Origin of Water in New Zealand Perlites

Transcription

1 Proceedings World Geothermal Congress 2015 Melbourne, Australia, April 2015 Origin of Water in New Zealand Perlites Jim Lawless 1 and Phil White 2 1: Lawless Geo-Consulting: Jlawless@clear.net.nz, 2: Panda Geoscience: pwhite@pandageoscience.co.nz, Keywords: New Zealand, perlite, petrogenesis, magma, permeability, rhyolite, obsidian, volcanic, hydrothermal ABSTRACT Perlites in the broad, industrial sense are volcanic glasses which contain enough water (typically 1 to 3%) that they expand when rapidly heated, usually to around 800 C or more. Most are of silicic composition. They differ from obsidian, which has a much lower water content (<0.5%). As most perlites are of Tertiary to recent age, they tend to occur in geological settings similar to those of active high temperature geothermal systems. The conventional wisdom about their origin, based mainly on work in the USA, is that perlites have formed by post-emplacement hydration of obsidian. Physical characteristics, field relations and textures suggest that the perlites in the Taupo Volcanic Zone of New Zealand differ from those in the US and elsewhere, having origins associated with water-rich magmas. New data support this interpretation because isotopic signatures of NZ perlites imply a magmatic origin for the water rather than a meteoric source. If correct, exploration for active geothermal systems may be impacted in two ways: (1) the presence of water-rich magmas implies a high potential for the creation of permeability through hydrothermal brecciation, and (2) as the New Zealand perlites appear to be very late-stage and structurally localised, associated magmas may have ascended along permeable structures coincident with zones of enhanced sub-surface permeability. 1. INTRODUCTION The term perlite can be used in several ways. It has a strict petrological meaning that refers to a certain texture within volcanic glass. In the present context, it is used in a more general industrial sense to refer to volcanic glasses that contain enough water (typically 1 to 3%) that they expand when rapidly heated, usually to around 800 C or more. Many of the materials that are used commercially as perlite do not have the typical pearly lustre from which the name is derived. In hand specimen the New Zealand examples may be identified by lower density and being softer than typical rhyolites, and have a dull woody sound when struck. Most perlites have silicic compositions, with a few verging on andesitic. Perlite differs from obsidian, which has a much lower water content (<0.5%). The water in perlite is not outgassed <110 C, but at much hotter temperatures, when the host glass begins softening ( C). Perlite-contained water forms part of the loose network of silica tetrahedra in the glass, but it is not systematically structured. The presence of water causes glass softening at cooler temperatures than otherwise would be the case, which is why the water released as steam can cause expansion, and then the resulting expanded rigid structure is preserved as the softening point is then higher. As most perlites are of Tertiary to recent age (older glasses having generally de-vitrified or been subject to alteration to clays), they tend to occur in similar geological settings to active high temperature geothermal systems. The conventional wisdom about their origin, based mainly on work in the USA and to a lesser extent in Greece and Turkey, has been that perlites have formed by postemplacement hydration of obsidian at <600 C. In part that was based on field observations that perlite in those settings tended to form a skin on the outside of domes or flows of obsidian ± lithic (in the sense of wholly crystalline) rhyolite, as well as measurements of the isotopic composition of the contained water (Friedman and Smith 1959), which indicated most of the water was meteoric. 2. PERLITE IN THE TAUPO VOLCANIC ZONE Following identification of a perlite resource by Thompson and Reed (1954), Crown Lynn Potteries Ltd., under the name of NZ Perlite, began to mine and process perlite from the Taupo Volcanic Zone (TVZ), New Zealand for industrial uses, in the late 1960 s. The industrial characteristics of New Zealand perlites differed from those being mined elsewhere in the world: the New Zealand material had a high expansion ratio, but the raw ore was relatively friable, which tended to make a fine grained but structurally weak product. Those characteristics rendered the TVZ perlite suitable for cryogenic, filter-aid or soil-mix applications, but limited its applicability as a lightweight concrete aggregate unless it was screened after expansion to remove fine material. The difference was not solely due to high water content: analyses indicate water contents similar to those of other perlites, albeit at the high end of the range. Crown Lynn Potteries initiated a wide-ranging geological exploration programme in the TVZ that was followed by a thesis project by Lawless (1975), which concentrated on a dome complex in an area south of Tokoroa (Figure 1). The area was re-mapped by the Institute of Geological and Nuclear Sciences (GNS) in the 2000 s. The same general area was explored by Mighty River Power Ltd. (MRP) for geothermal energy as part of the Mangakino project, which included re-mapping by Sinclair Knight Merz Ltd. (SKM). Four deep geothermal wells were then drilled by MRP, none of which was commercially successful. Perlites are widespread within the Haparangi Rhyolites, a diverse group of mainly Quaternary rhyolites within the TVZ,, but are volumetrically minor compared to lithic rhyolites. The term Haparangi Rhyolites has generally been applied to dome- or flowforming edifices, but these are commonly surrounded by aprons of crumble breccias formed during their emplacement. In early mapping, the crumble breccias were often lumped with more distally-source ignimbrites. Subsequent mapping in local areas (e.g., Nairn, 2002) has, in some cases, led to subdivision of this rock unit and application of local lithostratigraphic names. 1

