Ascent-driven crystallisation of dacite magmas at Mount St Helens, 1980–1986

We introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system Qz-Ab-Or-H₂O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) conditions in the sub-volcanic magma system. By this means an important distinction can be made between ascent-driven and cooling-driven crystallisation under water-saturated or undersaturated conditions. In samples containing feldspar and a silica phase (quartz or tridymite), quantitative P-T estimates of the conditions of last equilibrium between crystals and melt can be made. Formation of highly silicic melts (i.e. >77 wt% SiO₂) is a simple consequence of the contraction of the silica phase volume with decreasing pressure, such that high silica glasses can only form by crystallisation at low pressure. Resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation. Groundmass and inclusion glasses in dacites from the 1980-1986 eruption of Mount St Helens volcano (WA) span a wide range in SiO₂ (68-80 wt%, anhydrous). The compositions of the least evolved (SiO₂-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with Е.4 wt% water, corresponding to a water saturation pressure of ~200 MPa at 900 °C. The compositions of more evolved (higher SiO₂) plagioclase-hosted inclusions and groundmass glasses are consistent with extensive ascent-driven fractional crystallisation of plagioclase, oxide and orthopyroxene phenocrysts and microlites to low pressures. During this polybaric crystallisation, plagioclase phenocrysts trapped melts with a wide range of dissolved water contents (3.5-5.7 wt%). Magmas erupted during the Plinian phase of the 18 May 1980 eruption were derived from a large reservoir at depths of ̈́ km. Subsequent magmas ascended to varying depths within the sub-volcanic system prior to extraction. From glass chemistry and groundmass texture two arrest levels have been identified, at depths of 0.5-1 and 2-4 km. A single dome sample from February 1983 contains groundmass plagioclase, tridymite and quartz, testifying to temperatures of at least 885 °C at 11 MPa. These shallow storage conditions are comparable to those in the cryptodome formed during spring 1980. The corresponding thermal gradient, А.2 °C MPa^-1, is consistent with near-adiabatic magma ascent from ~8 km. We argue that the crystallisation history of Mount St Helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation. This challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980-1986 eruption. Because the crystallisation process is both polybaric and fractional, it cannot be simulated directly using isobaric equilibrium crystallisation experiments. However, calculation of the phase proportions in water-saturated 910ᆣ °C experiments by Rutherford et al. (1985) over the pressure range 220-125 MPa reproduces the crystallisation sequence and phenocryst modes of Mount St Helens dacites from 18 May 1980. By allowing for the effects of fractional versus equilibrium crystallisation, entrained residual source material, and small temperature differences between nature and experiment, phase compositions can also be matched to the natural samples. We conclude that decompression of water-saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres.     

