Origins of cold-wet-oxidizing to hot-dry-reducing rhyolite magma cycles and distribution in the Taupo Volcanic Zone, New Zealand

Slab-derived aqueous fluid components (Ba, Cl) correlate well with oxygen fugacity, and other well-defined characteristics of silicic magmas in the Taupo Volcanic Zone (TVZ) between a cold-wet-oxidizing magma type [R1: amphibole ± biotite; high Sr, low Zr and FeO*/MgO, depleted middle rare-earth elements (MREE)] and a hot-dry-reducing magma type (R2: orthopyroxene ± clinopyroxene; low Sr, high Zr, and FeO*/MgO, less depleted MREE). Oxygen fugacity was obtained from analysis of Fe–Ti oxides and ranges between −0.04 and +2.1 log units (ΔQFM, where QFM = quartz + fayalite + magnetite buffer) and is positively correlated with the bulk-rock Ba/La ratio, indicating that slab-derived fluid is the oxidizing agent in the rhyolites. Chlorine contents in hornblende also correlate with the bulk-rock Ba/La ratio. Hence, high-fluid flux typically correlates with the R1 and low-fluid flux with R2 rhyolite magma types. A geochemical evolution and distribution can be tracked in time and space throughout the central region of the TVZ from 550 ka to present and has revealed two distinct magmatic cycles that vary in length. The first cycle included widespread R1 type magmatism across the central TVZ beginning ca. 550 ka and was directly associated with previously unreported dome-building and ignimbrite-forming volcanism, and led to a voluminous (>3,000 km³) ignimbrite ‘flare-up’ between ca. 340 and 240 ka. The second cycle began roughly 180 ka, erupting ca. 800 km³ of magma, and continues to the present. The duration, rate, and composition of magma production within these cycles appears to be governed by the flux of fluid released from the subducting slab, while the distribution of magmas may be governed more by extension along the central rift axis. Shorter cycles have also been identified and are unrelated to subduction processes, but occur following large, caldera-forming events.     

High-alumina basalts of Taupo Volcanic Zone,New Zealand

High-alumina basalt occurs at eight localities in the Taupo Volcanic Zone associated with the Okataina, Maroa and Taupo calderas. At Tarawera the basalt is associated with a series of craters aligned NNE, and at K Trig. a series of scoria cones of similar alignment. On a broader scale the eruptive centres lie on three lines probably corresponding to major faults within the graben.Petrographically the rocks are plagioclase-olivine-augite basalts with varying percentages of each mafic phenocryst. Most are fine-medium grained, but the Ongaroto basalt is coarser-grained, and has a higher modal percentage of olivine. Chemically all are high-alumina basalts. The Ongaroto basalts has lower Al₂O₃ and higher MgO, Ni and Cr, and this, together with petrographic evidence, suggests that the rock is a cumulate.All the basalts are likely to be derived from the mantle, with intrusion into the crust controlled largely by faulting.     

Variation in rhyolitic magma composition linked with fractionation from a common source: insights from the Rotoiti eruption,Taupo Volcanic Zone,New Zealand

ABSTRACT

The Taupo Volcanic Zone (TVZ), New Zealand, is a well-documented volcanic arc characterized by explosive rhyolitic magmas within a series of caldera complexes that include the Okataina Volcanic Centre (OVC). New quartz melt inclusion and volcanic glass data from the 45 ka caldera-forming Rotoiti eruption within the OVC are compared to published studies. The new data are characterized by low K₂O (~1.5–3.5 wt.%), Rb (~30–70 ppm), Sr (~40–90 ppm), U (~0.5–2.5 ppm), and Ba (~300–1000 ppm) ranges that differ significantly from other OVC systems (~3.0–4.5 wt.% K₂O, ~80–150 ppm Rb, and ~2.5–5.0 ppm U). Most interestingly, the Rotoiti melt inclusion data measured in this study show a decrease in Rb, Sr, and U, although the fractionation trends originate from the same source point as published OVC data. This progressive decreasing trend is interpreted as an interaction with a less enriched rhyolitic melt (represented by the low Rb, Sr, and U of glasses) during fractionation processes from a common TVZ source. The established model for TVZ rhyolites is that they are extracted from a middle or upper crustal source (‘mush’ zone) prior to eruption. Adding to this model, new melt inclusion data suggest that all TVZ rhyolites are fractionated from this common TVZ source and, prior to eruption, the Rotoiti system was rejuvenated by this source (evidenced by the low REE glasses). Exactly what triggers the common TVZ source to fractionate remains unclear, but a proposed mechanism to account for this involves the successive melting of the upper crust by upwelling mantle induced by incremental subduction.     

