Phenocryst Assemblages Research Articles

Impact of volcanism on the evolution of Lake Van I: evolution of explosive volcanism of Nemrut Volcano (eastern Anatolia) during the past >400,000 years

The historically active Nemrut Volcano (2,948 m asl) (Eastern Anatolia), rising close to the western shore of huge alkaline Lake Van, has been the source of intense Plinian eruptions for >530,000 years (drilled lake sediments). About 40 widespread, newly recognized trachytic and less common rhyolitic fallout tephras and ca. 12 interbedded ignimbrites, sourced in Nemrut Volcano, are documented in stratigraphic traverses throughout an area of >6,000 km2 mostly west of Lake Van. Phenocrysts in the moderately peralkaline trachytes and rarer large-volume comenditic rhyolites comprise anorthoclase, hedenbergite-augite, fayalite and, especially in trachytic units, augite, minor aenigmatite, apatite and quartz, and rare chevkinite and zircon. Dacitic to rhyolitic tephras from nearby calcalkalic Suphan Volcano (4,058 m asl), locally interbedded with Nemrut tephras, are characterized by disequilibrium phenocryst assemblages (biotite, augitic clinopyroxene and hypersthene, minor olivine, common crystal clots and/or, in some deposits, amphibole). The magma volume (DRE) of the largest Nemrut tephra sheet (AP-1) described in detail may exceed 30 km3. Extreme facies and systematic compositional changes are documented in the ca. 30 ka Nemrut Formation (NF) deposits formed from one large and complex eruption (thick rhyolitic fallout overlain by ignimbrite, welded agglutinate, overbank surge deposits, and final more mafic fallout deposits). Common evidence of magma mixing in Nemrut ignimbrites reflects eruption from compositionally zoned magma reservoirs. Several young Cekmece Formation trachytes overlying ca. 30 ka old NF deposits and the late trachytes of the NF deposits show compositional affinities to tephra from Suphan Volcano possibly due to temporary influx of Suphan magmas into the Nemrut system following the evacuation of >10 km3 magma (DRE) during the caldera-forming NF eruption. Axes of large fallout fans are dominantly SW–NE but W–E in the younger sheets resembling the direction of the present dominant wind field. Growth of Nemrut volcanic edifice and its peripheral domes since before 0.5 Ma in the hinge area between the Van and Mus tectonic basins is likely to have been the major factor in isolating Lake Van basin thus initiating the origin and subsequent alkaline evolution of the lake. This alkalinity was later significantly controlled by climate forcing. Internal forcing mechanisms (volcanic and geodynamic) may also have contributed to major lake level changes in addition to climate forcing.

Storage and eruption of near-liquidus rhyolite magma at Cordón Caulle, Chile

The last three eruptions at the Cordon Caulle volcanic complex, Chile, have been strikingly similar in that they have started with relatively short pre-eruptive warning and produced chemically homogeneous rhyolite to rhyodacite magma with glassy to aphyric texture. These characteristics collectively call for an understanding of the storage conditions leading to the rise and extraction of crystal-poor silicic magma from volcanoes. We have analyzed and experimentally reproduced the mineral assemblage and glass chemistry in rhyolite magma produced in the most recent eruption of Cordon Caulle, and we use these to infer magma storage and ascent conditions. Fe–Ti oxide mineral geothermometry suggests that the rhyolite was stored at ∼870–920 °C. At these temperatures, the phenocryst assemblage (plag∼An37 > cpx + opx > mag + ilm) can be reproduced under H2O-saturated conditions of between 100 and 50 MPa, corresponding to crustal depths between about 2.5 and 5.0 km. The shallow and relatively hot magma storage conditions have implications for the rapid onset, degassing efficiency, and progression from explosive to mixed pyroclastic-effusive eruption style at Cordon Caulle.

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.

Textural control on the weathering of basic igneous rocks: A micromorphological approach