Petrological imaging of an active pluton beneath Cerro Uturuncu,Bolivia

Uturuncu is a dormant volcano in the Altiplano of SW Bolivia. A present day ~70 km diameter interferometric synthetic aperture radar (InSAR) anomaly roughly centred on Uturuncu’s edifice is believed to be a result of magma intrusion into an active crustal pluton. Past activity at the volcano, spanning 0.89 to 0.27 Ma, is exclusively effusive and almost all lavas and domes are dacitic with phenocrysts of plagioclase, orthopyroxene, biotite, ilmenite and Ti-magnetite plus or minus quartz, and microlites of plagioclase and orthopyroxene set in rhyolitic groundmass glass. Plagioclase-hosted melt inclusions (MI) are rhyolitic with major element compositions that are similar to groundmass glasses. H₂O concentrations plotted versus incompatible elements for individual samples describe a trend typical of near-isobaric, volatile-saturated crystallisation. At 870 °C, the average magma temperature calculated from Fe–Ti oxides, the average H₂O of 3.2 ± 0.7 wt% and CO₂ typically <160 ppm equate to MI trapping pressures of 50–120 MPa, approximately 2–4.5 km below surface. Such shallow storage precludes the role of dacite magma emplacement into pre-eruptive storage regions as being the cause of the observed InSAR anomaly. Storage pressures, whole-rock (WR) chemistry and phase assemblage are remarkably consistent across the eruptive history of the volcano, although magmatic temperatures calculated from Fe–Ti oxide geothermometry, zircon saturation thermometry using MI and orthopyroxene-melt thermometry range from 760 to 925 °C at NNO ± 1 log. This large temperature range is similar to that of saturation temperatures of observed phases in experimental data on Uturuncu dacites. The variation in calculated temperatures is attributed to piecemeal construction of the active pluton by successive inputs of new magma into a growing volume of plutonic mush. Fluctuating temperatures within the mush can account for sieve-textured cores and complex zoning in plagioclase phenocrysts, resorption of quartz and biotite phenocrysts and apatite microlites. That Fe–Ti oxide temperatures vary by ~50–100 °C in a single thin section indicates that magmas were not homogenised effectively prior to eruption. Phenocryst contents do not correlate with calculated magmatic temperatures, consistent with crystal entrainment from the mush during magma ascent and eruption. Microlites grew during ascent from the magma storage region. Variability in the proportion of microlites is attributed to differing ascent and effusion rates with faster rates in general for lavas >0.5 Ma compared to those <0.5 Ma. High microlite contents of domes indicate that effusion rates were probably slowest in dome-forming eruptions. Linear trends in WR major and trace element chemistries, highly variable, bimodal mineral compositions, and the presence of mafic enclaves in lavas demonstrate that intrusion of more mafic magmas into the evolving, shallow plutonic mush also occurred further amplifying local temperature fluctuations. Crystallisation and resorption of accessory phases, particularly ilmenite and apatite, can be detected in MI and groundmass glass trace element covariation trends, which are oblique to WRs. Marked variability of Ba, Sr and La in MI can be attributed to temperature-controlled, localised crystallisation of plagioclase, orthopyroxene and biotite within the evolving mush.     

Volcanic facies and mineral chemistry of Tertiary volcanics in the northern part of the Eastern Pontides,northeast Turkey: implications for pre-eruptive crystallization conditions and magma chamber processes

Tertiary volcanics in the northern zone of the Eastern Pontides are characterized by subaerial and shallow-subaqueous facieses, and are divided into three volcanic suites: Eocene aged (1) basalt-trachybasalt-basaltic trachyandesite (BTB) and (2) trachyte-trachyandesite (TT), and Miocene aged (3) basanite-tephrite (BT) suites. Clinopyroxene is a common phase in all three volcanic suites, and has different compositions with Mg# varying from 0.57 to 0.91 in BTB suite and 0.57–0.84 in TT suite to 0.65–0.90 in BT suite. Feldspars in all suites generally exhibit wide range of compositions from sanidine to albite or anorthite and have weak normal and reverse compositional zoning. Olivines in BTB and BT suites have Fo_60–92. Hornblendes in BTB, TT and BT suites are commonly magnesio-hastingsite and rare pargasite in composition (Mg#: 0.67–0.90). Brown mica is mainly phlogopite with Mg# ranging from 0.56 to 0.92 in the BTB suite, 0.59–0.84 in the TT suite, and 0.75–0.93 in the BT suite. Analcime is present only in the BT suite rocks. Fe–Ti oxides in all suites are mainly composed of magnetite and titanomagnetite. Textural petrographic and mineral chemical data suggest that magmas had undergone hydrous and anhydrous crystallizations in deep-, mid-, and shallow-crustal magma chambers. Clinopyroxene thermobarometric calculations show that Eocene magma chambers were characterized by temperature ranging from 1,100 to 1,244 °C and pressure ranging from 1.84 to 5.39 kbar. Similarly, the Miocene magma chambers were characterized by temperature ranging from 1,119 to 1,146 °C and pressure ranging from 4.23 to 4.93 kbar. Hornblende thermobarometry, oxygen fugacity, and hygrometer reveal that the crystallization temperature of Eocene volcanics range from 956 to 959 °C at pressure ranging from 6.49 to 6.52 kbar. Eocene volcanics were characterized by water content ranging from 7.83 to 8.57 wt.% and oxygen fugacity of 10^?9.36 to 10^?9.46 (ΔNNO+2). Miocene volcanics had crystallization temperature ranging from 970 to 978 °C at pressure ranging from 8.70 to 9.00 kbar with water content ranging from 8.04 to 8.64 wt.% and oxygen fugacity ranging from 10^?8.75 to 10^?8.87 (ΔNNO+2). Brown mica thermobarometric data show that Eocene volcanics were characterized by relatively high oxygen fugacity varying from 10^-10.32 to 10^-12.37 (HM) at temperature ranging from 858 to 953 °C and pressure ranging from 1.08 to 1.41 kbar. Miocene volcanics were crystallized at highly oxidized conditions, which are characterized by high oxygen fugacity of 10^?12.0 (HM) at temperature of 875 °C and pressure of 2.09 kbar. The wide range of obtained temperatures for clinopyroxenes of the suites denotes that the equilibration of clinopyroxene crystals initiates from depth until close to the surface before magma eruption. The compositional variations, resorbed core and reverse zoning patterns in clinopyroxene phenocrysts, as well as variable pressures of crystallization, further indicate that the magmas that formed the suites were polybaric in origins and were composite products of more than one petrogenetic stage. The observed range of phenocryst assemblage and different compositional trends possibly originated from fractionation of magmas with different initial water contents under variable pressures of crystallization. The repeated occurrence of magmas from different suites during a single period of activity suggests that the magmatic system consists of several conduit systems and that magma reservoirs are dispersed at different levels of crustal magma chambers.     

