A cascade of magmatic events during the assembly and eruption of a super-sized magma body

We use comprehensive geochemical and petrological records from whole-rock samples, crystals, matrix glasses and melt inclusions to derive an integrated picture of the generation, accumulation and evacuation of 530 km³ of crystal-poor rhyolite in the 25.4 ka Oruanui supereruption (New Zealand). New data from plagioclase, orthopyroxene, amphibole, quartz, Fe–Ti oxides, matrix glasses, and plagioclase- and quartz-hosted melt inclusions, in samples spanning different phases of the eruption, are integrated with existing data to build a history of the magma system prior to and during eruption. A thermally and compositionally zoned, parental crystal-rich (mush) body was developed during two periods of intensive crystallisation, 70 and 10–15 kyr before the eruption. The mush top was quartz-bearing and as shallow as ~3.5 km deep, and the roots quartz-free and extending to >10 km depth. Less than 600 year prior to the eruption, extraction of large volumes of ~840 °C low-silica rhyolite melt with some crystal cargo (between 1 and 10%), began from this mush to form a melt-dominant (eruptible) body that eventually extended from 3.5 to 6 km depth. Crystals from all levels of the mush were entrained into the eruptible magma, as seen in mineral zonation and amphibole model pressures. Rapid translation of crystals from the mush to the eruptible magma is reflected in textural and compositional diversity in crystal cores and melt inclusion compositions, versus uniformity in the outermost rims. Prior to eruption the assembled eruptible magma body was not thermally or compositionally zoned and at temperatures of ~790 °C, reflecting rapid cooling from the ~840 °C low-silica rhyolite feedstock magma. A subordinate but significant volume (3–5 km³) of contrasting tholeiitic and calc-alkaline mafic material was co-erupted with the dominant rhyolite. These mafic clasts host crystals with compositions which demonstrate that there was some limited pre-eruptive physical interaction of mafic magmas with the mush and melt-dominant body. However, the mafic magmas do not appear to have triggered the eruption or controlled magmatic temperatures in the erupted rhyolite. Integration of textural and compositional data from all available crystal types, across all dominant and subordinate magmatic components, allow the history of the Oruanui magma body to be reconstructed over a wide range of temporal scales using multiple techniques. This history spans the tens of millennia required to grow the parental magma system (U–Th disequilibrium dating in zircon), through the centuries and decades required to assemble the eruptible magma body (textural and diffusion modelling in orthopyroxene), to the months, days, hours and minutes over which individual phases of the eruption occurred, identified through field observations tied to diffusion modelling in magnetite, olivine, quartz and feldspar. Tectonic processes, rather than any inherent characteristics of the magmatic system, were a principal factor acting to drive the rapid accumulation of magma and control its release episodically during the eruption. This work highlights the richness of information that can be gained by integrating multiple lines of petrologic evidence into a holistic timeline of field-verifiable processes.     

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.     

Textural and micro-analytical insights into mafic–felsic interactions during the Oruanui eruption,Taupo

Quenched juvenile mafic inclusions (enclaves) are an occasional but informative component in the deposits of large felsic eruptions. Typically, the groundmasses of these inclusions rapidly crystallize as the mafic magma is chilled against a more voluminous, cooler felsic host, providing a physical and chemical record of the nature and timing of mafic–felsic interactions. We examine mafic inclusions of two compositional lineages (tholeiitic and calc-alkaline) from deposits of the 25.4 ka Oruanui eruption (Taupo, New Zealand). 2-D quantitative textural data from analysis of back-scattered electron images reveal a marked diversity in the groundmass textures of the inclusions, including median crystal sizes (amphibole: 14–45 µm; plagioclase: 21–75 µm) and aspect ratios (amphibole: 1.7–4.2; plagioclase: 2.1–4.0), area number densities (amphibole: 122–2660 mm^?2; plagioclase: 117–2990 mm^?2), area fractions (?) of minerals (?_plag?=?23–45%, ?_amph?=?0–28%, ?_cpx?=?0–6%, ?_oxides?=?0.6–5.5%), and the relative abundance of plagioclase and amphibole (?_plag/?_amph?=?1.0–4.6). Textural parameters vary more significantly within, rather than between, the two compositional lineages, and in some cases show marked variations across individual clasts, implying that each inclusion’s cooling history, rather than bulk composition, was the dominant control on textural development. Groundmass mineral compositions are also diverse both within and between inclusions (e.g. plagioclase from An_34–92, with typical intra-clast variability of ~?20 mol%), and do not correlate with bulk chemistry. Diverse groundmass textures and mineral and glass chemistries are inferred to reflect complex interplay of a range of factors including the degree and rate of undercooling, bulk composition, water content and, possibly, intensive variables. Our data are inconsistent with breakup of a crystallizing ponded mafic layer at the base of the Oruanui melt-dominant body, instead implying that each inclusion partially crystallized as a discrete body with a unique cooling history. Extensive ingestion of mush-derived macro-crystals suggests that mechanical breakup of mafic feeder dikes occurred within a transition zone between the mush and melt-dominant magma body. In this zone, the mush lacked yield strength, as has been inferred from field studies of narrow (meters to few tens of meters) mush-melt transition zones preserved in composite intrusions. Evidence for plastic deformation of inclusions during eruption and the abundance of fresh residual glass in inclusions from all eruptive phases suggest that the inclusions formed syn-eruptively, and must have been formed recurrently at multiple stages throughout the eruption.     