The Puno minettes (Eastern Andean Cordillera, southern Peru) are phlogopite–phyric rocks with a groundmass consisting mainly of phlogopite, alkali feldspar and apatite microphenocrysts and/or microlites. They display the strong large-ion-lithophile-element enrichments that characterize potassic to ultrapotassic rocks of the lamprophyre clan (Rb, Ba, Sr up to 103× Primordial Mantle, REE up to 6.102× CI-chondrites). In addition to their close association with granitoids, the rocks of this series show many features of minettes occurring in arc-related collisional zones, i.e., absence of mantle xenoliths, up to 30 vol.% felsic xenocrysts and strong negative Nb and Ti anomalies. However, they are distinguished by unusual phenocryst phases, whole-rock compositions and groundmass mineralogy. Some minettes contain only a few per cent of biotite±quartz±plagioclase xenocrysts immersed in a typical minette groundmass whereas those richer in xenocrysts display the complete liquidus paragenesis of the coexisting peraluminous granite (biotite+quartz+plagioclase+alkali feldspar+cordierite+zircon+pyrite) in a high-K latitic to trachydacitic groundmass. Field relations, radiometric ages, whole-rock data and felsic xenocrysts all indicate that the Puno minettes result from a mixing process involving Al-rich lamproites and peraluminous monzogranites. The most common phenocryst assemblage (highly magnesian phlogopite+olivine±titanian Cr-spinel±Ca-rich enstatite±magnesian ilmenite±Al- and Ti-poor diopside) is similar to that reported in Al-rich phlogopite lamproites from southeastern Spain. Likewise, the host rocks have diagnostic features of Spanish phlogopite lamproites (e.g., K2O/Al2O3 ratios >9, CaO contents <2 wt.%, negative Ba and Sr anomalies in primitive mantle-normalized multi-element plots, LaN/NdN<1.5). Graphite in the groundmass and Cr-spinel and ilmenite compositions devoid of Fe3+ suggest strongly reduced conditions of formation. Another phenocryst assemblage (Ca–Al-rich enstatite+Ca-deficient highly magnesian augite+magnesian phlogopite±ilmenite±discrete olivine) is observed in one sample which derives from an Al-rich leucite lamproite.

Geochemistry, mineralogy and petrology of the Eocene potassic magmatism from the Milk River area, southern Alberta, and Sweet Grass Hills, northern Montana

New whole-rock geochemical, quantitative mineralogical and electron-microprobe data are presented from lamproite, olivine minettes and mafic trachyte of the Milk River area, southern Alberta, and latites, trachytes and high-K rhyolite of the Sweet Grass Hills, northern Montana. These rocks comprise dikes, small plugs and a pipe complex associated with mainly syenite and hornblende monzonite plutons that core several buttes in the Sweet Grass Hills and mark the northern extent of the Cenozoic Montana Alkaline Province. Minerals restricted to peralkaline lamproite at the 49th Parallel locality include Ti-rich phlogopite–annite with Fe3+ in tetrahedral coordination, Na-poor sanidine, Ba-rich and Cl-poor fluorapatite, K-rich titano-ferronyböite/ferro-obertiite, LREE-rich strontian fluorapatite (SrO=17.5–25.2wt.%, Ce2O3=2.9–3.3wt.%), ilmenite, strontian barite, barian celestine, and unidentified Zr–Ti phase (TiO2=76.7–88.0wt.%, ZrO2=2.1–7.2wt.%) perhaps replacing primary Zr–Ti silicates. The strontian fluorapatite has the highest SrO content thus far reported in the literature for apatites from lamproites. Olivine minettes and mafic trachyte of the Milk River area contain augite, Al-diopside, Ti-rich magnetite, sanidine with higher Na, hyalophane, Cl-bearing fluorapatite with negligible Ba, ferroan magnesite phenocrysts, and primary calcite, all of which are notably absent in the 49th Parallel lamproite. Mica displays distinct core-to-rim zoning trends in these rocks, characterized by increasing FeOt and TiO2 at decreasing Al2O3 in lamproite and increasing FeOt and TiO2 at increasing Al2O3 in minettes and trachytes. The 49th Parallel lamproite has higher Ti, K, P, Ba, Rb, Sr, Nb, Th, U, Zr, Hf and REE concentrations, higher K/Al, (Ce/Yb)CN, (La/Sm)CN, (Gd/Yb)CN, Rb/Cs, Ce/Pb, Th/Ta, Th/Yb, and lower Ba/La, Ba/Th and Nb/U relative to minettes of the Milk River area. Major-element, least-squares mass balance and trace-element models call for the origin of latites, trachytes and high-K rhyolite in the Sweet Grass Hills complex by two-stage fractional crystallization of primitive olivine minette magmas concurrent with negligible, if any, assimilation of crustal materials. Oscillatory and reverse zoning in phenocrysts and hybrid phenocryst assemblages record recharge of more evolved magma chambers with primitive minette magmas. The calculated evolutionary models are inconsistent with the compositional variations displayed by the olivine minettes and lamproite of the Milk River area. These rocks represent distinct low-degree partial melts of sub-lithospheric mantle enriched in K and volatiles by the recent (<100Ma) plume activity or convective upwelling of the upper mantle above subducted Farallon slab, which underwent variable assimilation of the ancient, LREE-Ba–Sr enriched and HFSE depleted Wyoming–Medicine Hat lithosphere during the post-Laramide extension. The olivine minettes derived from more oxidized, carbonated region of the mantle source, compared to the more reduced source of the 49th Parallel lamproite. The extremely negative εNd values, unradiogenic Pb and moderately radiogenic Sr isotopic compositions, correlated with LREE, Ba enrichment and relative Nb, Ta, Ti and other HFSE depletions of the Cenozoic K-rich rocks of the MAP, may reflect carbonatite metasomatism of the lithospheric mantle associated with the widespread Mesoproterozoic (1.5–1.4Ga) anorogenic magmatism and intracontinental rifting related to mantle upwelling and plume activity beneath a thick Precambrian supercontinent.

Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study

We experimentally investigated the phase rela- tions of a peralkaline phonolitic dyke rock associated with the Ilimaussaq plutonic complex (South Greenland). The extremely evolved and iron-rich composition (magnesium number = 2, alkalinity index = 1.44, FeO* = 12 wt%) may represent the parental magma of the Ilimaussaq com- plex. This dyke rock is therefore perfectly suited for per- forming phase-equilibrium experiments, since in contrast to the plutonic rocks of the complex, no major cumulate for- mation processes complicate defining a reasonable starting composition. Experiments were carried out in hydrothermal rapid-quench cold-seal pressure vessels at P = 100 MPa and T = 950-750 C. H2O contents ranging from anhy- drous to H2O saturated (*5 wt% H2O) and varying fO2 (*DlogFMQ - 3t o?1; where FMQ represents the fayalite-magnetite-quartz oxygen buffer) were applied. Reduced and dry conditions lead to substantial crystalliza- tion of alkali feldspar, nepheline, hedenbergite-rich clino- pyroxene, fayalite-rich olivine and minor amounts of ulvospinel-rich magnetite, which represent the phenocryst assemblage of the natural dyke rock. Oxidized and H2O-rich conditions, however, suppress the crystallization of olivine in favor of magnetite and clinopyroxene with less or no alkali feldspar and nepheline formation. Accordingly, combined low fO2 and aH2O force the evolution of the residual melt toward decreasing SiO2, increasing FeO* and alkalinity index (up to 3.55). On the contrary, high fO2 and aH2O produce residual melts with relatively low FeO*, high SiO2 and a relatively constant alkalinity index. We show that variations of aH2O and fO2 lead to contrasting trends regarding the liquid lines of descent of iron-rich silica- undersaturated peralkaline compositions. Moreover, the increase in FeO* and alkalinity index (reduced and dry conditions) in the residual melt is an important prerequisite to stabilize late-magmatic minerals of the dyke rock, for example, aenigmatite (Na2Fe5TiSi6O20), coexisting with the most evolved melts at 750 C. Contrary to what might be expected, experiments with high aH2O and interlinked high fO2 exhibit higher liquidus T's compared with exper- iments performed at low aH2O and fO2 for experiments where magnetite is liquidus phase. This is because ulvospinel-poor magnetite crystallizes at higher fO2 and has a higher melting point than ulvospinel-rich magnetite, which is favored at lower fO2.

Composition and origin of rhyolite melt intersected by drilling in the Krafla geothermal field, Iceland

The Iceland Deep Drilling Project Well 1 was designed as a 4- to 5-km-deep exploration well with the goal of intercepting supercritical hydrothermal fluids in the Krafla geothermal field, Iceland. The well unexpectedly drilled into a high-silica (76.5 % SiO2) rhyolite melt at approximately 2.1 km. Some of the melt vesiculated while extruding into the drill hole, but most of the recovered cuttings are quenched sparsely phyric, vesicle-poor glass. The phenocryst assemblage is comprised of titanomagne- tite, plagioclase, augite, and pigeonite. Compositional zoning in plagioclase and exsolution lamellae in augite and pigeonite record changing crystallization conditions as the melt migrated to its present depth of emplacement. The in situ temperature of the melt is estimated to be between 850 and 920 C based on two-pyroxene geothermometry and modeling of the crystallization sequence. Volatile content of the glass indicated partial degassing at an in situ pressure that is above hydrostatic (*16 MPa) and below lithostatic (*55 MPa). The major element and minor element com- position of the melt are consistent with an origin by partial melting of hydrothermally altered basaltic crust at depth, similar to rhyolite erupted within the Krafla Caldera.