Crystal reaming during the assembly, maturation, and waning of an eleven-million-year crustal magma cycle: thermobarometry of the Aucanquilcha Volcanic Cluster

Phenocryst assemblages of lavas from the long-lived Aucanquilcha Volcanic Cluster (AVC) have been probed to assess pressure and temperature conditions of pre-eruptive arc magmas. Andesite to dacite lavas of the AVC erupted throughout an 11-million-year, arc magmatic cycle in the central Andes in northern Chile. Phases targeted for thermobarometry include amphibole, plagioclase, pyroxenes, and Fe–Ti oxides. Overall, crystallization is documented over 1–7.5 kbar (~25 km) of pressure and ~680–1,110 °C of temperature. Pressure estimates range from ~1 to 5 kbar for amphiboles and from ~3 to 7.5 kbar for pyroxenes. Pyroxene temperatures are tightly clustered from ~1,000–1,100 °C, Fe–Ti oxide temperatures range from ~750–1,000 °C, and amphibole temperatures range from ~780–1,050 °C. Although slightly higher, these temperatures correspond well with previously published zircon temperatures ranging from ~670–900 °C. Two different Fe–Ti oxide thermometers (Andersen and Lindsley 1985; Ghiorso and Evans 2008) are compared and agree well. We also compare amphibole and amphibole–plagioclase thermobarometers (Ridolfi et al. 2010; Holland and Blundy 1994; Anderson and Smith 1995), the solutions from which do not agree well. In samples where we employ multiple thermometers, pyroxene temperature estimates are always highest, zircon temperature estimates are lowest, and Fe–Ti oxide and amphibole temperature estimates fall in between. Maximum Fe–Ti oxide and zircon temperatures are observed during the middle stage of AVC activity (~5–3 Ma), a time associated with increased eruption rates. Amphibole temperatures during this time are relatively restricted (~850–1,000 °C). The crystal record presented here offers a time-transgressive view of an evolving, multi-tiered subvolcanic reservoir. Some crystals in AVC lavas are likely to be true phenocrysts, but the diversity of crystallization temperatures and pressures recorded by phases in individual AVC lavas suggests erupting magma extensively reams and accumulates crystals from disparate levels of the middle to upper crust.     

Magmatic Conditions and Magma Ascent as Indicated by Hornblende Phase Equilibria and Reactions in the 1995-2002 Soufriere Hills Magma

A study of amphiboles and associated minerals in samples ofSoufrière Hills andesite erupted from 1995 to 2002 showssignificant compositional variations within hornblende phenocrysts,a separate set of small pargasitic crystals in the groundmass,and two types of reaction rims on the phenocrysts. The compositionof the amphiboles and coexisting phases defines the thermalhistory of the erupting magma. As many as seven zones (<200µm wide) in the hornblende phenocrysts begin with a sharpincrease in Mg and Si, and then change gradually to a more Fe-and Al-rich hornblende, a transition that is consistent witha temperature rise. Analyses of the hornblende phenocrysts andassociated Fe–Ti oxides verify previous conclusions thatthe pre-eruption magma was at 130 MPa and 830 ± 10°C,but was variably heated before eruption. The heating occurredwithin     

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.     