The 26{middle dot}5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body

The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km³ of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO₂ and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO₂. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and ⁸⁷Sr/⁸⁶Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO₂)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO₂) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO₂ content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics     

Timescales of mixing and mobilisation in the Bishop Tuff magma body: perspectives from diffusion chronometry

We present two-feldspar thermometry and diffusion chronometry from sanidine, orthopyroxene and quartz from multiple samples of the Bishop Tuff, California, to constrain the temperature stratification within the pre-eruptive magma body and the timescales of magma mixing prior to its evacuation. Two-feldspar thermometry yields estimates that agree well with previous Fe–Ti oxide thermometry and gives a ~80 °C temperature difference between the earlier- and later-erupted regions of the magma chamber. Using the thermometry results, we model diffusion of Ti in quartz, and Ba and Sr in sanidine as well as Fe–Mg interdiffusion in orthopyroxene to yield timescales for the formation of overgrowth rims on these crystal phases. Diffusion profiles of Ti in quartz and Fe–Mg in orthopyroxene both yield timescales of <150 years for the formation of overgrowth rims. In contrast, both Ba and Sr diffusion in sanidine yield nominal timescales 1–2 orders of magnitude longer than these two methods. The main cause for this discrepancy is inferred to be an incorrect assumption for the initial profile shape for Ba and Sr diffusion modelling (i.e. growth zoning exists). Utilising the divergent diffusion behaviour of Ba and Sr, we place constraints on the initial width of the interface and can refine our initial conditions considerably, bringing Ba and Sr data into alignment, and yielding timescales closer to 500 years, the majority of which are then within uncertainty of timescales modelled from Ti diffusion in quartz. Care must be thus taken when using Ba in sanidine geospeedometry in evolved magmatic systems where no other phases or elements are available for comparative diffusion profiling. Our diffusion modelling reveals piecemeal rejuvenation of the lower parts of the Bishop Tuff magma chamber at least 500 years prior to eruption. Timescales from our mineral profiling imply either that diffusion coefficients currently used are uncertain by 1–2 orders of magnitude, or that the minerals concerned did not experience a common history, despite being extracted from the same single pumice clasts. Introduction of the magma initiating crystallisation of the contrasting rims on sanidine, quartz, orthopyroxene and zircon was prolonged, and may be a marker of other processes that initiated the Bishop Tuff eruption rather than the trigger itself.     

Quartz zoning and the pre-eruptive evolution of the ~340-ka Whakamaru magma systems,New Zealand