Mineralogy, conditions of crystallization and melt generation of epidote-bearing porphyries from the Middle Urals, Russian federation

Epidote-bearing porphyritic dikes (whole rock analysis: SiO2 = 55–65 wt. %, MgO 17 wt. %, Na2O + K2O = 5.7–9.4 wt. %) situated in the continental margin zone, the Middle Urals, Russian Federation have been dated using SHRIMP U-Pb zircon techniques and give a Middle Devonian age of 388 ± 2 Ma and 389 ± 6 Ma. The porphyries contain phenocrysts of magmatic epidote (Ps = 17–25 %), Ca- and Mn-rich (CaO >9 wt. %; MnO >6 wt. %) almandine garnet, Al-rich (Al2O3 = 12–16 wt. %) amphibole, titanite, plagioclase, biotite, muscovite, apatite, and quartz. 60 to 70 % groundmass of the porphyritic dikes consists of fine-grained albite, quartz, and K-feldspar. A variety of thermobarometric estimations, plus comparison with published experimental data indicate that the phenocryst assemblage was stable between 5 and 11 kbar and 690 to 800 °C. Oxygen fugacity was close to or greater than logfo2 = Ni-NiO + 1. Later stage formation of the quartz-feldspar groundmass took place at hypabyssal conditions, corresponding to 1 to 2 kbar and 660 to 690 °C. The porphyritic dikes are metaluminous to slightly peraluminous (ACNK = 0.7–1.17). They are enriched in REE and depleted Nb and Ti. They show features typical of subduction-related magmas. Chemical composition and isotopic ratios of 86Sr/87Sri = 0.709–0.720 suggest that both mantle- and deep crustal-derived materials were involved in their petrogenesis.

Peralkaline magma evolution and the tephra record in the Ethiopian Rift

The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75–76 wt% SiO2) in comparison with the pumice (64–75 wt% SiO2), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50–60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P2O5 highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50–60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO2 values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal–proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit.

Magma Evolution in the Primitive, Intra-oceanic Tonga Arc: Petrogenesis of Basaltic Andesites at Tofua Volcano

Tofua volcano is situated midway along the Tonga oceanic arc and has undergone two phases of ignimbrite-forming activity. The eruptive products are almost entirely basaltic andesites (52·5–57 wt % SiO2) with the exception of a volumetrically minor pre-caldera dacite. The suite displays a strong tholeiitic trend with K2O <1 wt %. Phenocryst assemblages typically comprise plagioclase + clinopyroxene ± orthopyroxene with microlites of Ti-magnetite. Olivine (Fo83–88) is rare and believed to be dominantly antecrystic. An increase in the extent and frequency of reverse zoning in phenocrysts, sieve-textured plagioclase and the occurrence of antecrystic phases in post-caldera lavas record a shift to dynamic conditions, allowing the interaction of magma batches that were previously distinct. Pyroxene thermobarometry suggests crystallization at 950–1200°C and 0·8–1·8 kbar. Volatile measurements of glassy melt inclusions indicate a maximum H2O content of 4·16 wt % H2O, and CO2–H2O saturation curves indicate that crystallization occurred at two levels, at depths of 4–5·5 km and 1·5–2·5 km. Major and trace element models suggest that the compositions of the majority of the samples represent a differentiation trend whereby the dacite was produced by 65% fractional crystallization of the most primitive basaltic andesite. Trace element models suggest that the sub-arc mantle source is the residuum of depleted Indian mid-ocean ridge basalt mantle (IDMM-1% melt), whereas radiogenic isotope data imply addition of 0·2% average Tongan sediment melt and a fluid component derived from the subducted altered Pacific oceanic crust. A horizontal array on the U–Th equiline diagram and Ra excesses of up to 500% suggest fluid addition to the mantle wedge within the last few thousand years. Time-integrated (226Ra/230Th) vs Sr/Th and Ba/Th fractionation models imply differentiation timescales of up to 4500 years for the dacitic magma compositions at Tofua.

Magma storage conditions beneath Dabbahu Volcano (Ethiopia) constrained by petrology, seismicity and satellite geodesy

A variety of methods exist to constrain sub-volcanic storage conditions of magmas. Petrological, seismological and satellite geodetic methods are integrated to determine storage conditions of peralkaline magmas beneath Dabbahu Volcano, Afar, Ethiopia. Secondary ion mass spectrometry (SIMS) analysis of volatile contents in melt inclusions trapped within phenocrysts of alkali feldspar, clinopyroxene and olivine from pantellerite obsidians representing the youngest eruptive phase (<8 ka) show H2O contents ≤5.8 wt.% and CO2 contents generally below 500 ppm, although rarely as high as 1,500 ppm. Volatile saturation pressures (at 679–835°C) are in the range 43–207 MPa, consistent with published experimental data for similar pantellerites, which show that the phenocryst assemblage of alkali feldspar + cpx + aenigmatite ± ilmenite is stable at 100 to 150 MPa. Inferred magma storage depths for these historic eruptions are ~1–5 km below sea-level, consistent with the depths of earthquakes, associated with magma chamber deflation following a dyke intrusion in the period Oct 2005–Apr 2006. Interferometric synthetic aperture radar (InSAR) data for the same period reveal a broad ~20 km diameter area of uplift. Modelling of different geometries reveals that a series of stacked sills over a 1–5 km depth range best matches the InSAR data. The consistency of depth estimates based on petrological study of ancient eruptions and the seismicity, inflation and deflation of Dabbahu observed in relation to the dyking event of 2005, suggest a small but vertically extensive and potentially long-lived magma storage region.