2 Figure 1. Location Map, sampling sites shown as blue diamonds 3. ANALYTICAL METHODS Oxygen and hydrogen isotopic compositions of rock samples were determined by fluorination (Clayton and Mayeda, 1963) and online high-temperature reduction (Sharp et al., 2001), respectively. Results are reported in delta-notation, in permil ( ), relative to Vienna Standard Mean Ocean Water. Reproducibility was ±0.2 or better for oxygen and ±2 or better for hydrogen. 4. RESULTS 4.1 Field observations Field observations revealed a number of significant common features in the perlites, lithic rhyolites, and obsidians of the TVZ (Lawless, 1975) that did not necessarily apply to overseas examples. Apart from where they have been brecciated and re-deposited by post-eruption processes, most of the TVZ perlite occurrences form discrete domes marginal to larger masses of lithic rhyolite. The perlite domes commonly also have lower topographic profiles than the adjacent steep-sided rhyolite domes (Figures 2 & 3). However stratigraphic relationships between the two are generally obscured by overlying air-fall pyroclastics from more distant sources. 2

3 Perlite dome Lithic rhyolite dome Figure 2. A small, gently-sloped perlite dome adjacent to a larger, more steeply-sloped lithic rhyolite dome. Geological Map W C Sample locations: C = Cashmores W = Wawa Figure 3. Geological map of the Mangakino area, including the Cashmores and Wawa sample locations, showing the distribution of lithic and perlite rhyolite domes. Grid in metres. Used with permission from Mighty River Power. The TVZ perlite is geologically young. Some outcrops are overlain by products of the 1.8 ka Taupo eruption, but not the 26.5 ka Oruanui Formation, showing the perlites are less than 26.5 ka. Others are overlain by both units, showing they are at least 26.5ka, but are emergent from (and therefore presumably overlay) older widespread ignimbrites which are older than 300 ka. This relationship has been confirmed by geothermal drilling in the Mangakino area shown in Figure 3. The perlite does not appear to form a carapace on dome margins. Rather, it is massive, though admixed with other materials as described below. Unfortunately, none of the domes has been drilled, but subsequent mining operations indicate that perlite extends to at least 30m vertically, and 100m horizontally into the domes. One occurrence preserves perlitic clasts in a steeply dipping intrusive breccia cutting through younger pyroclastics and not part of a dome, possibly emplaced on a fault. Admixed with the perlite are masses of obsidian and crystalline (lithic) spherulites, commonly arranged in streaks and lenses following flow bands. The boundaries between perlite and obsidian are very sharp, and microscopic examination corroborates this relationship at the sub-millimeter scale.. In some places, tension gashes in the perlite (some arranged in an en-echelon fashion) are lined with obsidian and lithic material. The obsidian may be vesicular, but most perlite is not. 3