Cathodoluminescence (CL) zoning in quartz crystals from rhyolitic pumices in two ignimbrite members of the ~340-ka Whakamaru super-eruption deposits, Taupo Volcanic Zone, New Zealand, is investigated in conjunction with the analysis of Ti concentration in quartz to reconstruct the history of changing magma chamber conditions and to elucidate the eruption-triggering processes. CL intensity images are taken as a proxy for Ti concentration and thus temperature and/or pressure and/or compositional variations during crystal growth history. Estimates of the maximum temperature changes (i.e., assuming other factors influencing Ti uptake remain constant) are made using the TitaniQ geothermometer based on the Ti concentration in quartz. These results are reviewed in comparison with Fe–Ti oxide, feldspar-melt and amphibole geothermometry. Core-to-rim quartz Ti profiles record a marked change in conditions (temperature increase and/or pressure decrease and/or change in melt composition) causing and then following a significant resorption horizon in the outer parts of the crystals. Two alternative models that could explain the quartz Ti zonation invoke a temperature increase caused by mafic recharge and/or a pressure decrease due to magma ponding and re-equilibration at shallow crustal levels. Concomitant changes in melt composition and Ti activity may, however, also have strongly influenced Ti uptake into the quartz. Some crystals also show other marked increases in CL brightness internally, but any accompanying magmatic changes did not result in eruption. Diffusion modelling indicates that this significant change in conditions occurred over ~10–85 years prior to caldera-forming eruption. This rapid thermal pulse or pressure change is interpreted as evidence for open-system processes, and appears to record a magma chamber recharge event that rejuvenated the Whakamaru magma system (melt-dominant magma plus crystal mush), and potentially acted as a trigger for processes that led to eruption.     

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     

Multiple timescale constraints for high-flux magma chamber assembly prior to the Late Bronze Age eruption of Santorini (Greece)

The rhyodacitic magma discharged during the 30–80 km³ DRE (dense rock equivalent) Late Bronze Age (LBA; also called ‘Minoan’) eruption of Santorini caldera is known from previous studies to have had a complex history of polybaric ascent and storage prior to eruption. We refine the timescales of these processes by modelling Mg–Fe diffusion profiles in orthopyroxene and clinopyroxene crystals. The data are integrated with previously published information on the LBA eruption (phase equilibria studies, melt inclusion volatile barometry, Mg-in-plagioclase diffusion chronometry), as well as new plagioclase crystal size distributions and the established pre-LBA history of the volcano, to reconstruct the events that led up to the assembly and discharge of the LBA magma chamber. Orthopyroxene, clinopyroxene and plagioclase crystals in the rhyodacite have compositionally distinct rims, overgrowing relict, probably source-derived, more magnesian (or calcic) cores, and record one or more crystallization (plag???opx?>?cpx) events during the few centuries to years prior to eruption. The crystallization event(s) can be explained by the rapid transfer of rhyodacitic melt from a dioritic/gabbroic region of the subcaldera pluton (mostly in the 8–12 km depth range), followed by injection, cooling and mixing in a large melt lens at 4–6 km depth (the pre-eruptive magma chamber). Since crystals from all eruptive phases yield similar timescales, the melt transfer event(s), the last of which took place less than 2 years before the eruption, must have involved most of the magma that subsequently erupted. The data are consistent with a model in which prolonged generation, storage and segregation of silicic melts were followed by gravitational instability in the subcaldera pluton, causing the rapid interconnection and amalgamation of melt-rich domains. The melts then drained to the top of the pluton, at fluxes of up to 0.1–1 km³ year^??1, where steep vertical gradients of density and rheology probably caused them to inject laterally, forming a short-lived holding chamber prior to eruption. This interpretation is consistent with growing evidence that some large silicic magma chambers are transient features on geological timescales. A similar process preceded at least one earlier caldera-forming eruption on Santorini, suggesting that it may be a general feature of this rift-hosted magmatic system.     

Pan-African decompressional P-T path recorded by granulites from central Dronning Maud Land, Antarctica

A high-grade metamorphic complex is exposed in Filchnerfjella (6–8°E), central Dronning Maud Land. The metamorphic evolution of the complex has been recovered through a study of textural relationships, conventional geothermobarometry and pseudosection modelling. Relicts of an early, high-P assemblage are preserved within low-strain mafic pods. Subsequent granulite facies metamorphism resulted in formation of orthopyroxene in rocks of mafic, intermediate to felsic compositions, whereas spinel + quartz were part of the peak assemblage in pelitic gneisses. Peak conditions were attained at temperatures between 850–885 °C and 0.55–0.70 GPa. Reaction textures, including the replacement of amphibole and garnet by symplectites of orthopyroxene + plagioclase and partial replacement of garnet + sillimanite + spinel bearing assemblages by cordierite, indicate that the granulite facies metamorphism was accompanied and followed by decompression. The observed assemblages define a clock-wise P-T path including near-isothermal decompression. During decompression, localized melting led to formation of post-kinematic cordierite-melt assemblages, whereas mafic rocks contain melt patches with euhedral orthopyroxene. The granulite facies metamorphism, decompression and partial crustal melting occurred during the Cambrian Pan-African tectonothermal event.     