Petrology and Geochemistry of Adakitic Dacites and High-MgO Andesites, and Related Calc-alkaline Dacites from the Miocene Okoppe Volcanic Field, N Hokkaido, Japan

In the Okoppe area of North Hokkaido, Japan, the eruption of adakitic and calc-alkaline dacites was followed by high-Mg andesite (HMA) and calc-alkaline dacite during the Middle Miocene (12–10 Ma). Adakitic dacite is characterized by high Sr/Y and low Y, with Sr and Nd isotopic compositions identical to those of mid-ocean ridge basalt. It has higher MgO contents than adakites generated by experimental melting of metabasalt and amphibolite, and higher Ni and Cr contents than either Archean trondhjemite–tonalite–granodiorite or Early Cretaceous adakitic granites, which are considered to represent partial melts of subducted oceanic crust. This provides compelling evidence that adakitic dacite magma from Okoppe resulted from interaction of a melt derived from subducted oceanic basaltic crust and the overlying mantle wedge peridotite, with little modification to the adakitic melt signature and Sr and Nd isotopic values. The compositional variations in the Toyono adakitic dacite and associated calc-alkaline dacite probably resulted from mixing of the reacted magma and an evolved silicic dacite magma formed by fractional crystallization of the reacted magma. A disequilibrium phenocryst assemblage in the HMA may result from mixing of boninite and silicic andesite that resulted from crustal melting. Calc-alkaline dacites associated with the HMA were derived by fractional crystallization of silicic andesite and assimilation of crust with an enriched Sr isotopic signature. The most likely tectono-magmatic model for the production of adakitic dacite and HMA involves upwelling of hot asthenosphere into the subcontinental lithosphere beneath North Hokkaido and the back-arc side of the NE Japan arc, coincident with the spreading of the Kurile back-arc basin and Japan Sea back-arc basin. This resulted in a high geothermal gradient in the mantle wedge beneath North Hokkaido. The subsequent melting of a limited part of the cool oceanic crust subducting beneath Hokkaido produced adakitic magmas, which interacted with the overlying mantle wedge peridotite. These magmas subsequently reacted with an evolved calc-alkaline melt en route to the surface. Boninitic magma derived from the ascending hot asthenosphere in part reacted with crust-derived silicic andesitic magma, undergoing simultaneous fractional crystallization.

Andesites of the 2009 eruption of Redoubt Volcano, Alaska

Crystal-rich andesites that erupted from Redoubt Volcano in 2009 range from 57.5 to 62.5wt.% SiO2 and have phenocryst and phenocryst-melt relations consistent with staging in the upper crust. Early explosive products are low-silica andesites (LSA, <58wt.% SiO2) that ascended from deeper crustal levels during or before the 6months of precursory activity, but a broad subsequent succession to more evolved and cooler products, and predominantly effusive dome growth, are interpreted to result from progressive mobilization and mixing with differentiated magmas tapped from pre-2009 Redoubt intrusions at ~3–6km depth. Initial explosions on March 23–28 ejected predominantly LSA with a uniform phenocryst assemblage of high-Al amphibole, ~An70 plagioclase, ortho- and clinopyroxene, FeTi oxides (890 to 960°C), and traces of magmatic sulfide. Melt in the dominant microlite-poor LSA was compositionally uniform dacite (67–68wt.% SiO2) but ranged to rhyolite with greater microlite growth. Minor amounts of intermediate- to high-silica andesite (ISA, HSA; 59–62.5wt.% SiO2) also erupted during the early explosions and most carried rhyolitic melt (72–74wt.% SiO2). A lava dome grew following the initial tephra-producing events but was destroyed by an explosion on April 4. Ejecta from the April 4 explosion consists entirely of ISA and HSA, as does a subsequent lava dome that grew April 4–July 1; LSA was absent. Andesites from the April 4 event and from the final dome had pre-eruptive temperatures of 725–840°C (FeTi oxides) and highly evolved matrix liquids (77–80wt.% SiO2), including in rare microlite-free pyroclasts. ISA has mixed populations of phenocrysts suggesting it is a hybrid between HSA and LSA. The last lavas from the 2009 eruption, effused May 1–July 1, are distinctly depleted in P2O5, consistent with low temperatures and high degrees of crystallization including apatite.Plagioclase–melt hygrometry and comparison to phase equilibrium experiments are consistent with pre-eruptive storage of all three magma types at 100–160MPa (4–6km depth), if they were close to H2O-saturation, coincident with the locus of shallow syn-eruptive seismicity. Deeper storage would be indicated if the magmas were CO2-rich. Relatively coarse-grained clinopyroxene-rich reaction rims on many LSA amphibole phenocrysts may result from slow ascent to, or storage at, depths shallow enough for the onset of appreciable H2O exsolution, consistent with pre-eruptive staging in the uppermost crust. We interpret that the 2009 LSA ascended from depth during the 8 or more months prior to the first eruption, but that the magma stalled and accumulated in the upper crust where its phenocryst rim and melt compositions were established. Ascent of LSA through stagnant mushy intrusions residual from earlier Redoubt activity mobilized differentiated magma pockets and interstitial liquids represented by HSA, and as LSA–HSA hybrids represented by ISA, that fed the subsequently erupted lava domes.