Geochemical constraints on the petrogenesis of high-Mg basaltic andesites from the Northern Taiwan Volcanic Zone

The Northern Taiwan Volcanic Zone (NTVZ) is a Late Pliocene–Quaternary volcanic field that occurred as a result of extensional collapse of the northern Taiwan mountain belt. We report here mineral compositions, major and trace element and Sr/Nd isotope data of high-Mg basaltic andesites from the Mienhuayu, a volcanic islet formed at ∼2.6 Ma in the central part of the NTVZ. The rocks are hypocrystalline, showing porphyritic texture with Mg-rich olivine (Fo≈81–80), bronzite (En≈82–79) and plagioclase (An≈66–58) as major phenocryst phases. They have uniform whole-rock compositions, marked by high magnesium (MgO≈5.9–8.1 wt.%, Mg value≈0.6) relative to accompanying silica contents (SiO₂≈52.8–54.5 wt.%). The high-Mg basaltic andesites contain the highest TiO₂(∼1.5 wt.%) and lowest K₂O (∼0.4 wt.%) among the NTVZ volcanic rocks. In the incompatible element variation diagram, these Mienhuayu magmas exhibit mild enrichments in large ion lithophile (LILE) and light rare earth elements (LREE), coupled with an apparent Pb-positive spike. They do not display depletions in high field strength elements (HFSE), a feature observed universally in the other NTVZ volcanics. The high-Mg basaltic andesites have rather unradiogenic Nd (εNd≈+5.1–7.2) but apparently elevated Sr (⁸⁷Sr/⁸⁶Sr≈0.70435–0.70543; leached values) isotope ratios. Their overall geochemical and isotopic characteristics are similar to mid-Miocene (∼13 Ma) high-Mg andesites from the Iriomote-jima, southern Ryukyus, Japan. Despite these magmas have lower LILE and LREE enrichments and Pb positive spike, their “intraplate-type” incompatible element variation patterns are comparable to those of extension-induced Miocene intraplate basalts emplaced in the Taiwan–Fujian region. Therefore, we interpret the Mienhuayu magmas as silica-saturated melts derived from decompression melting of the ascended asthenosphere that had been subtly affected by the adjacent Ryukyu subduction zone processes. This interpretation is consistent with the notion that in the northern Taiwan mountain belt post-orogenic lithospheric extension started in Plio–Pleistocene time.     

Mineralogical relations and magma mixing in calc-alkaline andesites from lake Atitlán,Guatemala

Summary Quaternary calc-alkaline andesites erupted form three neighboring volcanoes along the Guatemalan volcanic front have mineralogic compositions and textures which show varying degrees of disequilibrium. Basaltic andesites and andesites (SiO₂ % = 50–59), erupted from Atitlán volcano located nearer to the trench, have the lowest degree of disequilibrium. These lavas contain an anhydrous phenocryst assemblage of mildly bimodal plagioclase, olivine, augite opx, and magnetite. Orthopyroxene occurs at the expense of olivine with increasing whole rock SiO₂. Most pyroxene phenocrysts show a trend of Fe enrichment.Andesites from Tolimán (SiO₂% = 53–62) and San Pedro (Si0₂% = 54–67) volcanoes, located further away from the trench, show comparatively high and moderate degrees of disequilibrium, respectively. Tolimán andesites have bimodal plagioclase compositions and textures. Olivine persists with increasing whole rock Si0₂ and lacks clear modal relations with coexisting orthopyroxene and hornblende phenocrysts. When compared to Atitlán andesites, Toliman olivines are more forsteritic and pyroxenes contain higher proportions of Mg-rich rims, though normal zoned phenocrysts occur within the same rock. Tolimán andesite also have lower proportions of phenocrysts to microphenocrysts, more calcic plagioclase groundmass compositions, and higher modal phenocrystic magnetite. San Pedro andesites have disequilibrium assemblages similar to Tolimán andesites but are not as striking.Magma mixing is proposed as the dominant cause for observed disequilibrium. Disequilibrium features are preserved best in Tolimán and San Pedro andesites because inferred durations between mixing and eruption are shortest, and consequently, these mixed andesites more clearly record mafic and silicic endmember compositions. The mafic component is a relatively high temperature, high-Al basalt containing phenocrysts of Mg-rich olivine (Fo = 78–80), calcic plagioclase (An 70–80), augite and titanomagnetite. The silicic component contains quartz, sodic plagioclase (An 40–50), Fe-rich orthopyroxene and titanomagnetite. Short durations between mixing and eruption produce petrographic features which, in part, mimic the effects of increasing PH₂0 in a fractionating magma. Inferred mixing durations for Atitlánn andesites are longer and involve a less-silicic composition. The intervolcano disequilibrium relations suggest that as Si0₂ in a silicic endmember increases, the duration and efficiency of mixing decreases.