Amphibole perspective to unravel pre-eruptive processes and conditions in volcanic plumbing systems beneath intermediate arc volcanoes: a case study from Ciomadul volcano (SE Carpathians)

Ciomadul is the youngest volcano in the Carpathian–Pannonian region produced crystal-rich high-K dacites that contain abundant amphibole phenocrysts. The amphiboles in the studied dacites are characterized by large variety of zoning patterns, textures, and a wide range of compositions (e.g., 6.4–15 wt% Al₂O₃, 79–821 ppm Sr) often in thin-section scale and even in single crystals. Two amphibole populations were observed in the dacite: low-Al hornblendes represent a cold (<800 °C) silicic crystal mush, whereas the high-Al pargasites crystallized in a hot (>900 °C) mafic magma. Amphibole thermobarometry suggests that the silicic crystal mush was stored in an upper crustal storage (~8–12 km). This was also the place where the erupted dacitic magma was formed during the remobilization of upper crustal silicic crystal mush body by hot mafic magma indicated by simple-zoned and composite amphiboles. This includes reheating (by ~200 °C) and partial remelting of different parts of the crystal mush followed by intensive crystallization of the second mineral population (including pargasites). Breakdown textures of amphiboles imply that they were formed by reheating in case of hornblendes, suggesting that pre-eruptive heating and mixing could take place within days or weeks before the eruption. The decompression rim of pargasites suggests around 12 days of magma ascent in the conduit. Several arc volcanoes produce mixed intermediate magmas with similar bimodal amphibole cargo as the Ciomadul, but in our dacite the two amphibole population can be found even in a single crystal (composite amphiboles). Our study indicates that high-Al pargasites form as a second generation in these magmas after the mafic replenishment into a silicic capture zone; thus, they cannot unambiguously indicate a deeper mafic storage zone beneath these volcanoes. The simple-zoned and composite amphiboles provide direct evidence that significant compositional variations of amphiboles do not necessarily mean variation in the pressure of crystallization even if the Al-tschermak substitution can be recognized, suggesting that amphibole barometers that consider only amphibole composition may often yield unrealistic pressure variation.     

Ultrahigh-temperature metamorphism from an unusual corundum+orthopyroxene intergrowth bearing Al–Mg granulite from the Southern Marginal Zone,Limpopo Complex,South Africa

The Southern Marginal Zone of the Limpopo Complex is composed of granite-greenstone cratonic rocks reworked by a Neoarchean high-grade tectono-metamorphic event. Petrographic and mineral chemical characterization of an Al–Mg granulite from this zone is presented here. The granulite has a gneissic fabric with distinct Al-rich and Si-rich layers, with the former preserving the unusual lamellar (random and regular subparallel) intergrowths of corundum and symplectic intergrowth of spinel with orthopyroxene. The Al-rich layer preserves mineral assemblages such as rutile with orthopyroxene + sillimanite ± quartz, Al-rich orthopyroxene (~11 wt%), spinel + quartz, and corundum in possible equilibrium with quartz, while the Si-rich layer preserves antiperthites and orthopyroxene + sillimanite ± quartz, all considered diagnostic of ultrahigh-temperature metamorphism. Application of Al-in-opx thermometry, ternary feldspar thermometry and construction of suitable pressure–temperature phase diagrams, compositional and model proportion isopleth results indicate P–T conditions as high as ~1,050–1,100 °C, and ~10–12 kbars for the Al–Mg granulite. Our report of ultrahigh-temperature conditions is significant considering that the very high temperature was reached during decompression of an otherwise high-pressure granulite complex (clockwise P–T path), whereas most other ultrahigh-temperature granulites are linked to magma underplating at the base of the crust (counterclockwise P–T path).     

Storage conditions of Bezymianny Volcano parental magmas: results of phase equilibria experiments at 100 and 700 MPa