Mineral chemical constraints on the petrogenesis of mafic and intermediate volcanic rocks from the Erciyes and Hasandağ volcanoes,Central Turkey

Hasandağ and Erciyes stratovolcanoes, which produced both calc-alkaline and alkaline eruptive products, are the two important volcanic complexes in Central Anatolia. There are three geochemical evolution stages in the history of the Hasandağ strato volcanic complex: (1) Keçikalesi tholeiitic, (2) Hasandağ calc-alkaline and (3) Hasandağ alkaline. Volcanologic and petrologic characteristics of the Hasandağ and Erciyes calc-alkaline series show that water played an important role on the genesis of these rocks. These rocks are phenocryst-rich with vesicular texture, and contain hydrous mineral phases. The approximate pressure and temperature estimates obtained from the mineral chemistry studies of the Hasandağ strato volcanic complex indicate crystallization temperature of 1100 °C with 2.5–3.4 kbar pressure interval for the first stage of Keçikalesi tholeiitic volcanism, and about 850 °C temperatures with 4.3–9.6 kbar pressure intervals for the second stage of Hasandağ calc-alkaline volcanism.The geochemical evolution of Erciyes volcanic complex also exhibits three distinct evolutionary stages: (1) Koçdağ alkaline, (2) Koçdağ calc-alkaline and (3) Erciyes calc-alkaline. The temperature of Koçdağ alkaline volcanism is 1097–1181 °C and in a range of 5.1–6.7 kbar pressure, for Koçdağ calc-alkaline volcanism 850–1050 °C temperature to 2.0–6.6 kbar pressure interval, and for Erciyes calc-alkaline volcanism about 950 °C temperature, to 3.2–7.9 kbar pressure intervals were calculated. Polybaric origin of magma chambers for calc-alkaline and alkaline rocks and disequilibrium parameters observed in phenocrysts indicate that the rocks were affected by magma mixing processes in crustal magma chambers. The disequilibrium features of amphibole and plagioclase phenocrysts in these rocks point the latent heat in magma chambers and periodic recharging with mafic magma chambers and also show that magmas reequilibrate before the eruption.     

A new method to quantify the real supply of mafic components to a hybrid andesite

The eruption of Soufrière Hills Volcano, Montserrat, has been ongoing since 1995. The volcano is erupting a crystal-rich hornblende-plagioclase andesite with ubiquitous mafic inclusions, indicating mixing with mafic magma. This mafic magma is thought to be the driving force of the eruption, supplying heat and volatiles to the andesite resident in the magma chamber. As well as producing macroscopic mafic inclusions, the magma mixing process involves incorporation of phenocrysts from the andesite into the mafic magma. These inherited phenocrysts show clear disequilibrium textures (e.g. sieved plagioclase rims and thermal breakdown rims on hornblende). Approximately 25 % of all phenocrysts in the andesite show these textures, indicating very extensive mass transfer between the two magma types. Fragments of mafic inclusions down to sub-mm scale are found in the andesite, together with mafic crystal clusters, which are commonly found adhered to the rims of phenocrysts with disequilibrium features. Mineral chemistry also points to the transfer of microlites or microphenocrysts, initially formed in the mafic inclusions, into the andesite. This combined evidence suggests that some of the mafic inclusions disaggregate during mingling and/or ascent, possibly due to shearing, and raises the question: What proportion of the andesite ‘groundmass’ actually originated in the mafic inclusions, and thus, what is the true amount of mafic magma in the magmatic system? We present a new method for quantifying the relative proportions of groundmass plagioclase derived from mafic and andesitic magma, based on analysis of back-scattered electron images of the groundmass. Preliminary results indicate that approximately 16 % of all groundmass plagioclase belongs genetically to the mafic inclusions. Together with the crystal clusters, disequilibrium phenocryst textures and mm-scale inclusions, there is a ‘cryptic’ mafic component in the andesite of approximately 6 % by volume. This is significant compared with the proportion of macroscopic mafic inclusions (typically ~ 1–5 %). The new method has the potential to allow tracking of the mafic fraction through time and thus to yield further insights into magma hybridisation processes.     

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.     