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Mineral-Reaktionen und Magma-Mixing in Kalk-Alkali-Andesiten vom Atitlan See, Guatemala

Zusammenfassung Quartäre Kalk-Alkali-Andesite von drei benachbarten Vulkanen aus dem Guatemala Vulkan-Gürtel zeigen anhand ihrer mineralogischen Zusammensetzung und ihrer Textur variierende Bedingungen des Ungleichgewichts. Basalt-Andesite und Andesite (SiO₂% = 50–59), die aus dem dem Trench am nächsten gelegenen Vulkan Atitlan eruptierten, lassen den niedrigsten Grad an Ungleichgewicht erkennen. Diese Laven führen eine Phenokristall-Assoziation bestehend aus leicht bimodalem Plagioklas, Olivin, Augit, Orthopyroxen und Magnetit. In Gesteinen mit steigendem GesamtgesteinsSiO₂, tritt Orthopyroxen auf Kosten des Olivins auf. Die meisten Pyroxene zeigen einen Trend zur Fe-Anreicherung. Andesite vom Toliman (Si0₂% = 53–62) und vom Vulkan San Pedro (Si0₂% = 54-67), die beide weiter vom Trench entfernt liegen, zeigen hohen bzw. mittleren Grad an Ungleichgewicht. Die Toliman Andesite sind durch bimodale Zusammensetzung der Plagioklase und Textur gekennzeichnet. Olivin bleibt auch bei steigendem SiO₂-Gehalt bestehen, und zeigt hinsichlich seiner Zusammensetzung keine Verbindung mit koexistierenden Phenokristallen von Orthopyroxen und Hornblende. Im Vergleich zu den Atitlan Andesiten, weisen Toliman Olivine höheren Forsteritgehalt auf, die Pyroxene zeigen häufiger Mg-reiche Ränder, obwohl normal zonierte Phenokristalle auch im selben Gestein auftreten. Die Toliman Andesite sind durch ein kleineres Verhältnis von Phenokristalle/Mikrophenokristalle, höheren Ca-Gehalt der Matrix-Plagioklase und höheren Gehalt an Magnetit-Phenokristallen gekennzeichnet. Die San Pedro Andesite zeigen Ungleichgewichts-Paragenesen vergleichbar mit denen der Toliman Andesite, jedoch nicht so auffallend. Es ist zu vermuten, da\ Magma-Mixing vorwiegend für die UngleichgewichtsParagenesen verantwortlich ist. Die Toliman- und San Pedro Andesite zeigen die best erhaltenen Anzeichen für Ungleichgewicht, da vermutlich der Zeitraum zwischen Magma-Mixing und Eruption am kürzesten gewesen ist, weshalb diese Andesite die gemischten mafischen bzw. salischen Endglieder am besten widerspiegeln. Das mafische Endglied ist ein Al-reicher Hoch-Temperatur-Basalt, der sich aus Phenokristallen von Mg-reichem Olivin (Fo = 78–80), Ca-reichem Plagioklas (An 70–80), Fe-reichem Orthopyroxen und Titanomagnetit zusammensetzt. Das salische Endglied besteht aus Quarz, Na-reichem Plagioklas (An 40–50), Fe-reichem Orthopyroxen und Titanomagnetit. Aufgrund des kurzen Zeitabstandes zwischen Magma-Mixing und Eruption entstehen petrographische Strukturen, die den Effekt von steigendem PH₂0 in einem fraktionierenden Magma widerspiegeln. Es wird vermutet, da\ bei den Atitlan-Andesiten mehr Zeit zwischen Mixing und Eruption vergangen ist; dies manifestiert sich in einem geringeren Anteil der salischen Komponente. Die Zusammenhänge der Ungleichgewichts-Bedingungen innerhalb eines Vulkans lassen vermuten, daß die Dauer und Intensität des Mixing mit zunehmender salischer Komponente abnimmt.

     

Characteristics of three recent earthquake sequences in the Taupo Volcanic Zone, New Zealand

We have analysed three recent earthquake sequences in the northern part of the Taupo Volcanic Zone. A 1998 sequence at Haroharo with a largest event of M_L 4.8, and a 2004 sequence near Lake Rotoehu (largest event M_L 5.4), had normal b-values, and displayed an aftershock decay pattern, with most of the activity within the first few days. In contrast, a 2005 sequence a few tens of kilometres away at Matata (largest event M_L 4.1), had very different characteristics, with a slow development and decay, no tendency for enhanced seismicity after the larger events, and a very high b-value.The focal mechanisms of the Rotoehu and Matata events are normal, and have stress patterns consistent with the geodetically observed extension of the Taupo Volcanic Zone in a northwest–southeast direction. The extensive recent volcanism in the Okataina Volcanic Centre does not seem to have affected the stress pattern in this area.The Rotoehu sequence showed a strong resemblance, particularly in the time distribution of events, to the well-known swarm activity in the Vogtland region on the German/Czech border, in which larger events were followed by a burst of seismicity, as in a normal aftershock sequence. Some of the arguments that have been advanced to explain the Vogtland swarm as seismicity induced by fluid injection apply to Rotoehu, but there is no direct evidence of fluid involvement. The Matata sequence appears to have a continuing trigger mechanism, either a slow injection of fluid, or a slow slip event, in an environment in which opening pore spaces prevent high overpressures developing. The Matata sequence occurred close to the area of the 1987 M_L 6.3 Edgecumbe Earthquake, so exhibiting two extremes of seismic temporal pattern, namely mainshock–aftershock and a swarm with many events of similar magnitude, within a small area.     