Petrology of the Coyaguayma ignimbrite, northern Puna of Argentina: Origin and evolution of a peraluminous high-SiO2 rhyolite magma

The Coyaguayma ignimbrite is a strongly peraluminous (SP), sillimanite, garnet-bearing silicic rhyolite which erupted in the northern Puna segment of the central Andean plateau in the Upper Miocene (~11Ma), a period that was characterized by the eruption of voluminous (100s to 1000s km3) dacitic ignimbrites of high-K calc-alkaline affinity. In this region, the SP magmatic rocks are both rare and small, but their importance is potentially much greater as rocks of this type are usually interpreted as products of crustal melting and therefore useful for addressing mantle addition vs crustal recycling in the central Andes.The phenocryst assemblage of the Coyaguayma ignimbrite comprises plagioclase (An26-18), quartz, Ba-rich and Ba-poor sanidine, minor Al-rich ferromagnesian minerals (AlT- and AlVI-rich biotite, almandine–spessartine garnet) and sillimanite, as well as accessory zircon and monazite. Textural relations suggest that the accessory and ferromagnesian phases crystallized before quartz and feldspars. Mineral equilibria suggest that crystallization of the rhyolite magma began at ~5kbar and 800°C, and continued almost isobarically to 720°C, causing the residual liquid to increase H2O contents from ~4–5% to ~7.5% before eruption.Most major features (e.g., high SiO2, A/CNK >1.3, low CaO, MgO, TiO2 and FeO) and trace element patterns (low Ba, Sr, Th, LREE and Eu/Eu*; high Rb, U, Y and Nb), along with the thermobarometric constraints on magmatic P, T and water contents are consistent with an origin by mica dehydration melting of metapelitic sources (e.g., typical biotite-muscovite gneisses from the outcropping S Puna basement). However, the relatively low initial 87Sr/86Sr (~0.7125), and high 143Nd/144Nd (~0.512200) ratios invalidate a pure crustal origin. Instead, we propose contamination of calc-alkaline dacitic magmas similar to typical Puna ignimbrites by metapelite at mid-crustal settings (≥18km depth). Geochemical modeling that satisfies the chemical and isotopic data suggests mixing of 70% dacite and ~30% of metapelite partial melts, followed by extensive (70%) fractionation of plagioclase, K-feldspar, quartz and biotite, with minor magnetite and apatite from the hybrid magma. Sillimanite in the Coyaguayma rhyolite is interpreted as a restite mineral or a product of incongruent melting of the metapelite, which was preserved intact in the hybrid melt due to local equilibrium.This petrogenetic model explains most characteristics of crystal-poor SP rhyolites from the Puna plateau (e.g., Tocomar, Ramadas rhyolites) and it may be more generally applicable to occurrences of SP magmas in Andean-type continental arcs dominated by calc-alkaline magmatism.