Small volume andesite magmas and melt–mush interactions at Ruapehu, New Zealand: evidence from melt inclusions

Historical eruptions from Mt. Ruapehu (New Zealand) have been small (<0.001 km³ of juvenile magma) and have often occurred without significant warning. Developing better modelling tools requires an improved understanding of the magma storage and transport system beneath the volcano. Towards that end, we have analysed the volatile content and major element chemistry of groundmass glass and phenocryst-hosted melt inclusions in erupted samples from 1945 to 1996. We find that during this time period, magma has been stored at depths of ~2–9 km, consistent with inferences from geophysical data. Our data also show that Ruapehu magmas are relatively H₂O-poor (<2 wt%) and CO₂-rich (≤1,000 ppm) compared to typical arc andesites. Surprisingly, we find that melt inclusions are often more evolved than their transporting melt (as inferred from groundmass glass compositions). Furthermore, even eruptions that are separated by less than 2 years exhibit distinct major element chemistry, which suggests that each eruption involved magma with a unique ascent history. From these data, we infer that individual melt batches rise through, and interact with, crystal mush zones formed by antecedent magmas. From this perspective, we envision the magmatic system at Ruapehu as frequently recharged by small magma inputs that, in turn, cool and crystallise to varying degrees. Melts that are able to erupt through this network of crystal mush entrain (to a greater or lesser extent) exotic crystals. In the extreme case (such as the 1996 eruption), the resulting scoria contain melt inclusion-bearing crystals that are exotic to the transporting magma. Finally, we suggest that complex interactions between recharge and antecedent magmas are probably common, but that the small volumes and short time scales of recharge at Ruapehu provide a unique window into these processes.     

Geochemical and isotopic insights into the assembly,evolution and disruption of a magmatic plumbing system before and after a cataclysmic caldera-collapse eruption at Ischia volcano (Italy)

New geochemical and isotopic data on volcanic rocks spanning the period ~75–50 ka BP on Ischia volcano, Italy, shed light on the evolution of the magmatic system before and after the catastrophic, caldera-forming Monte Epomeo Green Tuff (MEGT) eruption. Volcanic activity during this period was influenced by a large, composite and differentiating magmatic system, replenished several times with isotopically distinct magmas of deep provenance. Chemical and isotopic variations highlight that the pre-MEGT eruptions were fed by trachytic/phonolitic magmas from an isotopically zoned reservoir that were poorly enriched in radiogenic Sr and became progressively less radiogenic with time. Just prior to the MEGT eruption, the magmatic system was recharged by an isotopically distinct magma, relatively more enriched in radiogenic Sr with respect to the previously erupted magmas. This second magma initially fed several SubPlinian explosive eruptions and later supplied the climactic, phonolitic-to-trachytic MEGT eruption(s). Isotopic data, together with erupted volume estimations obtained for MEGT eruption(s), indicate that >5–10 km³ of this relatively enriched magma had accumulated in the Ischia plumbing system. Geochemical modelling indicates that it accumulated at shallow depths (4–6 km), over a period of ca. 20 ka. After the MEGT eruption, volcanic activity was fed by a new batch of less differentiated (trachyte-latite) magma that was slightly less enriched in radiogenic Sr. The geochemical and Sr–Nd-isotopic variations through time reflect the upward flux of isotopically distinct magma batches, variably contaminated by Hercynian crust at 8–12 km depth. The deep-sourced latitic to trachytic magmas stalled at shallow depths (4–6 km depth), differentiated to phonolite through crystal fractionation and assimilation of a feldspar-rich mush, or ascended directly to the surface and erupted.     

Composition and conditions of formation of the parental melts of Jurassic dolerites of southwestern Crimea: Evidence from melt inclusions in olivine phenocrysts

This study focuses on Jurassic shallow intrusions and subvolcanic bodies from around Trudolyubovka village in the southwestern Crimea. All the rocks are similar in mineral composition and have similar geochemical features and occur in close spatial and geological association. This allows us to assign the intrusions to a single magmatic series and interpret them as differentiation products of a single parental melt. The investigation of melt inclusions in olivine from the most magnesian sample showed that the composition of igneous melts ranged from basalt to basaltic andesite of a moderately potassic subalkaline affinity. Compared with N-MORB, they are enriched in LILE, but have similar HFSE and REE contents. The early magmatic melts crystallized at temperatures ranging from 1240 to 1125°C, pressures of 6–8 kbar, and an oxygen fugacity of ΔQFM = +0.6; and later melts crystallized at 1090–940°C, ~1.5 kbar, and oxygen fugacity increasing from ΔQFM + 0.9 to ΔQFM + 2.3. The minimum pressure of groundmass crystallization was estimated as 40–60 bar. The primitive melts were formed in a mature island arc or an active continental margin setting by ~13% melting of a DMM-like source. The melting occurred at spinel-facies depths under the influence of a slab-derived fluid at a temperature 25°C below the dry peridotite solidus.     