Geothermal waters from the Taupo Volcanic Zone,New Zealand: Li,B and Sr isotopes characterization

Chemical and isotopic data for 23 geothermal water samples collected in New Zealand within the Taupo Volcanic Zone (TVZ) are reported. Major and trace elements including Li, B and Sr and their isotopic compositions (δ⁷Li, δ¹¹B, ⁸⁷Sr/⁸⁶Sr) were determined in high temperature geothermal waters collected from deep boreholes in different geothermal fields (Ohaaki, Wairakei, Mokai, Kawerau and Rotokawa geothermal systems). Lithium concentrations are high (from 4.5 to 19.9 mg/L) and Li isotopic compositions (δ⁷Li) are homogeneous, ranging between −0.5‰ and +1.4‰. In particular, it is noteworthy that, except for the samples from the Kawerau geothermal field having slightly higher δ⁷Li values (+1.4%), the other geothermal waters have a near constant δ⁷Li signature around a mean value of 0‰ ± 0.6 (2σ, n = 21). Boron concentrations are also high and relatively homogeneous for the geothermal samples, falling between 17.5 and 82.1 mg/L. Boron isotopic compositions (δ¹¹B) are all negative, and display a range between −6.7‰ and −1.9‰. These B isotope compositions are in agreement with those of the Ngawha geothermal field in New Zealand. Lithium and B isotope signatures are in a good agreement with a fluid signature mainly derived from water/rock interaction involving magmatic rocks with no evidence of seawater input. On the other hand, Sr concentrations are lower and more heterogeneous and fall between 2 and 165 μg/L. The ⁸⁷Sr/⁸⁶Sr ratios range from 0.70549 to 0.70961. These Sr isotope compositions overlap those of the Rotorua geothermal field in New Zealand, confirming that some geothermal waters (with more radiogenic Sr) have interacted with bedrocks from the metasedimentary basement. Each of these isotope systems on their own reveals important information about particular aspects of either water source or water/rock interaction processes, but, considered together, provide a more integrated understanding of the geothermal systems from the TVZ in New Zealand.     

Petrology of Calc-alkaline Lavas from Ruapehu Volcano and Related Vents, Taupo Volcanic Zone, New Zealand

Most Ruapehu lavas and those of related vents (Taupo VolcanicZone, New Zealand) are calc-alkaline, medium-K basic and acidandesites, though minor volumes of basalt and dacite occur.Nearly all are porphyritic with phenocrysts of plagioclase,augite, olivine (mainly in basalts and basic andesites), orthopyroxene(mainly in acid andesites and dacites), and titanomagnetite(chrome spinel in basic lavas). The lavas have been subdividedinto six groups, each petrographically, geochemically and isotopicallydistinct: Type 1 plagioclase-pyroxene phyric lavas dominate,and range from basalt to dacite. Least squares mass balancecalculations indicate that these lavas were probably generatedfrom low-alumina basalt by combined crystal fractionation (15–55per cent) and crustal assimilation (1–30 per cent). Xenolithstudies indicate that the assimilant is most likely to be apartial melt of gneiss, originally Torlesse terrane greywacke.Crystal accumulation occurs to a minor extent in Type 1 lavasand becomes important in Type 2 (plagioclase-phyric) and Type3 (pyroxene-phyric) lavas. Type 4 lavas are rare and of unknownorigin, though they may be similar to rare hornblende-bearingandesites from nearby Maungakatote volcano. Type 5 lavas areclinopyroxene-olivine-phyric andesites which were probably generatedfrom a primitive basalt by crystal fractionation without crustalassimilation. Type 6 lavas show strong evidence of disequilibriumand were probably generated by mixing Type 5 basalt with Type1 dacite in proportions of between 60:40 and 50:50. The assertion that the assimilant involved in contaminationof most Ruapehu andesites is a partial melt of basement greywackeis a significant departure from previously published theoriesand has important implications for trace element and isotopicmodelling.     