The crystallization sequence of a basaltic andesite from Bezymianny Volcano, Kamchatka, Russia, was simulated experimentally at 100 and 700 MPa at various water activities (aH₂O) to investigate the compositional evolution of residual liquids. The temperature (T) range of the experiments was 950–1,150 °C, aH₂O varied between 0.1 and 1, and the log of oxygen fugacity (fO₂) varied between quartz–fayalite–magnetite (QFM) and QFM + 4.1. The comparison of the experimentally produced liquids and natural samples was used to constrain the pressure (P)–T–aH₂O–fO₂ conditions of the Bezymianny parental magma in the intra-crustal magma plumbing system. The phase equilibria constraints suggest that parental basaltic andesite magmas should contain ~2–2.5 wt% H₂O; they can be stored in upper crustal levels at a depth of ~15 km, and at this depth they start to crystallize at ~1,110 °C. The subsequent chemical evolution of this parental magma most probably proceeded as decompressional crystallization occurred during magma ascent. The final depths at which crystallization products accumulated prior to eruption are not well constrained experimentally but should not be shallower than 3–4 km because amphibole is present in natural magmas (>150 MPa). Thus, the major volume of Bezymianny andesites was produced in a mid-crustal magma chamber as a result of decompressional crystallization of parental basaltic andesites, accompanied by mixing with silicic products from the earlier stages of magma fractionation. In addition, these processes are complicated by the release of volatiles due to magma degassing, which occurs at various stages during magma ascent.     

Mixing and differentiation in the Oruanui rhyolitic magma, Taupo, New Zealand: evidence from volatiles and trace elements in melt inclusions

Large pyroclastic rhyolites are snapshots of evolving magma bodies, and preserved in their eruptive pyroclasts is a record of evolution up to the time of eruption. Here we focus on the conditions and processes in the Oruanui magma that erupted at 26.5 ka from Taupo Volcano, New Zealand. The 530 km³ (void-free) of material erupted in the Oruanui event is comparable in size to the Bishop Tuff in California, but differs in that rhyolitic pumice and glass compositions, although variable, did not change systematically with eruption order. We measured the concentrations of H₂O, CO₂ and major and trace elements in zoned phenocrysts and melt inclusions from individual pumice clasts covering the range from early to late erupted units. We also used cathodoluminescence imaging to infer growth histories of quartz phenocrysts. For quartz-hosted inclusions, we studied both fully enclosed melt inclusions and reentrants (connecting to host melt through a small opening). The textures and compositions of inclusions and phenocrysts reflect complex pre-eruptive processes of incomplete assimilation/partial melting, crystallization differentiation, magma mixing and gas saturation. ‘Restitic’ quartz occurs in seven of eight pumice clasts studied. Variations in dissolved H₂O and CO₂ in quartz-hosted melt inclusions reflect gas saturation in the Oruanui magma and crystallization depths of ∼3.5–7 km. Based on variations of dissolved H₂O and CO₂ in reentrants, the amount of exsolved gas at the beginning of eruption increased with depth, corresponding to decreasing density with depth. Pre-eruptive mixing of magma with varying gas content implies variations in magma bulk density that would have driven convective mixing. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Evacuation of multiple magma bodies and the onset of caldera collapse in a supereruption,captured in glass and mineral compositions

Complexities in the nature of large-scale silicic eruptions and their magmatic systems can be discerned through micro-analytical geochemical studies. We present high-resolution, stratigraphically constrained compositional data on glassy matrix material and feldspar crystals from the initial fall deposits and earliest ignimbrite (base of member A) of the 2.08 Ma, ~?2500 km³ Huckleberry Ridge Tuff (HRT), Yellowstone. We use these data to document the nature of the magmatic system and compositional changes related to the transition from fall to widespread ignimbrite deposition, inferred to mark the onset of caldera collapse. Although major element glass compositions are relatively uniform, trace elements span a large range (e.g. Ba 10–900 ppm, Sr/Rb?=?0.005–0.09), with highly evolved glasses dominating in the fall deposits. Several trace elements (e.g. Ba and light rare earth elements) in the glass samples serve to define statistically significant compositional clustering in the fall deposits and basal ignimbrite. These clusters are inferred to reflect melt compositions controlled by fractional crystallisation processes and are interpreted to represent multiple, discrete melt-dominant domains that were tapped by multiple vents. The onset of widespread ignimbrite deposition is marked by an increase in the number of erupted melt compositional clusters from four in the fall deposits to eight, representing nine melt-dominant domains. There is an absence of geographical variation of glass compositions within the basal ignimbrite, with samples from proximal to distal localities north, west and south of the HRT caldera exhibiting similar variability. Pairing of glass analyses with sanidine major and minor element compositional data suggests that the nine melt compositional domains converged at depth into two compositionally distinct upper-crustal magmatic lineages that were both active during these early stages of the eruption. Our data collectively indicate the evacuation of an exceptionally complex and heterogeneous magma system. The onset of widespread ignimbrite deposition, inferred to relate to caldera collapse, occurred after ~ 50 km³ of magma had been discharged. Although external controls were important as an eruption trigger, depressurisation of the system led to caldera collapse with the eruption of numerous discrete melt-dominant domains.     

