Showing papers on "Phenocryst published in 2005"

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

Abstract: Young (<65 ka) explosive silicic volcanism at Taupo volcano, New Zealand, has involved the development and evacuation of several crustal magmatic systems. Up to and including the 26·5 ka 530 km3 Oruanui eruption, magmatic systems were contemporaneous but geographically separated. Subsequently they have been separated in time and have vented from geographically overlapping areas. Single-crystal (secondary ionization mass spectrometry) and multiple-crystal (thermal ionization mass spectrometry) zircon model-age data are presented from nine representative eruption deposits from 45 to 3·5 ka. Zircon yields vary by three orders of magnitude, correlating with the degrees of zircon saturation in the magmas, and influencing the spectra of model ages. Two adjacent magma systems active up to 26·5 ka show wholly contrasting model-age spectra. The smaller system shows a simple unimodal distribution. The larger system, using data 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 younger peak to represent zircons (phenocrysts) that grew in the magma body immediately prior to eruption. Post-26·5 ka magma batches show contrasting age spectra, consistent with a mixture of antecrysts, phenocrysts and, in two examples, xenocrysts from Quaternary plutonic and Mesozoic–Palaeozoic metasedimentary rocks. The model-age spectra, coupled with zircon-dissolution modelling, highlight contrasts between short-term silicic magma generation at Taupo, by bulk remobilization of crystal mush and assimilation of metasediment and/or silicic plutonic basement rocks, and the longer-term processes of fractionation from crustally contaminated mafic melts. Contrasts between adjacent or successive magma systems are attributed to differences in positions of the source and root zones within contrasting domains in the quartzo-feldspathic (<15 km deep) crust below the volcano.

Rapid decompression-driven crystallization recorded by melt inclusions from Mount St Helens volcano

Abstract: Crystals in hydrous magmas can form in response to falling temperature (magma cooling) or degassing (magma decompression). It remains unclear which process dominates beneath explosive silicic volcanoes. Because decompression and cooling operate on very different time scales, resolving the driving force behind crystallization is of fundamental importance for determining magma dynamics and eruption hazard. Here we use ion-microprobe measurements of dissolved H 2 O in phenocryst-hosted melt inclusions from pumices erupted between May and October 1980 at Mount St. Helens volcano to show that all microlites and a significant proportion of phenocrysts were formed by near isothermal decompression. Magmas erupted after 18 May show evidence for subsequent crystallization of both phenocrysts and microlites, indicating that the time scales of crystal nucleation and growth are on the order of months or less.

Magma evolution and the formation of porphyry Cu–Au ore fluids: evidence from silicate and sulfide melt inclusions

Abstract: Silicate and sulfide melt inclusions from the andesitic Farallon Negro Volcanic Complex in NW Argentina were analyzed by laser ablation ICPMS to track the behavior of Cu and Au during magma evolution, and to identify the processes in the source of fluids responsible for porphyry-Cu-Au mineralization at the 600 Mt Bajo de la Alumbrera deposit. The combination of silicate and sulfide melt inclusion data with previously published geological and geochemical information indicates that the source of ore metals and water was a mantle-derived mafic magma that contained approximately 6 wt.% H2O and 200 ppm Cu. This magma and a rhyodacitic magma mixed in an upper-crustal magma chamber, feeding the volcanic systems and associated subvolcanic intrusions over 2.6 million years. Generation of the ore fluid from this magma occurred towards the end of this protracted evolution and probably involved six important steps: (1) Generation of a sulfide melt upon magma mixing in some parts of the magma chamber. (2) Partitioning of Cu and Au into the sulfide melt (enrichment factor of 10,000 for Cu) leading to Cu and Au concentrations of several wt.% or ppm, respectively. (3) A change in the tectonic regime from local extension to compression at the end of protracted volcanism. (4) Intrusion of a dacitic magma stock from the upper part of the layered magma chamber. (5) Volatile exsolution and resorption of the sulfide melt from the lower and more mafic parts of the magma chamber, generating a fluid with a Cu/Au ratio equal to that of the precursor sulfide. (6) Focused fluid transport and precipitation of the two metals in the porphyry, yielding an ore body containing Au and Cu in the proportions dictated by the magmatic fluid source. The Cu/S ratio in the sulfide melt inclusions requires that approximately 4,000 ppm sulfur is extracted from the andesitic magma upon mixing. This exceeds the solubility of sulfide or sulfate in either of the silicate melts and implies an additional source for S. The extra sulfur could be added in the form of anhydrite phenocrysts present in the rhyodacitic magma. It appears, thus, that unusually sulfur-rich, not Cu-rich magmas are the key to the formation of porphyry-type ore deposits. Our observations imply that dacitic intrusions hosting the porphyry–Cu–Au mineralization are not representative of the magma from which the ore-fluid exsolved. The source of the ore fluid is the underlying more mafic magma, and unaltered andesitic dikes emplaced immediately after ore formation are more likely to represent the magma from which the fluids were generated. At Alumbrera, these andesitic dikes carry relicts of the sulfide melt as inclusions in amphibole. Sulfide inclusions in similar dykes of other, less explored magmatic complexes may be used to predict the Au/Cu ratio of potential ore-forming fluids and the expected metal ratio in any undiscovered porphyry deposit.