Occurrence of gold in hydrothermal pyrite,western Taupo Volcanic Zone,New Zealand

The Taupo Volcanic Zone (TVZ) on the North Island of New Zealand is located above the subducting Pacific slab and hosts hydrothermal systems related to subduction, arc magmatism and crustal extension. In these systems, gold is transported primarily as sulphide complexes, with gold being deposited in response to boiling and mixing of the deep geothermal fluids. Conglomerate cobbles and hydrothermal fumaroles from the upper Retaruke River in the western TVZ are mineralised deposits that have been eroded from the nearby Miocene alluvial sedimentary plateau. Abundant gold-bearing pyrite was precipitated in the conglomerates and fumaroles by late hydrothermal process, primarily occurring as veinlets, disseminations and fine-grained aggregates which consists discrete euhedral microcrystals. Scanning electron microscopy combined with X-ray energy disperses spectroscopy shows that pyrites are commendably affected by late hydrothermal fluids, possibly by the carbonate fluid. Electron probe microanalysis revealed that the pyrite contains 20–120 ppm Au (averaging 60 ppm). Wavelength dispersive spectral elemental mapping suggests that gold is distributed uniformly in pyrite, indicating structurally bound gold (solid solution) in pyrite. Gold mineralisation recognised in the conglomerates and fumaroles demonstrates that the upper Retaruke River is a promising target for future gold exploration.     

From 2D to 3D: Prospectivity modelling in the Taupo Volcanic Zone,New Zealand

A 2D prospectivity model of epithermal gold mineralisation has been completed over the Taupo Volcanic Zone (TVZ), using the weights of evidence modelling technique. This study was used to restrict a 3D geological interpretation and prospectivity model for the Ohakuri region. The TVZ is commonly thought of as a present-day analogue of the environment in which many epithermal ore deposits, such as in the Hauraki Goldfield, Coromandel Volcanic Zone, are formed. The models utilise compiled digital data including historical exploration data, geological data from the Institute of Geological and Nuclear Sciences Ltd. Quarter Million Mapping Programme, recent Glass Earth geophysics data and historic exploration geochemical data, including rock-chip and stream sediment information. Spatial correlations between known deposits and predictive maps are determined from the available data, which represent each component of the currently accepted mineral system model for epithermal gold. The 2D prospectivity model confirms that the TVZ has potential for gold mineralisation. However, one of the weaknesses of this weights of evidence model is that the studies are carried out in 2D, with an approximation of 3D provided by geophysical and drilling data projected to a 2D plane. Consequently, a 3D prospectivity model was completed over the Ohakuri area, constrained by the results of the 2D model and predictive maps. The 3D model improved the results allowing more effective exploration targeting. However, the study also highlighted the main issues that need to be resolved before 3D prospectivity modelling becomes standard practise in the mineral exploration industry. The study also helped develop a work flow that incorporates preliminary 2D spatial data analysis from the weights of evidence technique to more effectively restrict and develop 3D predictive map interpretation and development.     

Phenocryst complexity in andesites and dacites from the Tequila volcanic field,Mexico: resolving the effects of degassing vs. magma mixing

The petrology of five phenocryst-poor (2–5%) andesites and dacites, all of which were erupted from different short-lived, monogenetic vents, is compared to that of phenocryst-rich (10–25%) andesites erupted from the adjacent stratovolcano, Volcán Tequila, in the Mexican arc. Despite differences in phenocryst abundances, these magmas have comparable phase assemblages (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± augite ± hornblende), and similarly wide variations in phenocryst compositions, coupled to complex zoning patterns. For the phenocryst-poor lavas, equilibrium pairs of two Fe–Ti oxides lead to a narrow range of calculated temperatures for each sample that range from 934 (±24) to 1,073 (±6)°C and oxygen fugacities that range from +0.1 to +0.7 log units relative to the Ni–NiO buffer. Application of the plagioclase-liquid hygrometer to each sample at these calculated temperatures leads to maximum melt water concentrations of 4.6–3.1 wt% during plagioclase crystallization, indicating that the magmas were fluid saturated at depths ≥6.4–4.5 km. There is a wide, continuous range in the composition of plagioclase (≤44 mol% An) and orthopyroxene (≤16% Mg#) phenocrysts in each sample, which is consistent with a loss of dissolved water (≤2.8 wt%) from the melt phase during degassing as the magmas ascended rapidly to the surface. Evidence is presented that shows the effect of dissolved water is to reduce the activity of MgO relative to FeO in the melt phase, which indicates that degassing will also affect the Mg# of pyroxene phenocrysts, with higher melt water concentrations favoring Fe-rich pyroxene. Both plagioclase and orthopyroxene commonly display diffusion-limited growth textures (e.g., skeletal and hopper crystals, large interior melt hollows, and swallow tails), which are consistent with large undercoolings produced by degassing-induced crystallization. Therefore, degassing is proposed as a possible cause for the phenocryst compositional diversity documented in the phenocryst-poor andesite and dacite lavas erupted from peripheral vents, including the coexistence of normally zoned plagioclase and reversely zoned orthopyroxene. Degassing-induced crystallization may also explain some of the phenocryst complexity in crystal-rich andesites erupted from large stratovolcanoes, including Volcán Tequila.     

