Isabelle Chambefort

Magmatic volatile distribution as recorded by rhyolitic melt inclusions in the Taupo Volcanic Zone, New Zealand

Abstract The central Taupo Volcanic Zone (TVZ) is an actively rifting continental arc and is well known for its exceptionally high rate of rhyolitic magma generation and frequent caldera-forming eruptions. Two end-member types of rhyolites (R1 and R2) have been previously identified based on differences in their bulk-rock chemistry and mineral assemblage with hydrous phases crystallizing in the R1 type, which are not present or only rare in R2 rhyolites. Here we present new trace element and volatile data from rhyolitic melt inclusions measured in several representative eruptive deposits (R1 and R2 rhyolites) from the central TVZ to examine their volatile concentrations and origin. R1 and R2 show very distinct Cl concentrations, with R2 rhyolites being enriched in Cl by c. 1000 ppm. H2O is slightly higher in the R1 rhyolites, whereas CO2 concentrations are similar between the two end-member types. The origin of these volatile disparities between R1 and R2 melts is assigned to differences in the initial bulk volatile content of the parental magma, possibly associated with distinct input of fluids from the subduction zone. These disparities in bulk volatile concentrations can lead to variations in relative timing of exsolution of volatile phase(s) prior to melt inclusion entrapment. Supplementary material: Major, trace and volatile composition for the analysed central TVZ rhyolites, and comparison of H2O data between the transmission and reflectance FTIR are available at http://www.geolsoc.org.uk/SUP18767.

Interaction between ferruginous clay sediment and an iron-reducing hyperthermophilic Pyrobaculum sp. in a terrestrial hot spring

Green-coloured sediments in low-temperature geothermal surface features are typically indicative of photosynthetic activity. A near-boiling (89-93°C), alkali-chloride spring in the Taupō Volcanic Zone, New Zealand, was observed to have dark green sediments despite being too hot to support any known photosynthetic organisms. Analysis of aqueous and sediment microbial communities via 16S rRNA amplicon sequencing revealed them to be dominated by Aquifex spp., a genus of known hyperthermophilic hydrogen-oxidisers (69%-91% of operational taxonomic units (OTUs)), followed by groups within the Crenarchaeota (3%-20%), including the known iron-reducing genus Pyrobaculum. Cultivation experiments suggest that the green colouration of clay sediments in this spring may be due in part to ferruginous clays and associated compounds serving as substrates for the iron-reducing activity of low-abundance Pyrobaculum spp. These findings demonstrate the dynamic nature of microbe-mineral interactions in geothermal environments, and the potential ability of the rarer biosphere (1%-2% of observed sequences, cell densities of 450-33 000 g-1 sediment) to influence mineral formation at a macro-scale.

Controls on hydrothermal fluid flow in caldera-hosted settings: Evidence from Lake City caldera, USA

Silicic caldera volcanoes are often associated with hydrothermal systems economically important for electricity generation and localization of ore deposits. Despite their potential importance, the poor exposure that is typical in caldera settings has limited the number of detailed studies of the relationship between caldera structures and fluid flow. We use field mapping, outcrop scale scanline transects, and petrographic analyses to characterize fault rocks, alteration, and veins in the well-exposed 22.9 Ma Lake City caldera fossil hydrothermal system. The caldera margin consists of relatively straight segments linked by more structurally complex intersections; these structural intricacies produce a zone of deformation that can reach >300 m wide. Structural analyses show that the wide (up to ~60 m) fault core of the ring fault contains abundant subparallel veins, with orientations similar to that of the caldera margin. Smaller displacement faults inside the caldera generally have narrow (<1 m), hydrothermally cemented fault cores with more variably oriented veins in the surrounding damage zone. These findings at Lake City illustrate that fluid flow is controlled by lithology and the location and displacement of faults, e.g., ring fault versus intracaldera fault. Fault connectivity is another key control. We propose a conceptual model where fluid flow in caldera-hosted settings is influenced by: (1) the presence of favorable lithologies (proximity to magmatic intrusions and/or the presence of permeable lithologies), (2) a high density of faults and fractures, and (3) favorable orientations of faults and fractures that promote the formation of discontinuity intersections.

Application of Raman spectroscopy to distinguish adularia and sanidine in drill cuttings from the Ngatamariki Geothermal Field, New Zealand

Adularia and sanidine are polymorphs of potassium feldspar commonly present in felsic, hydrothermally altered volcanic deposits. Sanidine is a high-temperature volcanic mineral, whereas adularia forms post deposition by hydrothermal processes. Petrographically differentiating between these polymorphs in hydrothermally altered volcanic rocks may be utilised to distinguish geological units as well as provide insights into fluid–rock interactions. However, petrographic identification may be difficult or not possible in fine-grained drill cuttings. Here, polymorphic-sensitive, Raman spectroscopy and electron microprobe analyses are utilised to characterise adularia and potential sanidine in drill cuttings from the Ngatamariki Geothermal Field, Taupo Volcanic Zone, New Zealand. Differences in Raman spectra are capable of distinguishing between adularia and sanidine whether using peak positions or principal component analysis. All the Ngatamariki Geothermal Field potassium feldspars analysed by Raman spectroscopy were found to be adularia, as expected, with typical high K, low Ca compositions between Or94 and Or99 confirmed with electron microprobe analyses. This applied approach demonstrates that Raman spectroscopy is a fast and effective method for lending confidence to adularia and sanidine identification, which can be utilised in geothermal fields worldwide.