Magnesian andesite and dacite lavas from Mt. Shasta, northern California: products of fractional crystallization of H 2 O-rich mantle melts

Abstract: Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10–15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption.

Insights into shallow magmatic processes in large silicic magma bodies: the trace element record in the Fish Canyon magma body, Colorado

Abstract: Highly evolved rhyolite glass plus near-solidus mineral assemblages in voluminous, dacitic, crystal-rich ignimbrites provide an opportunity to evaluate the late magmatic evolution of granodiorite batholiths. This study reports laser-ablation ICP-MS analyses of trace element concentrations in feldspars, hornblende, biotite, titanite, zircon, magnetite, and interstitial glass of the crystal-rich Fish Canyon Tuff. The high-silica rhyolite glass is characterized by relatively high concentrations of feldspar-compatible elements (e.g., 100 ppm Sr and 500 ppm Ba) and low concentrations of Y ( 40) compared to many well-studied high-silica rhyolite glasses and whole-rock compositions. Most minerals record some trace element heterogeneities, with, in particular, one large hornblende phenocryst showing four- to six-fold core-to-rim increases in Sr and Ba coupled with a decrease in Sc. The depletions of Y and HREE in the Fish Canyon glass relative to the whole-rock composition (concentrations in glass ~30% of those in whole rocks) reflect late crystallization of phases wherein these elements were compatible. As garnet is not stable at the low-P conditions at which the Fish Canyon magma crystallized, we show that a combination of modally abundant hornblende (~4%) + titanite (~0.5–1%) and the highly polymerized nature of the rhyolitic liquid led to Y and HREE depletions in melt. Relatively high Sr and Ba contents in glass and rimward Sr and Ba increases in euhedral, concentrically zoned hornblende suggest partial feldspar dissolution and a late release of these elements to the melt as hornblende was crystallizing, in agreement with textural evidence for feldspar (and quartz) resorption. Both observations are consistent with thermal rejuvenation of the magma body prior to eruption, during which the proportion of melt increased via feldspar and quartz dissolution, even as hydrous and accessory phases were crystallizing. Sr/Y in Fish Canyon glass (13–18) is lower than the typical “adakitic” value (>40), confirming that high Sr/Y is a reliable indicator of high-pressure magma generation and/or differentiation wherein garnet is implicated.

Controls on conduit magma flow dynamics during lava dome building eruptions

Abstract: [1] Lava dome eruptions commonly display fairly regular alternations between periods of high and low or no activity with timescales typically of weeks to years and sudden transitions between effusive and explosive activity. We develop a transient model of the magma flow in a volcanic conduit from an open-system magma chamber with continuous replenishment. The model incorporates gas exsolution, bubble growth, gas escape through the magma, and decompression-induced crystallization and considers variations in magma temperature, water content, conduit diameter, phenocryst size, chamber volume, and magma rheology. Calculations show the presence of periodic variations in discharge rate due to the transition from a stable regime, when discharge rate is low and crystals grow efficiently leading to high magma viscosity, to another stable state, when discharge rate is high and crystallization is negligible. The difference in discharge rates between these regimes can be several orders of magnitude. Periods are similar to the observed timescales and mainly depend on the chamber volume. The system shows strongly nonlinear responses to the variation of governing parameters. If magma has a Bingham rheology pauses in discharge rate occur between peaks of discharge and the peaks are much higher than for the case of Newtonian rheology. Large changes in discharge rate and eruptive behavior can occur as the consequence of small changes in magma temperature, water content, phenocryst size distribution, or conduit diameter. The system can fluctuate between low and high discharge rates with transitions to explosive activity.