辽西中生代陆内底侵作用背景下形成的安山岩

本文选择了辽西地区中生代230Ma、190Ma、159Ma、123Ma、116Ma等不同时期的安山岩及其所含的辉石,通过岩石学、岩相学、地球化学和同位素的研究,发现它们同时具有壳源和幔源的特征,而且大多数单斜辉石记录了两种或多种熔体的相互作用.据此作者认为安山岩是壳幔岩浆混熔成因的,而如此长期的热演化与大陆内部伸展背景下的底侵作用有关.     

Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS. Part 2: application to Taupo Volcanic Zone rhyolites

Constraining the pressure of crystallisation of large silicic magma bodies gives important insight into the depth and vertical extent of magmatic plumbing systems; however, it is notably difficult to constrain pressure at the level of detail necessary to understand shallow magmatic systems. In this study, we use the recently developed rhyolite-MELTS geobarometer to constrain the crystallisation pressures of rhyolites from the Taupo Volcanic Zone (TVZ). As sanidine is absent from the studied deposits, we calculate the pressures at which quartz and feldspar are found to be in equilibrium with melt now preserved as glass (the quartz +1 feldspar constraint of Gualda and Ghiorso, Contrib Mineral Petrol 168:1033. doi: 10.1007/s00410-014-1033-3. 2014). We use glass compositions (matrix glass and melt inclusions) from seven eruptive deposits dated between ~320 and 0.7 ka from four distinct calderas in the central TVZ, and we discuss advantages and limitations of the rhyolite-MELTS geobarometer in comparison with other geobarometers applied to the same eruptive deposits. Overall, there is good agreement with other pressure estimates from the literature (amphibole geobarometry and H₂O–CO₂ solubility models). One of the main advantages of this new geobarometer is that it can be applied to both matrix glass and melt inclusions—regardless of volatile saturation. The examples presented also emphasise the utility of this method to filter out spurious glass compositions. Pressure estimates obtained with the new rhyolite-MELTS geobarometer range between ~250 to ~50 MPa, with a large majority at ~100 MPa. These results confirm that the TVZ hosts some of the shallowest rhyolitic magma bodies on the planet, resulting from the extensional tectonic regime and thinning of the crust. Distinct populations with different equilibration pressures are also recognised, which is consistent with the idea that multiple batches of eruptible magma can be present in the crust at the same time and can be tapped simultaneously by large eruptive events.     

The origin of lavas and ignimbrites of the Taupo Volcanic Zone,New Zealand,in the light of oxygen isotope data

The Taupo Volcanic Zone consists predominantly of andesites at each end and about a 20times larger volume of rhyolites (incl. ignimbrites and pumice) in the center. In a δ¹⁸O vs SiO₂ diagram the rhyolites do not continue the increasingly ¹⁸O-contaminated andesite (basalt to dacite) trend, but have lower δ¹⁸O values than the most differentiated andesites. Comparison with δ¹⁸O of greywackes and conjectural granitoids seems to rule out a partial fusion origin. In retrospect the Sr isotope data of Ewart and Stipp (1968) are seen to confirm this conclusion. Isotopic adjustment of source rocks before partial fusion, by exchange with circulating subsurface waters, is highly unlikely because of the combined demands for O and Sr isotope shifts, and the constraint offered by the normal igneous to somewhat enriched δ¹⁸O values of the rhyolites. Oxygen isotope results confirm separate origins or at least different modes of ascent of basalt and andesites on one hand and rhyolites on the other. Both magma types appear to have originated below crustal formations and both have become contaminated by greywacke through which they passed to reach the surface.     

The Petrology of the Rotoiti Eruption Sequence, Taupo Volcanic Zone: an Example of Fractionation and Mixing in a Rhyolitic System

The Rotoiti eruption from the Taupo Volcanic Zone (TVZ) in northernNew Zealand produced voluminous pyroclastic deposits. The ferromagnesianmineral assemblage in these dominantly consists of cummingtonite+ hornblende + orthopyroxene with uniform magnesium/iron ratios;a second assemblage of biotite + hornblende + orthopyroxene,also with uniform Fe/Mg ratios, appears midway through the eruptionsequence and, thereafter, increases in abundance. These contrastingmineral assemblages, together with pumice clast and groundmassglass compositions, provide evidence for mingling of two discretemagmas. Similarities in the chemical characteristics of thetwo magmas suggest that they developed from a similar source.The eruption initially tapped relatively homogeneous magma thatwas erupted throughout most of this phase of activity. The middlestages of the eruption included some mixed magma. The finalstages of the eruption were dominated by a second magma composition,which was probably injected into the bottom of the main magmabody as the eruption proceeded. The source that fed the eruptionwas complex, and discrete magma bodies existed and evolved separatelyprior to the eruption. We conclude that eruptions in the TVZare fed from a diffuse upper-crustal zone of partially interconnected,and at times physically separate, magma bodies rather than fromcentralized and necessarily large long-lived magma chambers. KEY WORDS: Taupo Volcanic Zone; Okataina Volcanic Centre; Rotoiti eruption; rhyolite system; magma mixing     