4 Flow banding is commonly steeply dipping or contorted, which indicates that the rhyolite lava very viscous when emplaced. Much of the perlite is intensely fractured and in some cases slickensided, but not in a fashion consistent with a fault association. Rather, it appears that some lava flowage continued after the magma was sufficiently solid to sustain brittle fractures and adjacent block record movement between them by polished surfaces. 4.2 Petrography and whole rock geochemistry The perlites, lithic rhyolites and obsidian from the principal perlite quarry at Cashmore s Rd, Atiamuri and the dome complex that it is part of are composed of glassy rhyolite that contains up to a few percent of phenocrysts, including quartz, feldspar, biotite, hypersthene and hornblende. Textural features indicate that the hypersthene is more altered in perlites than in the lithic rhyolites, which may imply a higher volatile content. The perlite and obsidian are minimally affected by either devitrification or hydrothermal alteration (e.g. to clays). Where discrete spherulites, typically a few mm in diameter, occur within the perlite or obsidian they are crystalline composed of radial needles of feldspar and quartz. Elsewhere sub-millimetre crystals of cristobalite rarely occur within cavities; these crystals appear to reflect deuteric rather than hydrothermal processes since they are present only in cavities and do not penetrate or replace the rock fabric. The perlites have average water contents (mass lost at up to 800 C, after drying at 110 C) of 2.4 wt% (range ), whereas the obsidians have 0.1% (range ), and the lithic rhyolites 0.5% ( ), with no overlap of the ranges. Water content correlates approximately with the expansion ratio of the perlites. Refractive indices of the obsidian and perlite glasses are systematically different: the perlite averaged 1.493, obsidian Both glasses are isotropic. There is little sign of strain birefingence in either. Obsidian within the perlite domes occurs texturally in two ways: (1) Obsidian occurring in larger bodies (metre scale) within the dome but lacking perlite within the obsidian, and (2) Obsidian from small lenses (milllimetre to centimetre scale) within perlite bodies. Compositional differences between plagioclase feldspar in the perlite and the two types of obsidian are small but significant. Obsidiian in larger bodies has few spherulites and phenocrysts, where the plagioclase is An28.5 (SD 6, noting that as individual crystals are compositionally zoned the mean observation does not necessarily reflect the bulk composition). Obsidian from smaller lenses has many spherulites and phenocrysts, with plagioclase An29.75, similar to those in the perlite at An30.0 (SD 4.5). Since the processes of hydration or dehydration cannot affect the composition of already formed feldspar crystals, the perlite cannot have been formed by hydration of the larger obsidian bodies. Presumably, this obsidian has never been hydrated. The smaller obsidian lenses may either represent dehydrated perlite, or they could represent residual obsidian that has been left behind when the rest of it has been converted to perlite through hydration, though the textural relations make that unlikely.. Based both on petrographic observations and on XRD analyses of whole rocks and spherulites/phenocryst assemblages separated from the glasses using density separation with heavy liquids, the lithic rhyolites and obsidians both contain cristobalite, but the perlite contains only trace amounts; cristolbalite contents appear to correlate with spherulite abundances so is probably present within them in sub-microscopic crystals. In contrast, quartz is rare in the obsidian and lithics but forms about of the perlites contain about 2% quartz phenocrysts. This is consistent with a greater water vapour pressure in perlites at the time of initial partial crystallisation. Minor cristobalite in vapour cavities as noted above may simply reflect greater vapour pressure in those cavities due to vapour exsolution during solidification Major oxide compositions from whole rock analyses indicate that lithic rhyolites have greater Fe, Mn, and Ti, and lesser Ca than the perlites, and that the perlites contain more Fe and K than obsidian. This is consistent with magmatic evolution from the rhyolites to the perlites. However, these analyses were based on un-calibrated XRF on powders in the 1970 s, which may not be reliable. These results characterize the mineralogical similarities between the perlite, obsidian, and lithic rhyolite within a single dome complex, which suggests that each is part of a single magmatic system (Lawless, 1975). However, systematic differences in phenocryst abundance and composition cannot be accounted for by post-emplacement hydration, since that should leave phenocrysts unchanged. Some of these differences are consistent with greater water-vapour pressure in the perlites during their (partial) crystallisation and solidification. 4.3 Stable isotope analyses Stable isotope values for obsidian and perlite are inconsistent with significant meteoric water input (Table 1). The perlite samples isotopically analysed contain 3.0 to 4.0 wt% water, whereas the obsidian contains 0.7 wt% water. The δd values of the perlite range from -98 to -84 permil, and the obsidian has a δd value of -86 permil. The δ 18 O values of the perlite range from 8.1 to 8.7 permil, and the obsidian has a more depleted δ 18 O value of 7.2 permil. The isotopic data indicate that water contained in the analyzed TVZ rhyolites are consistent with the derivation of these perlites from a magmatic source (Figure 4). Note that in Figure 4a distinction is made between the perlite rock compositions that were determined, and the compositions of the water present at the time of hydration, based on isotopic fractionation between rhyolite melt and water at 800 C. 4