Petrogenesis of Mafic Inclusions in Rhyolitic Lavas from Narugo Volcano, Northeastern Japan

Mafic inclusions present in the rhyolitic lavas of Narugo volcano,Japan, are vesiculated andesites with diktytaxitic texturesmainly composed of quenched acicular plagioclase, pyroxenes,and interstitial glass. When the mafic magma was incorporatedinto the silica-rich host magma, the cores of pyroxenes andplagioclase began to crystallize (>1000°C) in a boundarylayer between the mafic and felsic magmas. Phenocryst rim compositionsand interstitial glass compositions (average 78 wt % SiO₂) inthe mafic inclusions are the same as those of the phenocrystsand groundmass glass in the host rhyolite. This suggests thatthe host felsic melt infiltrated into the incompletely solidifiedmafic inclusion, and that the interstitial melt compositionin the inclusions became close to that of the host melt (c.850°C). Infiltration was enhanced by the vesiculation ofthe mafic magma. Finally, hybridized and density-reduced portionsof the mafic magma floated up from the boundary layer into thehost rhyolite. We conclude that the ascent of mafic magma triggeredthe eruption of the host rhyolitic magma. KEY WORDS: mafic inclusion; stratified magma chamber; magma mixing; mingling; Narugo volcano; Japan     

Lamprophyres of the Tomtor Massif: A result of mixing between potassic and sodic alkaline mafic magmas

The paper presents data on inclusions in minerals of the least modified potassic lamprophyres in a series of strongly carbonatized potassic alkaline ultramafic porphyritic rocks. The rocks consist of diopside, kaersutite, analcime, apatite, and rare phlogopite and titanite phenocrysts and a groundmass, which is made up, along with these minerals, of potassic feldspar and calcite. The diopside and kaersutite phenocrysts display unsystematic multiple zoning. Chemically and mineralogically, the rock is ultramafic foidite and most likely corresponds to monchiquite. Primary and secondary melt inclusions were found in diopside, kaersutite, apatite, and titanite phenocrysts and are classified into three types: sodic silicate inclusions with analcime, potassic silicate inclusions with potassic feldspar, and carbonate inclusions, which are dominated by calcite. Heating and homogenization of the inclusions show that the potassic lamprophyres crystallized from a heterogeneous magma, with consisted of mixing mafic sodic and potassic alkaline magmas enriched in a carbonatite component. The composition of the magmas was close to nepheline and leucite melanephelinite. The minerals crystallized at 1150–1090°C from the sodic melts and at 1200–1250°C from the potassic ones. The sodic mafic melts were richer in Fe than the potassic ones, were the richest in Al, Mn, SO₃, Cl, and H₂O and poorer in Ti and P. The potassic mafic melts were not lamproitic, as follows from the presence of albite in the crystallized primary potassic melt inclusions. The diopside, the first mineral to crystallize in the rock, started to crystallize in the magmatic chamber from sodic mafic melt and ended to crystallize from mixed sodic–potassic melts. The potassic mafic melts were multiply replenished in the chamber in relation to tectonic motions. The ascent of the melts to the surface and rapidly varying P–T parameters of the magma were favorable for multiple separations of carbonatite melts from the alkaline mafic ones and their mixing and mingling.     

Petrology of mafic enclaves in the 2006–2012 eruptive products of Bezymianny Volcano,Kamchatka

This paper reports the results of the first comprehensive petrological study of mafic enclaves widespread in the products of recent (2006–2012) eruptions of Bezymianny Volcano, Kamchatka. Four types of mafic enclaves were distinguished on the basis of the composition and morphology of minerals, P–T conditions of formation of mineral assemblages, and structural and textural characteristics of the rocks. Disequilibrium assemblages of mafic enclaves indicate a complex structure of the magmatic plumbing system of the volcano, including a shallow chamber with andesite–basaltic andesite magmas and a deep reservoir filled in part with plagioclase–hornblende cumulates and fed by basic magmas with mantle harzburgite xenoliths. The mafic enclaves were formed at different levels of the magmatic plumbing system of the volcano and correspond to different degrees of mixing of interacting magmas. The most abundant enclaves were formed during magma ascent from the deep reservoir (960–1040°C, 5–9 kbar) into the shallow andesitic chamber (940–980°C). Enclaves of plagioclase–hornblende cumulates from the basic magmas feeding the deep reservoir (T > 1090°C and P > 9 kbar) are much less common.     