Quaternary acid magma in New Zealand

The voluminous Pleistocene—Recent Taupo rhyolites typically contain phenocrysts of plagioclase (oligoclase-labradorite), quartz, titanomagnetite, ilmenite, and ferromagnesian silicates. Ferromagnesian assemblages correlate with well defined Fe-Ti oxide equilibration temperature ranges and allow the rhyolites to be subdivided as follows: (1) Cummingtonite (c)—calcic hornblende (hb)—orthopyroxene (opx); 725–755°C, (2) Hb-opx, 750–825°C, (3) Biotite-hb-(c-opx), 720–765°C, (4) Opx-clinopyroxene (cpx), 860–915°C, (5) Fe olivine-opx-cpx, one sample with temperature of 900°C. Plagioclase and orthopyroxene phenocryst compositions typically exhibit a range of composition up to ～20 mol.%. Calculated average phenocryst equilibration pressures (P _total) range between 0.5–4.9 kb, and average 2.2 kb (～7–8 km depth), indicating upper crustal crystallization. These calculations are very sensitive to variations in phenocryst composition. Calculated \(/_{{\text{H}}_2 {\text{O}}} \) for the amphibole and biotite-bearing rhyolites indicate phenocryst equilibration under \(P_{{\text{H}}_2 {\text{O}}} \simeq P_{{\text{total}}} \) , with \(X_{{\text{H}}_2 {\text{O}}} \) ～0.17–0.24 (5–8 wt. %). The precipitation of cummingtonite is thus temperature dependent, the upper limit being close to 760°C. Eruptive mechanisms of the lavas, pumices, and ash-flow deposits are evidently not primarily controlled by temperature, P _total, \(P_{{\text{H}}_2 {\text{O}}} \) , or crystal content of the magmas, and explanations must lie in kinetic and fluid dynamic behavior of the magmas. For the Taupo rhyolites, it is suggested that the critical size of a magma body (i.e. Rayleigh number) is a controlling factor in that it will influence the convective regime; fully turbulent convection is deduced to have occurred within the larger magma bodies. One consequence is intense vesiculation, prior to eruption, within the uppermost zones of these magma chambers, and the voluminous pumice deposits are believed to emanate from such chambers. Oscillatory compositional zoning within pyroxene phenocrysts is consistent with magma convection.     

The Bishop Tuff giant magma body: an alternative to the Standard Model

The Bishop Tuff, one of the most extensively studied high-silica rhyolite bodies in the world, is usually considered as the archetypical example of a deposit formed from a magma body characterized by thermal and compositional vertical stratification—what we call the Standard Model for the Bishop magma body. We present here new geothermometry and geobarometry results derived using a large database of previously published quartz-hosted glass inclusion compositions. Assuming equilibrium between melt and an assemblage composed of quartz, ±plagioclase, ±sanidine, +zircon, ±fluid, we use Zr contents in glass inclusions to derive quartz crystallization temperatures, and we use (1) silica contents in glass, (2) projection of glass compositions onto the haplogranitic (quartz-albite-orthoclase) ternary, and (3) phase equilibria calculations using rhyolite-MELTS, to constrain crystallization pressures. We find crystallization temperatures of ~740–750 °C for all inclusions from both early- and late-erupted pumice. Crystallization pressures for both early- and late-erupted inclusions are also very similar to each other, with averages of ~175–200 MPa. We find no evidence of late-erupted inclusions having been entrapped at higher temperatures or pressures than early-erupted inclusions, as would be expected by the Standard Model. We argue that the thermal gradient inferred from Fe–Ti oxides—the backbone of the Standard Model—does not reflect equilibrium pre-eruptive conditions; we also note that H₂O–CO₂ systematics of glass inclusions yields overlapping pressure ranges for early- and late-erupted inclusions, similar to the results presented here; and we show that glass inclusion and phenocryst compositions show bimodal distributions, suggestive of compositional separation between early- and late-erupted populations. These findings are inconsistent with the Standard Model. The similarity in crystallization conditions and the compositional separation between early- and late-erupted magmas suggest that two laterally juxtaposed independent magma reservoirs existed in the same region at the same time and co-erupted to form the Long Valley Caldera and the Bishop Tuff. This hypothesis would explain the lack of mixing between early- and late-erupted crystal populations in pumice clasts; it could also explain the inferred eruption pattern—which resulted in early-erupted magmas being deposited only to the south of the caldera—if the early-erupted magma body resided to the south and the late-erupted magma body was located to the north. Our alternative model is consistent with the patchy distribution of thermal anomalies and the inference of co-eruption of distinct magma types in active volcanic areas such as the central Taupo Volcanic Zone.     

