Volcanic Zone each exhibited biogeochemical anomalies, which successfully highlighted most of the known quartz veins and provided additional anomalies for further investigation. Rough tree fern was the most common species at Goldmine Hill, Puhipuhi (Taupo Volcanic Zone). Although this species absorbs lower concentrations of many elements than the silver fern, the spatial distribution of elements is of greater significance than their absolute concentrations. The highest Au, Ag, As and Al concentrations occurred in samples from a ridge extending WNW from Goldmine Hill. Sb and Bi were at anomalous levels in an area peripheral to the precious metal anomalies, indicating the potential zonation of elements distal from the Au and Ag deposits. Supplementary material: The full datasets on the fern and tea tree chemistry, including quality assurance/quality control and multi-element plots, are available free of charge through the GNS Science website (search for Dunn) at http://shop.gns.cri.nz/publications/science-reports/ .

Tectonic Controls on Taupo Volcanic Zone Geothermal Expression: Insights From Te Mihi, Wairakei Geothermal Field

AbstractInformation on structure, stress, and their interrelationship is essential for understanding structurally controlled geothermal permeability. Active fault mapping, borehole image analysis, and well testing in the Te Mihi geothermal area, New Zealand, allows us to refine structural and fluid flow architecture of this resource. The Te Mihi area is structurally complex, comprising a set of NW dipping master faults containing pervasive SE dipping antithetic and splay structures in their hanging walls. These faults are also intersected by E‐W striking faults. A localized, N‐S striking structural trend is also observed at Te Mihi. In consideration with Global Navigation Satellite System velocity vectors, both active NE‐SW and E‐W striking faults create biaxial extension at Te Mihi, though the observed NE‐SW SHmax direction suggests that contemporary extension is NW‐SE dominated. Stress field perturbations coincide with structural complexities like fault splays and intersections and/or proximity to recently active E‐W and NE‐SW striking structures. Borehole fluid flow at Te Mihi is concentrated at NW dipping master fault intersections, travel time fractures on acoustic image logs, halo fractures on resistivity image logs, NE‐SW and E‐W striking fractures, intervals of high fracture density, and spatial concentrations of wide aperture fractures and recently active NE‐SW and E‐W striking fractures. This study suggests Te Mihi geothermal expression results from biaxial extension evident from active structural trend intersections and the predominance of NE‐SW and E‐W striking structures within permeable well zones. Biaxial extension is therefore an important control on crustal fluid flow within the Taupo Volcanic Zone and thus geothermal resource delineation.

Magma extraction pressures and the architecture of volcanic plumbing systems

The deposits of large volcanic eruptions provide the sole record of the architecture of magmatic plumbing systems in the moments when large pools of crystal-poor, eruptible magma are present in the crust. It is widely accepted that silicic magmas form by segregation of melt-rich, crystal-poor magma from a crystal-rich source; however, the depths at which segregation takes place and the distribution of the magma within the crust are not well constrained. We present a new approach to calculate pressures at which crystal-poor, eruptible magma is extracted from a crystal-rich source (i.e. mush). We apply the approach to a sequence of large (>50 km3) eruptions from the Taupo Volcanic Zone (TVZ), New Zealand, which were part of a volcanic flare-up. We compare the calculated extraction pressures with pre-eruptive storage pressures for the same units. Our results show that storage and extraction pressures do not always coincide. Instead, eruptible magma can be completely segregated from the crystal-rich source, and stored at shallower levels in the crust prior to eruption. In the case of the TVZ flare-up, repeated input of material and heat – probably coupled with tectonic extension – gradually conditioned the crust and allowed extraction of eruptible magma over a growing range of pressures with time. Our approach has the potential to reveal important information on the structure and distribution of magmatic systems within the shallow crust.

Ar-Ar age constraints on the timing of Havre Trough opening and magmatism

ABSTRACTThe age and style of opening of the Havre Trough back-arc system is uncertain due to a lack of geochronologic constraints for the region. 40Ar/39Ar dating of 19 volcanic rocks from across the southern Havre Trough and Kermadec Arc was conducted in three laboratories to provide age constraints on the system. The results are integrated and interpreted as suggesting that this subduction system is young (<2 Ma) and coeval with opening of the continental Taupo Volcanic Zone of New Zealand. Arc magmatism was broadly concurrent across the breadth of the Havre Trough.

Selective chemical degradation of silica sinters of the Taupo Volcanic Zone (New Zealand). Implications for early Earth and Astrobiology.