Surface heat flow and CO2 emissions within the Ohaaki hydrothermal field,Taupo Volcanic Zone,New Zealand

Carbon dioxide emissions and heat flow have been determined from the Ohaaki hydrothermal field, Taupo Volcanic Zone (TVZ), New Zealand following 20 a of production (116 MW_e). Soil CO₂ degassing was quantified with 2663 CO₂ flux measurements using the accumulation chamber method, and 2563 soil temperatures were measured and converted to equivalent heat flow (W m⁻²) using published soil temperature heat flow functions. Both CO₂ flux and heat flow were analysed statistically and then modelled using 500 sequential Gaussian simulations. Forty subsoil CO₂ gas samples were also analysed for stable C isotopes. Following 20 a of production, current CO₂ emissions equated to 111 ± 6.7 T/d. Observed heat flow was 70 ± 6.4 MW, compared with a pre-production value of 122 MW. This 52 MW reduction in surface heat flow is due to production-induced drying up of all alkali–Cl outflows (61.5 MW) and steam-heated pools (8.6 MW) within the Ohaaki West thermal area (OHW). The drying up of all alkali–Cl outflows at Ohaaki means that the soil zone is now the major natural pathway of heat release from the high-temperature reservoir. On the other hand, a net gain in thermal ground heat flow of 18 MW (from 25 MW to 43.3 ± 5 MW) at OHW is associated with permeability increases resulting from surface unit fracturing by production-induced ground subsidence. The Ohaaki East (OHE) thermal area showed no change in distribution of shallow and deep soil temperature contours despite 20 a of production, with an observed heat flow of 26.7 ± 3 MW and a CO₂ emission rate of 39 ± 3 T/d. The negligible change in the thermal status of the OHE thermal area is attributed to the low permeability of the reservoir beneath this area, which has limited production (mass extraction) and sheltered the area from the pressure decline within the main reservoir. Chemistry suggests that although alkali–Cl outflows once contributed significantly to the natural surface heat flow (∼50%) they contributed little (<1%) to pre-production CO₂ emissions due to the loss of >99% of the original CO₂ content due to depressurisation and boiling as the fluids ascended to the surface. Consequently, the soil has persisted as the major (99%) pathway of CO₂ release to the atmosphere from the high temperature reservoir at Ohaaki. The CO₂ flux and heat flow surveys indicate that despite 20 a of production the variability in location, spatial extent and magnitude of CO₂ flux remains consistent with established geochemical and geophysical models of the Ohaaki Field. At both OHW and OHE carbon isotopic analyses of soil gas indicate a two-stage fractionation process for moderate-flux (>60 g m⁻² d⁻¹) sites; boiling during fluid ascent within the underlying reservoir and isotopic enrichment as CO₂ diffuses through porous media of the soil zone. For high-flux sites (>300 g m⁻² d⁻¹), the δ¹³CO₂ signature (−7.4 ± 0.3‰ OHW and −6.5 ± 0.6‰ OHE) is unaffected by near-surface (soil zone) fractionation processes and reflects the composition of the boiled magmatic CO₂ source for each respective upflow. Flux thresholds of <30 g m⁻² d⁻¹ for purely diffusive gas transport, between 30 and 300 g m⁻² d⁻¹ for combined diffusive–advective transport, and ?300 g m⁻² d⁻¹ for purely advective gas transport at Ohaaki were assigned. δ¹³CO₂ values and cumulative probability plots of CO₂ flux data both identified a threshold of ∼15 g m⁻² d⁻¹ by which background (atmospheric and soil respired) CO₂ may be differentiated from hydrothermal CO₂.     

Role of cryptic amphibole crystallization in magma differentiation at Hudson volcano, Southern Volcanic Zone, Chile