Quenched mafic inclusions in ≤2200 years B.P. deposits at Augustine Volcano, Alaska

This article describes results from morphological, textural, mineralogical, and compositional analyses of lava clasts and quenched mafic inclusions from 10 Holocene debris-avalanche deposits (2200–125 years B.P.) that form the lower flanks of Augustine Volcano, Alaska. Mafic inclusions collected from the Rocky Point pyroclastic-flow deposit emplaced during the 2006 eruption are included for comparison. All deposits contain evidence for mixing between basalt and high-silica andesite prior to and during eruption, including compositional banding, disequilibria phenocryst assemblages, linear variation in major- and trace-element concentrations, and quenched mafic inclusions (51.3–57.3 wt.% SiO2) hosted by andesite lavas (59.1–62.6 wt.% SiO2). Generally, inclusions from all deposits share many morphologic and petrologic characteristics. Inclusions range in diameter from <1 to >36 cm, although 87% of the 959 inclusions analysed are less than 5 cm in diameter. Mafic inclusions account for an average of 8 vol.% of host andesitic lava clasts included in the oldest debris-avalanche deposits compared with subsequently emplaced deposits that contain an average of only 1–3 vol.%. Inclusions contain phenocrysts of plagioclase, amphibole, clinopyroxene, olivine, and rare orthopyroxene as well as microphenocrysts of plagioclase, amphibole, clinopyroxene, olivine, magnetite, ilmenite, and apatite in a glassy, vesicular, and acicular groundmass. Plagioclase phenocrysts in inclusions typically have a 30–150 μm-thick, fine-grained ‘dusty sieved’ rim superimposed over oscillatory zoned texture, suggesting that they existed in the host andesite magma prior to basaltic intrusion, but were engulfed during the intrusion and inclusion formation process. Mafic inclusions are calc-alkaline, low-K (0.45–0.82 wt.% K2O) basalts to basaltic andesites that are altogether different in terms of mineralogy and composition from olivine basalt contained in late Pleistocene-age fragmental deposits described by Plank et al. (2006, The Augustine basalt: Eos (American Geophysical Union Transactions), v. 87, V42B-06) and Larsen et al. (2010, Petrology and geochemistry of the 2006 eruption of Augustine Volcano, in Power, J.A., Coombs, M.L., and Freymueller, J.T., eds., The 2006 eruption of Augustine Volcano, Alaska: US Geological Survey Professional Paper 1769). Thus, the most reliable approximation of the basaltic endmember composition involved in magma mixing processes at Augustine Volcano over the past ∼2200 years are the most mafic inclusions, which consistently possess the (1) highest abundance of vesicles, (2) fewest number of phenocryst-sized plagioclase, and (3) largest volume of microlites compared with more contaminated inclusions.

Temporal Evolution of a High-K Andesitic Magmatic System: Taranaki Volcano, New Zealand

Taranaki (Mt. Egmont) in the western North Island of New Zealand is a high-K andesite volcano with an eruptive history extending over more than 200 kyr. In general, petrological research has concentrated on the post-10 ka record of the modern edifice. This study focuses on the earlier history, which is recorded in 11 major pre-7 ka debris avalanche deposits. Each of these formed as a result of a catastrophic collapse of the edifice of the time. The clast assemblages of these deposits provide insights into the chemical compositions of magmas erupted during the earlier stages of activity of the volcano and form the basis for a new chemo-stratigraphic analysis of the pre-10 ka volcanic succession. Sample suites from the studied debris avalanche deposits show a progressive enrichment in K2O and large ion lithophile elements (LILE), reflecting a gradual evolution to high-K andesite. The early magmatic system (pre-100 ka) produced a wide range of compositions including relatively primitive basalts and basaltic andesites.These rocks contain phenocryst assemblages that indicate crystallization within the lower crust or mantle, including a broad range of clinopyroxene compositions, high-Al2O3 hornblende, olivine and phlogopite. A higher proportion of high-silica compositions in the younger sample suites and the appearance of late-stage, low-pressure mineral phases, such as high-TiO2 hornblende, biotite and Fe-rich orthopyroxene, reflect a gradual shift to more evolved magmas with time.These new data are interpreted to reflect a multi-stage origin for Taranaki andesites. Parental magmas were generated within a lower crustal ‘hot zone’, which formed as a result of repeated intrusions of primitive melts into the lower crust. The geochemical and mineralogical evidence indicates that prior to 100 ka this zone was relatively thin and cold, so that primitive magmas were able to rise rapidly through the crust without significant interaction and modification. As the hot zone evolved, larger proportions of intruded and underplated mafic material were partially remelted, and interaction of these melts with fractionating mantle-derived magmas generated progressively more K- and LILE-enriched compositions. A complex and dispersed magma assembly and storage system developed in the upper crust where the hot-zone melts were further modified by fractional crystallization and magma mixing and mingling.