Most organic matter (OM) on Earth occurs as kerogen-like materials, that is naturally formed macromolecules insoluble with standard organic solvents. The formation of this insoluble organic matter (IOM) is a topic of much interest, especially when it limits the detection of compounds of geomicrobiological interest. For example, studies that search for biomarker evidence of life on early Earth or other planets usually use solvent-based extractions. This leaves behind a pool of OM as unexplored post-extraction residues, potentially containing diagnostic biomarkers. Since the IOM has an enhanced potential for preservation compared to soluble OM, analysing IOM-released biomarkers can also provide even deeper insights into the ecology of ancient settings, with implications for early Earth and Astrobiology investigations. Here, we analyse the prokaryotic lipid biosignature within soluble and IOM of the Taupo Volcanic Zone (TVZ) silica sinters, which are key analogues in the search for life. We apply sequential solvent extractions and a selective chemical degradation upon the post-solvent extraction residue. Moreover, we compare the IOM from TVZ sinters to analogous studies on peat and marine sediments to assess patterns in OM insolubilisation across the geosphere. Consistent with previous work, we find significant but variable proportions-1%-45% of the total prokaryotic lipids recovered-associated with IOM fractions. This occurs even in recently formed silica sinters, likely indicating inherent cell insolubility. Moreover, archaeal lipids seem more prone to insolubilisation as compared to the bacterial analogues, which might enhance their preservation and also bias overall biomarkers interpretation. These observations are similar to those observed in other settings, confirming that even in a setting where the OM derives predominantly from prokaryotic sources, patterns of IOM formation/occurrence are conserved. Differences with other settings, however, such as the occurrence of archaeol in IOM fractions, could be indicative of different mechanisms for IOM formation that merit further exploration.

Whanganui Basin, an archive of prehistoric earthquakes? Waitapu Shell Conglomerate (c. 0.9 Ma), North Island, New Zealand

ABSTRACTThe Waitapu Shell Conglomerate is an important marker horizon in the eastern Whanganui Basin, occurring within a Pleistocene volcaniclastic record that contains early eruption products from the Taupo Volcanic Zone. The unit comprises a cross-bedded pebbly-shell conglomerate containing the first influx of Kaukatea Pumice (c. 0.9 Ma) within the Rangitikei succession. We document soft sediment deformation structures that occur in close stratigraphic proximity to the Waitapu Shell Conglomerate and other laterally equivalent units within the basins Castlecliffian outcrop belt. Soft sediment deformation structures formed through a combination of liquefaction and fluidisation, triggered by a range of mechanisms, including evidence of high sedimentation rates, loading, slope instability and potential for wave and earthquake-induced seismicity. Lateral changes in depositional style toward the basins eastern margin relate to relative position on the paleo-shelf, reduction of accommodation space, intermittent preservation of low stand deposits and proximity to the uplifting paleo-axial range.

Testing the tunable diode laser system in extreme environments: Measuring high and low CO2 concentrations in both active volcanic and geothermal settings

Measuring CO2 emissions in geothermal and volcanic areas is sometimes difficult because of large areas to cover and sites often inaccessible. Measuring high levels of CO2 concentration can provide information on hidden structure in geothermal areas and recording changes in CO2 concentration on volcanic areas can help monitor the level of volcanic activity. The purpose of this study was to use the Tunable Laser Diode (TDL) absorption spectroscopy method to test levels of CO2 concentrations at two extreme environments: White Island volcano, the most active volcano of New Zealand, with large and concentrated gas fluxes, and Ngapouri geothermal area, a small geothermal area in the Taupo Volcanic Zone, New Zealand, with relatively low and diffuse gas emissions. In 2017, for the first time using TDL at White Island, CO2 concentration measurements performed across the active fumarole fields had the highest CO2 concentrations of 657 ppm. TDL survey measurements were also conducted across fault strands near the Ngapouri geothermal area, and the results complemented CO2 flux results obtained with the accumulation chamber method. Higher CO2 concentrations were measured close to the mapped Ngapouri splays with a maximum of 484 ppm. The maximum CO2 flux measured in the same area was 100 g m−2 day−1 however the highest CO2 fluxes measured along the transects and by the mapped faults were less clear, but the CO2 concentrations increased closer to the fault splays. Advantages and disadvantages of using a TDL system have been described and compared to the accumulation chamber method. The results from the TDL system demonstrated that CO2 concentrations can be used as a tool, with other geophysical tools, for both detecting and highlighting geological structures where no obvious thermal activity is present and for monitoring purposes on active volcanoes.