Rapid open-system assembly of a large silicic magma body: time-resolved evidence from cored plagioclase crystals in the Oruanui eruption deposits, New Zealand

Rhyolitic pumices in the 26.5 ka Oruanui eruption (Taupo volcano, New Zealand) contain an average of 10 wt% crystals. About 2 wt% of the crystal population is feldspar crystals that display bluish–grey cloudy cores, the colour being imparted by exsolved needles of rutile. The volume of cloudy-cored feldspars thus amounts to ~1.0 km³ in a total magma volume of ~530 km³. The cored feldspars show great variability in detail, but in general have a rounded cloudy core bounded by a zone rich in glass and mineral inclusions, that was then overgrown by a euhedral clear rim. Sr-isotopic variations in eight representative crystals were measured on micromilled samples of selected growth zones in the cores and rims, and linked to feldspar compositions through microprobe traverses. The cloudy cores range from ⁸⁷Sr/⁸⁶Sr = 0.70547 to 0.70657, with compositions of An₄₃ to An₇₈. The overgrowth rims display wider variations: inner parts show extreme ranges in composition (maxima ⁸⁷Sr/⁸⁶Sr = 0.70764 and An₇₈), while outer parts in seven of eight crystals are zoned, with outward-decreasing Sr-isotopic and An values to figures that are in accord with the bulk pumice and other, clear-feldspar values, respectively. The three parts of the crystals represent distinct regimes. The cloudy cores are inherited from an intermediate plutonic protolith that has been subjected to melting. The inner overgrowth rims were crystallised from a high temperature, relatively radiogenic melt derived from Mesozoic-Palaeozoic metasedimentary rocks (“greywacke”). The outer euhedral rims reflect mixing into and continued growth within the growing Oruanui magma body. The cloudy-cored feldspars also contain rare zircon inclusions. Twenty one zircons were recovered by HF digestion of a bulk sample of cloudy feldspars and analysed by SHRIMP for U–Th isotopes with which to calculate model ages. Eighteen of 21 crystals returned finite ages, the model-age spectrum of which is similar to the age spectra from free zircons in Oruanui pumices. Assembly of the Oruanui magma body was not only rapid (over ~40 kyr, as shown by other data) but involved a wide open system, with significant contributions from partly-melted intermediate-composition igneous intrusions (cloudy cores) and greywacke melts (inner overgrowths) being introduced into the magma body up to the point of eruption. Such open system behaviour contrasts with that proposed in models for comparably voluminous silicic magmas derived dominantly by fractionation (such as the Bishop Tuff) where the magma and its crystal cargo were better insulated thermally and chemically from country-rock interaction.     

Plagioclase nucleation and growth kinetics in a hydrous basaltic melt by decompression experiments

Isothermal single-step decompression experiments (at temperature of 1075 °C and pressure between 5 and 50 MPa) were used to study the crystallization kinetics of plagioclase in hydrous high-K basaltic melts as a function of pressure, effective undercooling (ΔT _eff) and time. Single-step decompression causes water exsolution and a consequent increase in the plagioclase liquidus, thus imposing an effective undercooling (?T _eff), accompanied by increased melt viscosity. Here, we show that the decompression process acts directly on viscosity and thermodynamic energy barriers (such as interfacial-free energy), controlling the nucleation process and favoring the formation of homogeneous nuclei also at high pressure (low effective undercoolings). In fact, this study shows that similar crystal number densities (N _a) can be obtained both at low and high pressure (between 5 and 50 MPa), whereas crystal growth processes are favored at low pressures (5–10 MPa). The main evidence of this study is that the crystallization of plagioclase in decompressed high-K basalts is more rapid than that in rhyolitic melts on similar timescales. The onset of the crystallization process during experiments was characterized by an initial nucleation event within the first hour of the experiment, which produced the largest amount of plagioclase. This nucleation event, at short experimental duration, can produce a dramatic change in crystal number density (N _a) and crystal fraction (?), triggering a significant textural evolution in only 1 h. In natural systems, this may affect the magma rheology and eruptive dynamics on very short time scales.     