Hudson volcano (Chile) is the southern most stratovolcano of the Andean Southern Volcanic Zone and has produced some of the largest Holocene eruptions in South America. There have been at least 12 recorded Holocene explosive events at Hudson, with the 6700 years BP, 3600 years BP, and 1991 eruptions the largest of these. Hudson volcano has consistently discharged magmas of similar trachyandesitic and trachydacitic composition, with comparable anhydrous phenocryst assemblages, and pre-eruptive temperatures and oxygen fugacities. Pre-eruptive storage conditions for the three largest Holocene events have been estimated using mineral geothermometry, melt inclusion volatile contents, and comparisons to analogous high pressure experiments. Throughout the Holocene, storage of the trachyandesitic magmas occurred at depths between 0.2 and 2.7 km at approximately ~972°C (±25) and log fO₂ −10.33–10.24 (±0.2) (one log unit above the NNO buffer), with between 1 and 3 wt% H₂O in the melt. Pre-eruptive storage of the trachydacitic magma occurred between 1.1 and 2.0 km, at ~942°C (±26) and log fO₂ −10.68 (±0.2), with ~2.5 wt% H₂O in the melt. The evolved trachyandesitic and trachydacitic magmas can be derived from a basaltic parent primarily via fractional crystallization. Entrapment pressures estimated from plagioclase-hosted melt inclusions suggest relatively shallow levels of crystallization. However, trace element data (e.g., Dy/Yb ratio trends) suggests amphibole played an important role in the differentiation of the Hudson magmas, and this fractionation is likely to have occurred at depths >6 km. The absence of a garnet signal in the Hudson trace element data, the potential staging point for differentiation of parental mafic magmas [i.e., ~20 km (e.g., Annen et al. in J Petrol 47(3):505–539, 2006)], and the inferred amphibolite facies [~24 km (e.g., Rudnick and Fountain in Rev Geophys 33:267–309, 1995)] combine to place some constraint on the lower limit of depth of differentiation (i.e., ~20–24 km). These constraints suggest that differentiation of mantle-derived magmas occurred at upper-mid to lower crustal levels and involved a hydrous mineral assemblage that included amphibole, and generated a basaltic to basaltic andesitic composition similar to the magma discharged during the first phase of the 1991 eruption. Continued fractionation at this depth resulted in the formation of the trachyandesitic and trachydacitic compositions. These more evolved magmas ascended and stalled in the shallow crust, as suggested by the pressures of entrapment obtained from the melt inclusions. The decrease in pressure that accompanied ascent, combined with the potential heating of the magma body through decompression-induced crystallization would cause the magma to cross out of the amphibole stability field. Further shallow crystallization involved an anhydrous mineral assemblage and may explain the lack of phenocrystic amphibole in the Hudson suite.     

Volcanic sequence and magma formation in the Oslo region

The Permian activity in the Oslo region started with lava effusions. Monzonitic rhomb porphyry flows predominate, with basaltic flows inbetween. Then a number of basalt volcanoes formed. This phase ended in explosive volcanism, producing ignimbrites, and the explosive activity is considered the primary cause for formation of at least four large and a few smaller cauldrons (or calderas). Below the lava surface monzonitic magma and associated syenitic and granitic magmas crystallized to larvikite, nordmarkitic and granitic rocks. These magmas are assumed to be formed by local melting of portions of the lower crust. The mode of emplacement is stoping.     

Evolution of the Taupo Volcanic Center, New Zealand: petrological and thermal constraints from the Omega dacite

The 20 ka ~0.1 km³ Omega dacite, which erupted shortly after the 26.5 ka Oruanui super-eruption, compositionally stands out among Taupo Volcanic Zone (TVZ) magmas, which are overwhelmingly characterized by rhyolites (>90 % by volume). The previously reported presence of inherited zircons in this zircon-undersaturated magma has provided unequivocal evidence for the involvement of upper-crustal material in a 1–10 year timescale prior to the Omega eruption. However, whether this crustal involvement is characterized by wholesale, melting of preexisting crust or subordinate bulk assimilation into an already differentiated magma body remains unclear. To disentangle these processes, we describe the mineral chemistry of the major phases present in the Omega dacite and determine intensive parameters describing magma chamber conditions. Dominantly unimodal populations of plagioclase (An_50–60), orthopyroxene (Mg# from 58 to 68), and clinopyroxene (Mg# from 65 to 73), along with coexisting equilibrium pairs of Fe–Ti oxides, constrain pre-eruptive temperatures to 850–950 °C, a pressure between ~3 and 7 kbars, and an oxygen fugacity of ~NNO. MELTS thermodynamic modeling suggests that this phase assemblage is in equilibrium with the bulk rock and glass compositions of the Omega dacite at these estimated P–T–fO₂ pre-eruptive conditions. Combining these petrological observations with insights into conductive thermal models of magma–crust interactions, we argue that the Omega dacite more likely formed in the mid-to-lower crust via protracted processing through fractional crystallization coupled with some assimilation (AFC). Incorporation of crustal material is likely to have occurred at various stages, with the inherited zircons (and potentially parts of glomerocrysts) representing late and subordinate upper-crustal assimilants. This petrogenetic model is consistent with the presence of a differentiating crustal column, consisting of a polybaric fractional crystallization and assimilation history. On the basis of petrological, thermal, and geophysical considerations, upper-crustal reservoirs, which feed large-scale rhyolitic volcanism in the TVZ, most likely take the form of large, long-lived crystal mush zones. Following large eruptions, such as the Oruanui event, this mush is expected to crystallize significantly (up to 70–80 vol% crystals) due to syn-eruptive decompression. Hence, the Omega dacite, immediately post-dating the Oruanui event, potentially represents incoming deeper recharge of less-evolved magma that was able to penetrate the nearly solidified upper-crustal mush. Over the past 20,000 years, similar intermediate recharge magmas have incrementally reheated, reconstructed, and reactivated the upper-crustal mush zone, allowing a gradual return to rhyolitic volcanism at the Taupo Volcanic Center.