Chemical composition of rock-forming minerals and crystallization physicochemical conditions of the Middle Eocene I-type Haji Abad pluton,SW Buin-Zahra,Iran

The Haji Abad intrusion is a well-exposed Middle Eocene I-type granodioritc pluton in the Urumieh–Dokhtar magmatic assemblage (UDMA). The major constituents of the investigated rocks are K-feldspar, quartz, plagioclase, pyroxene, and minor Fe–Ti oxide and hornblende. The plagioclase compositions fall in the labradorite, andesine, and oligoclase fields. The amphiboles range in composition from magnesio-hornblende to tremolite–hornblende of the calcic-amphibole group. Most pyroxenes principally plot in the field of diopside. The calculated average pressure of emplacement is 1.9 kbar for the granodioritic rocks, crystallizing at depths of about 6.7 km. The highest pressure estimated from clinopyroxene geobarometry (5 kbar) reflects initial pyroxene crystallization pressure, indicating initial crystallization depth (17.5 km) in the Haji Abad granodiorite. The estimated temperatures using two-feldspar thermometry give an average 724 °C. The calculated average temperature for clinopyroxene crystallization is 1090 °C. The pyroxene temperatures are higher than the estimated temperature by feldspar thermometry, indicating that the pyroxene and feldspar temperatures represent the first and late stages of magmatic crystallization of Haji Abad granodiorite, respectively. Most pyroxenes plot above the line of Fe³⁺?=?0, indicating they crystallized under relatively high oxygen fugacity or oxidized conditions. Furthermore, the results show that the Middle Eocene granitoids crystallized from magmas with H₂O content about 3.2 wt%. The relatively high water content is consistent with the generation environment of HAG rocks in an active continental margin and has allowed the magma to reach shallower crustal levels. The MMEs with ellipsoidal and spherical shapes show igneous microgranular textures and chilled margins, probably indicating the presence of magma mixing. Besides, core to rim compositional oscillations (An and FeO) for the plagioclase crystals serve as robust evidence to support magma mixing. The studied amphiboles and pyroxenes are grouped in the subalkaline fields that are consistent with crystallization from I-type calc-alkaine magma in the subduction environment related to active continental margin. Mineral chemistry data indicate that Haji Abad granodiorites were generated in an orogenic belt related to the volcanic arc setting consistent with the subduction of Neo-Tethyan oceanic crust beneath the central Iranian microcontinent.     

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.     

Peralkaline felsic magmatism at the Nemrut volcano,Turkey: impact of volcanism on the evolution of Lake Van (Anatolia) IV

Nemrut volcano, adjacent to Lake Van (Turkey), is one of the most important peralkaline silicic centres in the world, where magmatism for ~570,000 years has been dominated by peralkaline trachytes and rhyolites. Using onshore and Lake Van drill site tephra samples, we document the phenocryst and glass matrix compositions, confirming a complete spectrum from very rare mafic to dominantly silicic magmas. Magma mixing has been common and, along with the multi-lineage nature of the magmas, indicates that Nemrut has been a very open system where, nevertheless, compositionally zoned caps developed during periods of relative eruptive quiescence. Geothermometry suggests that the intermediate-silicic magmas evolved in an upper crustal magma reservoir at temperatures between 1100 and 750 °C, at fO₂ close to the FMQ buffer. The silicic magmas either were halogen poor or exsolved a halogen-rich phase prior to or during eruption. An unusual Pb-rich phase, with up to 98.78 wt% PbO, is interpreted as having exsolved from the intermediate-rhyolitic magmas.     