Magma Generation at a Large, Hyperactive Silicic Volcano (Taupo, New Zealand) Revealed by U-Th and U-Pb Systematics in Zircons

Young (<65 ka) explosive silicic volcanism at Taupo volcano,New Zealand, has involved the development and evacuation ofseveral crustal magmatic systems. Up to and including the 26·5ka 530 km³ Oruanui eruption, magmatic systems were contemporaneousbut geographically separated. Subsequently they have been separatedin time and have vented from geographically overlapping areas.Single-crystal (secondary ionization mass spectrometry) andmultiple-crystal (thermal ionization mass spectrometry) zirconmodel-age data are presented from nine representative eruptiondeposits from 45 to 3·5 ka. Zircon yields vary by threeorders of magnitude, correlating with the degrees of zirconsaturation in the magmas, and influencing the spectra of modelages. Two adjacent magma systems active up to 26·5 kashow wholly contrasting model-age spectra. The smaller systemshows a simple unimodal distribution. The larger system, usingdata from three eruptions, shows bimodal model-age spectra.An older 100 ka peak is interpreted to represent zircons (antecrysts)derived from older silicic mush or plutonic rocks, and a youngerpeak to represent zircons (phenocrysts) that grew in the magmabody immediately prior to eruption. Post-26·5 ka magmabatches show contrasting age spectra, consistent with a mixtureof antecrysts, phenocrysts and, in two examples, xenocrystsfrom Quaternary plutonic and Mesozoic–Palaeozoic metasedimentaryrocks. The model-age spectra, coupled with zircon-dissolutionmodelling, highlight contrasts between short-term silicic magmageneration at Taupo, by bulk remobilization of crystal mushand assimilation of metasediment and/or silicic plutonic basementrocks, and the longer-term processes of fractionation from crustallycontaminated mafic melts. Contrasts between adjacent or successivemagma systems are attributed to differences in positions ofthe source and root zones within contrasting domains in thequartzo-feldspathic (<15 km deep) crust below the volcano. KEY WORDS: zircon; U-series dating; rhyolite; Taupo Volcanic Zone; Taupo volcano     

In situ major and trace element analysis of amphiboles in quartz monzodiorite porphyry from the Tonglvshan Cu–Fe (Au) deposit,Hubei Province,China: insights into magma evolution and related mineralization

The Tonglvshan deposit is the largest Cu–Fe (Au) skarn deposit in the Edong district, which is located in the westernmost part of the Middle and Lower Yangtze River metallogenic belt, China. In this study, we performed a detailed in situ analysis of major and trace elements in amphiboles from the ore-related Tonglvshan quartz monzodiorite porphyry using electron microprobe (EMPA) analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Two distinct populations of amphiboles, which can be distinguished by their aluminum content, are found in the quartz monzodiorite porphyry. The low-aluminum (Low-Al) amphiboles are subhedral or anhedral and formed at 46.3–73.5 MPa and 713–763 °C. In contrast, the high-aluminum (High-Al) amphiboles are euhedral and formed at 88–165 MPa and 778–854 °C. Some euhedral amphiboles are partially or completely replaced by Low-Al amphibole. The compositions of parental melts in equilibrium with the High-Al amphibole (Melt 1) and Low-Al amphibole (Melt 2) were computed by applying solid/liquid partition coefficients. This modeling shows that magma in equilibrium with High-Al amphibole (Melt 1) underwent 40% fractional crystallization of amphibole, plagioclase and apatite at a depth of ~5 km to evolve to magma in equilibrium with Low-Al amphibole (Melt 2). Copper enrichment occurred in the magma after undergoing fractional crystallization. The magma had a high oxygen fugacity, increasing from NNO + 1 (Melt 1) through NNO + 2 to HM (Melt 2), which could have prevented the loss of Cu (and possibly Au) to sulfide minerals during crystallization. Finally, the evolved magma intruded to shallower depths, where it presumably exsolved aqueous ore-forming fluids. Therefore, the large Cu–Fe–Au reserves of the Tonglvshan deposit can likely be attributed to a combination of controlling factors, including high oxygen fugacity, fractional crystallization, fluid exsolution, and a shallow emplacement depth.