Integrating Ultramafic Lamprophyres into the IUGS Classification of Igneous Rocks: Rationale and Implications

Abstract: RECEIVED JULY 16, 2004; ACCEPTED MARCH 16, 2005ADVANCE ACCESS PUBLICATION APRIL 29, 2005We introduce a modification to the current IUGS classificationsystem for igneous rocks to include ultramafic lamprophyres, whicharecurrentlyentirelyomitted.Thisisdonebyincludinganewstepinthe sequential system, after the assignment of pyroclastic rocks andcarbonatites, that considers ultramafic inequigranular textured rockswith olivine and phlogopite macrocrysts and/or phenocrysts. At thisstepultramaficlamprophyresareconsideredtogetherwithkimberlites,orangeites (former Group 2 kimberlites) and olivine lamproites.This proposal allows the correct identification and classificationof ultramafic lamprophyres within the IUGS scheme. Only threeend-members are required for describing the petrographic and com-positional continuum of ultramafic lamprophyres: alno¨ite (essentialgroundmass melilite), aillikite (essential primary carbonate) anddamtjernite (essentialgroundmass nepheline and/or alkali feldspar).It is argued that all ultramafic lamprophyre rock types canbe relatedto a common magma type which differs in important petrogeneticaspects from kimberlites, orangeites, olivine lamproites and theremainder of lamprophyres such as alkaline and calc-alkaline vari-eties. Ultramafic lamprophyres can be readily distinguished fromolivine lamproites by the occurrence of primary carbonates, and fromkimberlites by the presence of groundmass clinopyroxene. In othercases distinction between aillikites, kimberlites and orangeites mustrely on mineral compositions in order to recognize their petrogeneticaffinities.

Reactive bulk assimilation: A model for crust-mantle mixing in silicic magmas

Abstract: Bulk assimilation of small (millimeters to ∼1 km) fragments of crust—driven and (ultimately) masked by reactions during xenolith melting and magma crystallization—is an important mechanism for crust-mantle mixing. Xenoliths containing mica or amphibole undergo dehydration melting when incorporated into a host magma, yielding mainly plagioclase, pyroxene, Fe-Ti oxides, and hydrous melt. The xenolith is physically compromised by partial melting and begins to disintegrate; xenolithic melt and crystals are mixed into the host magma. Xenocrystic zircon is liberated at this stage. The cryptic character of assimilation is greatly enhanced in any hydrous magma by hydration crystallization reactions (the reverse of dehydration melting). All pyroxenes and oxides (phenocrysts, xenocrysts, or crystals having a hybrid signature) will be subject to these reactions, producing feldspars, amphiboles, and micas that incorporate material from several sources, a particularly effective mixing mechanism. Implicit in the model is a reduced energy penalty for bulk assimilation—much of the assimilant remains in solid form—compared to melt-assimilation models. A large role for bulk assimilation supports stoping as a credible mechanism for the ascent of magmas. While the assimilation of low-density crust and concomitant fractionation provide the isostatic impetus for ascent, the wholesale incorporation and processing of crustal rocks in the magma chamber helps create the room for ascent.

Crystal size distributions (CSD) in three dimensions : insights from the 3D reconstruction of a highly porphyritic rhyolite.

Abstract: Growth histories and residence times of crystals in magmatic systems can be revealed by studying crystal sizes, size distributions and shapes. In this contribution, serial sectioning has been employed on a sample of porphyritic rhyolite from a Permo-Carboniferous laccolith from the Halle Volcanic Complex, Germany, to reconstruct the distribution of felsic phenocrysts in three dimensions in order to determine their true shapes, sizes and three-dimensional size distributions. A model of all three phenocryst phases (quartz, plagioclase, K-feldspar) with 217 crystals, and a larger model containing 1599 K-feldspar crystals was reconstructed in three dimensions. The first model revealed a non-touching framework of crystals in three dimensions, suggesting that individual crystals grew freely in the melt prior to quenching of the texture. However, crystal shapes are complex and show large variation on a Zingg diagram (intermediate over long axis plotted against short over intermediate axis). They often do not resemble the crystallographic shapes expected for phenocrysts growing unhindered from a melt, indicating complex growth histories. In contrast, the three-dimensional size distribution is a simple straight line with a negative slope. Stereologically corrected size distributions from individual sections compare well with stereologically corrected size distributions obtained previously from the same sample. However, crystal size distribution (CSD) data from individual sections scatter considerably. It is shown that CSDs can be robustly reproduced with a sampling size of greater than � 200 crystals. The kind of shape assumed in stereological correction of CSDs, however, has a large influence on the calculation and estimation of crystal residence times.