Mineral stability in peralkaline silicic rocks: Information from trachytes of the Menengai volcano, Kenya

Electron microprobe analyses are presented for phenocrysts and matrix glass in peralkaline, silica-oversaturated trachytes from the Menengai volcano, Kenya Rift Valley. The dominant phenocryst assemblage is alkali feldspar–hedenbergite–fayalite–titanomagnetite–apatite. Aenigmatite, amphibole and quartz occur rarely in more peralkaline rocks. QUILF calculations indicate that the trachytic magmas crystallised at temperatures of 854–870°C, at relatively low oxidation states (ΔFMQ −1.6 to −1.7) and silica activity (aSiO2 (Qtz) of 0.60–0.66). The new analyses are used, along with published data, to outline the distribution of the main phases in the compositional spectrum of peralkaline quartz trachytes and rhyolites. There is uncertainty about the nature, or even existence, of a low-temperature zone in the alkali feldspar primary phase region, the equivalent of the thermal valley in the haplogranite system. Quartz phenocrysts may appear early or late in the crystallisation sequence, even in rocks of similar bulk composition, its appearance perhaps being a function of the F content of the melts. Whereas hedenbergite and fayalite show fairly systematic compositional trends with increasing host rock peralkalinity, amphibole compositions are variable, for reasons not yet understood. Aenigmatite crystallisation is at least partly controlled by oxygen fugacity and silica activity. With rare exceptions, ilmenite and titanomagnetite are incompatible phases but the factors controlling their relative stabilities are not clear. It appears that peralkaline trachyte–rhyolite sequences evolve along many crystallisation paths, the paths perhaps being strongly influenced by pH2O, pF2, melt F/Cl ratios and perhaps total pressure.

Mineral Chemistry and Petrography of Lava Successions From Kepsut-Dursunbey Volcanic Field, NW Turkey: Implications For Magmatic Processes and Crystallization Conditions

Kepsut-Dursunbey volcanic field (KDVF) is located in NW Turkey and contains various products of the post-collisional Neogene magmatic activity. Two distinct volcanic suites have been recognized; the Kepsut volcanic suite (KVS) and the Dursunbey volcanic suite (DVS). The KVS includes basaltic trachyandesite- basaltic andesite-andesite lavas and associated pyroclastic rocks. The DVS consists of dacite-rhyodacite lavas and extensive pumice-ash fall and flow deposits. Petrographical features (i.e. existence of xenocrysts, glomerocrysts, and mixing-compatible textures) and mineral chemistry of phenocryst assemblages of both suites provide evidence for magma mixing/AFC. Calculated crystallization pressures and temperatures give values of 5.7-7.0 kbar and 927-982 °C for the KVS and 3.7-5.3 kbar and 783-787°C for the DVS, indicating separate magma reservoirs and crystallization in magma chambers at deep and mid crustal levels, respectively. These observations support the establishment and evolution of KDVF magma system promoted by episodic basaltic inputs which may generate and mix with crustal melts.

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.

The petrology, geochronology and geochemistry of Hauhungatahi volcano, S.W. Taupo Volcanic Zone

Hauhungatahi volcano is an eroded andesitic edifice 10 km west of Ruapehu volcano constructed on an upfaulted block of Mesozoic marine sediments. Hauhungatahi andesites are distinctively clinopyroxene-phyric with high ratios of clinopyroxene:plagioclase. This contrasts with the plagioclase-phyric dominated assemblages in the stratovolcanoes such as Ruapehu and Tongariro. Hauhungatahi andesites show high MgO (> 8.0% wt), Sr (> 400 ppm), Ni (> 100 ppm) and Cr (> 400 ppm) and low Si, Rb, Ba and Zr relative to andesites from Ruapehu volcano, with fractionated LREE (Ce/Sm) n ~ 2 and flat HREE (Dy/Yb) n ~ 1. Sr-isotope ratios are lower at equivalent Nd isotope ratios than Ruapehu andesites of the (oldest) Te Herenga Formation. 40Ar/ 39Ar step-heating experiments of groundmass concentrates of 4 samples have yielded ages between 881 ± 83 ka and 961 ± 59 ka, with a weighted mean age of 933 ± 46 ka, indicating that Hauhungatahi is significantly older than Ruapehu Volcano where volcanism commenced ~ 250–300 ka. Hauhungatahi is therefore similar in age to the older andesitic edifices that are marginal to the TVZ (eg Titiraupenga, Pureora and Rolles Peak). We suggest that these high-Mg andesites hold clues to the early conditioning of the lithosphere beneath TVZ, prior to the establishment of the large andesite stratovolcanoes that presently dominate the skyline at the southern end of TVZ. The relatively low abundance of plagioclase in the phenocryst assemblages hints at high P H 2O and possible involvement of amphibole in the source.