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MATERIALS AND METHODSIn 2004, the study site was mapped to determine sinter microfacies types, vent locations, fluid flow directions, temperatures, and pHs of discharged thermal waters, water level changes, and silica accretion rates. The site was visited several times that year to note any environmental changes, especially of water levels. Mapping was undertaken with tape and compass. Temperatures and pHs were measured with a portable battery operated OrionTM (model 250A) pH/ISE meter with automatic temperature compensation. Silica accretion rates were measured by placing glass slides (25 x 75 mm) vertically in the discharge channels for 67 days (cf. Mountain et al. 2003; Handley 2004; Handley et al. 2005). Filtered (<0.2 µm) water samples were collected in February 2006 to determine anion (unacidified) and cation (acidified by 0.5% HNO3) concentrations. For HCO3 and H2S concentrations, samples were collected in rubber-sealed bottles. The waters were analysed by the Institute of Geologic and Nuclear Sciences (IGNS), New Zealand. The silica mineralogy of the sinters was determined by X-ray powder diffractometry (XRPD). Each sample was air-dried and ground to a fine powder in a mortar and pestle. XRPD was conducted with a Philips diffraction goniometer fitted with a graphite monochromator. Samples were scanned at 1.95° 2θ/min with a step size of 0.02° from 2 to 62° 2θ. Operating conditions were 40 kV and 20 mA using CuKα radiation (λα1 = 1.54051 Å; λα2 = 1.5443 Å). Sinter composition and water content was determined by combined differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Sinters were air-dried, ground to a fine powder, placed into pre-weighed platinum crucibles (~0.22 g), and heated to 1400°C in a Polymer Laboratories Simultaneous Thermal Analyser 1500 (Epsom, Surrey, UK) equipped with PLus V software (v.5.40) and a type R (Pt-13% Rh/Pt) flat-plate thermocouple system. Heating was in dry air or nitrogen. The former allowed the combustion of organic material to be gauged, while the latter measured water loss. Fine-scale sinter textures and potential microbial involvements were examined by Scanning Electron Microscopy (SEM). Samples were stored in a fixative of 2.5% glutaraldehyde immediately upon field collection. Prior to imaging, samples were critical-point dried to prevent the effects of surface tension from destroying delicate biofilms. The samples were rinsed twice in deionised water followed by dehydration through an ethanol series (30%, 50%, 70%, 90%, and 100% x 2). Ethanol was exchanged for liquid CO2 at 5500 kPa by flushing and soaking for one hour in a Polaron E3000 Series II critical point drier (Polaron Equipment Ltd, Watford, UK). Liquid CO2 was subsequently vaporised at 31.5° C. Critical-point dried samples were mounted on aluminium stubs and coated with platinum using a Polaron SC 7640 Sputter Coater (Quorum Technologies Ltd, Newhaven, UK). Samples were examined with a Philips SEM XL30S (Eindhoven, Netherlands) fitted with SiLi (lithium drifted) electron-dispersive X-ray spectrometer (EDS) with a Supra Ultra Thin Window (EDAX, Mahwah, New Jersey, USA) for assessing elemental composition of sinter components. To examine broad sinter textures and allow for comparisons with fine-scale SEM images, thin sections of the sinters were constructed. Samples were first embedded overnight in an epoxy resin of methyl methacrylate, and the dried resin was subsequently polymerised using cobalt-60 radiation. Sections were examined under a Nikon Labophot-POL petrographic microscope (Tokyo, Japan) equipped with a Nikon Coolpix 4500 digital camera (Tokyo, Japan). |
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