Oxygen isotope constraints on the sources of Central American arc lavas

Abstract: Oxygen-isotope ratios of olivine and plagioclase phenocrysts in basalts and basaltic andesites from the Central American arc vary systematically with location, from a minimum δ18Oolivine value of 4.6 (below the range typical of terrestrial basalts) in Nicaragua near the center of the arc to a maximum δ18Oolivine value of 5.7 (above the typical range) in Guatemala near the northwest end of the arc. These oxygen-isotope variations correlate with major and trace element abundances and with Sr and Nd isotope compositions of host lavas, defining trends that suggest variations in δ18O reflect slab contributions to the mantle sources of these lavas. These trends can be explained by a model in which both a low-δ18O, water-rich component and a high-δ18O, water-poor component are extracted from the subducting Cocos slab and flux melting in the overlying mantle wedge. The first of these components dominates slab fluxes beneath the center of the arc and is the principal control on the extent of melting of the mantle wedge (which is highest in the center of the arc); the second component dominates slab fluxes beneath the northwestern margin of the arc. Fluxes of both components are small or negligible beneath the southeastern margin of the arc. We suggest that the low-δ18O component is a solute-rich aqueous fluid produced by dehydration of hydrothermally altered rocks deep within the Cocos slab (perhaps serpentinites produced in deep normal faults offshore of Nicaragua) and that the high-δ18O component is a partial melt of subducted sediment on top of the plate.

Experimental Constraints on the Role of Garnet Pyroxenite in the Genesis of High-Fe Mantle Plume Derived Melts

Abstract: The anhydrous phase relations of an uncontaminated (primitive), ferropicrite lava from the base of the Early Cretaceous Parana´- Etendeka continental flood basalt province have been determined between 1 atm and 7GPa. The sample has high contents of MgO (� 14� 9 wt %), FeO* (14� 9 wt %) and Ni (660ppm). Olivine phenocrysts have maximum Fo contents of 85 and are in equilibrium with the bulk rock, assuming a K Olliquid DFeMg of 0� 32. A comparison of our results with previous experimental studies of high-Mg rocks shows that the high FeO content of the ferropicrite causes an expan- sion of the liquidus crystallization field of garnet and clinopyroxene relative to olivine; orthopyroxene was not observed in any of our experiments. The high FeO content also decreases solidus temper- atures. Phase relations indicate that the ferropicrite melt last equili- brated either at � 2� 2GPa with an olivine-clinopyroxene residue, or at � 5GPa with a garnet-clinopyroxene residue. The low bulk-rock Al2O3 content (9 wt %) and high (Gd/Yb)n ratio (3� 1) are consistent with the presence of residual garnet in the ferropicrite melt source and favour high-pressure melting of a garnet pyroxenite source. The garnet pyroxenite may represent subducted oceanic litho- sphereentrainedbytheupwellingTristanstartingmantleplumehead. During adiabatic decompression, intersection of the garnet pyroxenite solidus at � 5GPa would occur at a mantle potential temperature of � 1550 � C and yield a ferropicrite primary magma. Subsequent melting of the surrounding peridotite at � 4� 5GPa may be restricted by the thickness of the overlying sub-continental lithosphere, such that dilution of the garnet pyroxenite melt component would be signific- antly less than in intra-oceanic plate settings (where the lithosphere is thinner). This model may explain the limited occurrence of ferropicrites at the base of continental flood basalt sequences and their apparent absence in ocean-island basalt successions.

Sr isotope disequilibrium in Columbia River flood basalts: Evidence for rapid shallow-level open-system processes

Abstract: Geochemical variability among Columbia River Basalt Group flood lavas has been attributed to two different origins: derivation from heterogeneous mantle and modification of mantle-derived magmas by open-system processes involving continental crust. We present in situ analyses of Sr isotopes from core-to-rim transects of plagioclase phenocrysts and groundmass from each major Columbia River Basalt Group formation and show that plagioclase crystals are usually internally zoned in 87 Sr/ 86 Sr and are in 87 Sr/ 86 Sr disequilibrium with their host groundmass. These data unequivocally demonstrate that Columbia River basalt magmas, regardless of the nature of mantle sources, were modified by opensystem processes operating at crustal depths. One-dimensional diffusion modeling indicates that observed isotopic heterogeneities cannot have existed at magmatic temperatures for more than a few years or decades. In general, results indicate that these flood basalt magmas were erupted while still in the process of assembly. A typical Columbia River flood basalt magma (melt plus phenocrysts) therefore only attains its final geochemical identity just before or during eruption, a fact that is generally obscured when evaluating conventional whole-rock isotope analyses.