Melt Inclusions in Olivine Phenocrysts: Using Diffusive Re-equilibration to Determine the Cooling History of a Crystal, with Implications for the Origin of Olivine-phyric Volcanic Rocks

A technique is described for determining the cooling historyof olivine phenocrysts. The technique is based on the analysisof the diffusive re-equilibration of melt inclusions trappedby olivine phenocrysts during crystallization. The mechanismof re-equilibration involves diffusion of Fe from and Mg intothe initial volume of the inclusion. The technique applies toa single crystal, and thus the cooling history of differentphenocrysts in a single erupted magma can be established. Weshow that melt inclusions in high-Fo olivine phenocrysts frommantle-derived magmas are typically partially re-equilibratedwith their hosts at temperatures below trapping. Our analysisdemonstrates that at a reasonable combination of factors suchas (1) cooling interval before eruption (<350°C), (2)eruption temperatures (>1000°C), and (3) inclusion size(<70 µm in radius), partial re-equilibration of upto 85% occurs within 3–5 months, corresponding to coolingrates faster than 1–2°/day. Short residence timesof high-Fo phenocrysts suggest that if eruption does not happenwithin a few months after a primitive magma begins cooling andcrystallization, olivines that crystallize from it are unlikelyto be erupted as phenocrysts. This can be explained by efficientseparation of olivine crystals from the melt, and their rapidincorporation into the cumulate layer of the chamber. Theseresults also suggest that in most cases erupted high-Fo olivinephenocrysts retain their original composition, and thus compositionsof melt inclusions in erupted high-Fo olivine phenocrysts donot suffer changes that cannot be reversed. Short residencetimes also imply that large unzoned cores of high-Fo phenocrystscannot reflect diffusive re-equilibration of originally zonedphenocrysts. The unzoned cores are a result of fast efficientaccumulation of olivines from the crystallizing magma, i.e.olivines are separated from the magma faster than melt changesits composition. Thus, the main source of high-Fo crystals inthe erupted magmas is the cumulate layers of the magmatic system.In other words, olivine-phyric rocks represent mixtures of anevolved transporting magma (which forms the groundmass of therock) with crystals that were formed during crystallizationof more primitive melt(s). Unlike high-Fo olivine phenocrysts,the evolved magma may reside in the magmatic system for a longtime. This reconciles long magma residence times estimated fromthe compositions of rocks with short residence times of high-Foolivine phenocrysts. KEY WORDS: melt inclusions; olivine; picrites; residence time; diffusion     

Contrasting compositional trends of rocks and olivine-hosted melt inclusions from Cerro Negro volcano (Central America): implications for decompression-driven fractionation of hydrous magmas

Melt inclusions in olivine Fo_83–72 from tephras of 1867, 1971 and 1992 eruptions of Cerro Negro volcano represent a series of basaltic to andesitic melts of narrow range of MgO (5.6–8 wt %) formed by ~46 wt % fractional crystallization of olivine (~6 wt %), plagioclase (~27 wt %), pyroxene (~13 wt %) and magnetite (<1 wt %) from primitive basaltic melt (average SiO₂ = 49 wt %, MgO = 7.6 wt %, H₂O = 6 wt %) as it ascended to the surface from the depth of about 14 km. The crystallization occurred at increasing liquidus temperature from 1,050 to 1,090 °C in the pressure range from 400 to 50 MPa and was induced by release of mixed H₂O–CO₂ fluid from the melt at decreasing pressure. Matrix glass compositions fall at the high-Si end of the melt inclusion trend and represent the final stage of melt crystallization during and after eruption. The bulk compositions of erupted Cerro Negro magmas (tephras and lavas) range from high- to low-MgO (3–10 wt %) basalts, which form a compositional array crossing the trend of melt inclusions so that virtually no rock from Cerro Negro has composition akin to true melt represented by the inclusions. The variations of the bulk magma (rocks) and melt (melt inclusions) compositions can be generated in a dyke connecting a deep primitive magma reservoir with the Cerro Negro edifice. While the melt inclusions represent the compositional trend of instantaneous melts along the magma pathway at decreasing pressure and H₂O content, occurrence of low-Mg to high-Mg basalts reflects the process of phenocryst re-distribution in progressively evolving melt. The crystallization scenario is anticipated to operate everywhere in dykes feeding basaltic volcanoes and can explain the predominance of plagioclase-rich high-Al basalts in island arc as well as typical compositional variations of magmas during single eruptions.