5 Table 1: Results of isotopic analyses of NZ perlite and obsidian Sample Sample locations, UTM delta D delta 18-O wt% H2O me mn Wawa obsidian Wawa perlite Pukatarata perlite Cashmore perlite Figure 4. Isotopic composition of waters from NZ perlites. TVZ water compositions from Hulston (1983): TVZ lake waters lie off the usual meteoric water line due to minor evaporation. 5. DISCUSSION AND CONCLUSIONS Available data indicate that water contained in the TVZ perlites was derived from magma that contained as much as 4% waters. Different pathways for formation of each of the TVZ rhyolitic rocks are possible and relate to the water content and cooling rate of the magma (Fig. 5). With sufficiently high water content and/or eruption rate, magmas vessiculate and are erupted as pumiceous pyroclastic deposits. With medium water contents and eruption rates and cooling is rapid perlite forms. Local vapour loss leads to the formation of obsidian and/or crystallisation, i.e. patches of obsidian within perlite domes are formed from perlite, not vice versa. 5

6 The observation that the perlite domes were marginal to, smaller than and had lower profiles than the lithic rhyolite domes would be consistent with them being late stage, possibly localised on ring fractures or other structures, and that the water-rich lavas had a lower viscosity than those forming the lithic rhyolites (therefore having a lower angle of repose). The late-stage, water-rich magma could either be formed by fractional crystallisation within a magma chamber, or it could be that additional water was acquired during magmatic ascent. A further hypothesis, not essential to the proposed genetic mechanism but consistent with the observations, was that the obsidian and perlite maybe represent two immiscible glasses formed by de-pressurization as proposed by Steiner (1960) for ignimbrites. Figure 5. Genetic relationship of Perlite, Obsidian and Lithic Rhyolite (Lawless, 1975) Some indirect supporting evidence supporting the existence of contrasting silicic magma types with high and low water contest has recently been provided by the work of Begue et al. (2013), following Deering et al. (2010) on ignimbrites from the TVZ. She pointed out that there were two types of ignimbrite in the region, one having apparently been derived from a more water-rich magma and with a more hydrous mafic phenocryst assemblage (hornblende ± cummingtonite ± biotite) very similar to the perlites studied, and a higher volatile content, compared to the other type which had less hydrous phenocrysts (pyroxenes) and lower volatiles. The existence of different water contents in the parent magmas was supported by B isotope analyses. If the hypothesis of direct formation of perlites from water-rich magmas is correct, rather than post-emplacement hydration, there are commercial implications: TVZ perlite deposits are likely to be much larger than those in the USA since they form the core of domes, not skins. More significant from the geothermal perspective is that if perlites form from water-rich, late stage magmas they may be associated with magmatic activity that is more prone to form intrusive or hydrothermal breccias than would be the case with lower water contents. As they also appear to be structurally localised, they may be associated with fault permeability as well. Being late stage, perlites may be more related to the youngest heat sources and structures than older domes. For all of these reasons, perlites may be a positive indicator for geothermal heat sources and permeability. REFERENCES Begue, F., Chambefort, I., Gravely, D., Kennedy, B, and Deering, C.: Magmatic Volatiles in the Taupo Volcanic Zone, NZ. Proceedings of the New Zealand Geothermal Workshop (Poster) (2013). Clayton R.N. and Mayeda T.K.: The use of bromine-pentafluoride in the extraction of oxygen from oxides and silicates for isotopic analysis: Geochimica et Cosmochimica Acta 27, (1963). Deering. C.D. Gravley, D.M, Vogel, T.A., Cole. J.W., Leonard, G.S. Origins of cold-wet-oxidizing to hot-dry-reducing rhyolite magma cycles and distribution in the Taupo Volcanic Zone, New Zealand. Contrib Mineral Petrol (2010) 160:

7 Friedman, I. and Smith, R.L.: The Deuterium Content of Water in Some Volcanic Glasses. Geochim. et Cosmochim. Acta, 15: (1958). Hulston J.R. Environmental isotope investigations of New Zealand geothermal waters - a review. Geothermics, Vol. 12, No. 2/3.: (1983) Lawless, J.V; The geology of the Cashmores Road Perlite Occurrence, Atiamuri. M.Sc Thesis, Waikato University (1975). Nairn, I. A.. Geology of the Okataina Volcanic Centre, scale 1:50,000. Institute of Geological & Nuclear Sciences geological map sheet pp. Lower Hutt, New Zealand: Institute of Geological & Nuclear Sciences Ltd (2002) Sharp Z.D., Atudorei V., and Durakiewicz T.; A rapid method for determination of hydrogen and oxygen isotope ratios from water and hydrous minerals: Chemical Geology 178, (2001). Steiner, A.; Origin of Ignimbrites of the North Island, New Zealand: A New Petrogenetic Concept. New Zealand Geological Survey Bulletin New Series 68, 42pp. (1960) Thompson, B.N. and Reed, J.J.; Perlite deposits in NZ. NZ J Sci Tech Section B v. 36 (3) p (1954). 7