Contrasting processes in silicic magma chambers: evidence from very large volume ignimbrites

Abstract: Very large volume (> 1000 km 3 of magma) crystal-rich dacitic ignimbrites that lack pronounced evidence of fractional crystallization or vertical zonation erupt in some continental magmatic arcs (e.g. the Lund Tuff of the Great Basin and the Fish Canyon Tuff of Colorado in western USA). Apparently, their magma chambers were only modestly heterogeneous and not systematically zoned from top to bottom. These ignimbrites have 40 to 50 % phenocrysts set in a high-silica rhyolite glass. Mineral assemblages and mineral compositions suggest pre-eruption temperatures were 730 to 820 °C and water and oxygen fugacities were relatively high. We have speculated that these very large volume ignimbrites are unzoned because crystallization and convection in slab-shaped magma chambers inhibited separation of crystals from liquids and resulted in a chamber filled with compositionally heterogeneous magma that lacked systematic chemical zonation or strong fractionation. However, many other very large volume silicic ignimbrites are strongly fractionated and may be vertically zoned (e.g. tuffs related to the Yellowstone hotspot). These rhyolitic tuffs typically have few phenocrysts, anhydrous mineral assemblages, low oxygen fugacities, crystallization temperatures of 830 to 1050 °C, and a strong imprint of fractional crystallization. Yet these Yellowstone-type rhyolites are derived from chambers 40 to 70 km across which have sill-like shapes (depth/diameter ratios much less than 1). Thus, factors other than chamber shape must be important for establishing the degree of evolution and nature of zonation in silicic magma chambers. Here, the role of crystallinity-dependent viscosity on the evolution of these two types of contrasting magmas is explored. Calculated magma viscosities for the hot, dry, crystal-poor rhyolites are significantly lower than for the cooler, wetter, crystal-rich dacites. Perhaps these hot rhyolites had low enough crystal contents and viscosities to allow efficient crystal–liquid separation, probably by a combination of unhindered crystal-settling, floor crystallization (including compaction), and crystallization on the walls of large chambers. Clean separation of melt from residual solids at their sources may have been promoted by their high temperatures and low viscosities ( 4.5 Pa s). In contrast, monotonous dacitic magmas may never have been crystal-free near-liquidus magmas. Their large magma chambers may have developed by progressive growth at a shallow level with repeated input of intermediate to silicic magma. Crystallization of the water-enriched dacitic magmas occurred at lower temperatures ( 10 6.5 Pa s) were significantly higher. These characteristics inhibited all forms of crystal–liquid separation, hindered development of systematic vertical zonation, and promoted quasi-equilibrium crystallization in small domains within large heterogeneous magma chambers. Eruptions of these crystal-rich dacites may only occur if the roof fails over a growing magma chamber that is becoming increasingly molten.

Trace-element distribution coefficients for pyroxenes, plagioclase, and olivine in evolved tholeiites from the 1955 eruption of Kilauea Volcano, Hawai’i, and petrogenesis of differentiated rift-zone lavas

Abstract: Reliable values for mineral-melt trace-element distribution coefficients ( D ) are essential for constructing realistic models of magma evolution based on trace elements. We have determined D -values for an extensive set of compatible and incompatible trace elements in clinopyroxene, orthopyroxene, plagioclase, and olivine phenocrysts in two moderately evolved (5.4 and 6.6 wt% MgO), tholeiitic lavas from the 1955 eruption of Kilauea volcano, Hawai’i, using laser ablation-ICPMS. Coexisting melt compositions were obtained by analyses of quenched mesostasis. These D -values are consistent with experimental results when major element variations in the host phase are considered. Lattice strain models reproduce many of the partitioning characteristics. The distribution coefficients determined here can be applied to understanding the petrogenesis of evolved tholeiitic magmas from two recent Kilauea eruptions. Trace-element compositions of the 1955 lavas are consistent with 30–40% fractional crystallization of a gabbroic assemblage from an olivine tholeiite parental magma. The reduced influx of melt to Kilauea during the late 19th and early 20th centuries may have allowed the formation of evolved magmas in the rift zone.

Magma ascent and eruptive processes from textural and compositional features of Monte Nuovo pyroclastic products, Campi Flegrei, Italy

Abstract: Geochemical and textural studies were carried out on alkaline products of the AD 1538 Monte Nuovo eruption. Due to the integration of the volcanological study with eyewitness reports, the dynamics and timing of each phase of the eruption and the volume of emitted magmas are known in detail. On this basis, unique in Campi Flegrei, the relations between magma chamber mechanisms, eruptive styles, magma ascent dynamics and volatile exsolution processes have been explored. Glass and phenocryst compositions indicate that the erupted magma has a homogeneous phono-trachytic composition. Textures and compositions of phenocrysts indicate that they crystallised at equilibrium with the melt in the magma chamber, likely as a mushy boundary layer along the chamber wall, where the temperature was below the liquidus temperature of the crystal free-chamber core. The estimated crystallisation temperature is 850±40°C. The magma phase relations in Petrogeny’s Residua System suggest that phenocryst crystallisation occurred at % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeiuamaaBa % aaleaacaqGibWaaSbaaWqaaiaaikdaaeqaaSGaae4taaqabaaaaa!3979! $$\text{P}_{\text{H}_2 \text{O}}$$ between 100 and 200 MPa, corresponding to depths ranging from 3 to 8 km. The microlite composition and their close genetic relations with vesicles indicate that groundmass crystallisation occurred during the eruption as a consequence of magma degassing and vesiculation induced by decompression during its ascent toward the surface. Crystal size distributions reveal that microlites grew in two stages of undercooling that we define as: (1) magma migration onset upward from the chamber and (2) magma rising through the conduit to the surface, possibly lasting tens of days and few days, respectively. These results provide information on the physical conditions that characterise pre- and syn-eruptive processes, which may be useful in order to define eruptive scenarios and to evaluate short-term precursors. Furthermore, the collected data provide for the first time information on degassing-induced crystallisation during the eruption of a highly evolved alkaline magma.

Evolution of the Magma Chamber beneath Usu Volcano since 1663: a Natural Laboratory for Observing Changing Phenocryst Compositions and Textures

Abstract: We have investigated the evolution of an active silicic magma-feeding system beneath Usu volcano, Japan, where eight eruptions have been recorded since AD 1663. All magmatic products contain similar types of plagioclase and orthopyroxene phenocrysts that consist of homogeneous cores with uniform compositions, and a zoned mantle that increases in size with time. The compositions of plagioclase and orthopyroxene phenocrysts vary gradually and regularly with time, as do the bulk-rock compositions. The texture of these phenocrysts also changes systematically, caused by progressive crystal growth, dissolution and diffusion. On the basis of these observations, we conclude that the same magma-feeding system has persisted at Usu volcano since AD 1663. Compositional variation of magnetite phenocrysts differs from that of plagioclase and orthopyroxene, because magnetite has large diffusion coefficients and should represent magmatic conditions immediately before the eruption. Most pumices from Usu volcano contain two types of magnetite phenocryst, each with a different composition and crystallization temperature, indicating that two magmas mixed before each eruption (approximately several days before). The end-members changed with time: rhyolite þ basaltic andesite (1663); dacite rhyolite (1769, 1822, 1853); dacite dacite (1977, 2000). The temperature of the magma apparently increases with time, and the increase can be explained by sequential tapping from a magma chamber with a thermal and chemical gradient in addition to injection of high-temperature magma.

Petrological constraints on the origin of arc picrites, New Georgia Group, Solomon Islands

Abstract: Subduction related picrites from the New Georgia archipelago (Solomon Islands) range in bulk MgO from 13 to 30 wt%. Two generations of olivine are identified based on CaO contents: High-CaO olivine phenocrysts from the picritic parental melt and low-CaO olivine xenocrysts incorporated from either lithospheric or asthenospheric upper mantle. There is also evidence that some of the low-CaO olivines are boninitic in origin. The bulk MgO range in the picrites is largely controlled by assimilation of low-CaO olivine xenocrysts. Oxidation states of the melt (ΔFMQ+2.2), calculated from magnetite activities in liquidus chromites, constrain the MgO content of the parental melt to 13 wt%, assuming Fe–Mg exchange equilibrium between melt and liquidus olivine composition. The dry liquidus temperature of the parent melt based on this MgO content is 1340°C, about 80°C above the temperature obtained with the olivine–clinopyroxene Ca-exchange thermobarometer. The residence time of the low-CaO olivine xenocrysts in the magma, estimated from Ca- and Fe–Mg interdiffusion profiles, did not exceed 1 year.