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Roland Maas - One of the best experts on this subject based on the ideXlab platform.
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final magma storage depth modulation of explosivity and trachyte Phonolite genesis at an intraplate volcano a case study from ulleung island south korea
Journal of Petrology, 2014Co-Authors: Marco Brenna, Richard C Price, Shane J Cronin, Ian E M Smith, Young Kwan Sohn, Gi Bom Kim, Roland MaasAbstract:Ulleung Island is the top of a 3000 m (from sea floor) intraplate alkalic volcanic edifice in the East Sea/Sea of Japan. The emergent 950 m consist of a basaltic lava and agglomerate succession (Stage 1, 1·37^0·97Ma), intruded and overlain by a sequence of trachytic lavas and domes, which erupted in two episodes (Stage 2, 0·83^ 0·77Ma; Stage 3, 0·73^0·24Ma). The youngest eruptions, post 20 ka BP, were explosive, generating thick tephra sequences of phonolitic composition (Stage 4), which also entrained phaneritic, porphyritic and cumulate accidental lithics. Major element chemistry of the evolved products shows a continuous spectrum of trachyte to Phonolite compositions, but these have discordant trace element trends and distinct isotopic characteristics, excluding a direct genetic relationship between the two end-members. Despite this, the Stage 3 trachytes and some porphyritic accidental lithics have chemical characteristics transitional between Stage 2 trachytes and Stage 4 Phonolites. Within the phonolitic Stage 4 tephras three subgroups can be distinguished. The oldest, Tephra 5, is considerably enriched in incompatible elements and chondrite-normalized rare earth element (REE) patterns display negative Eu anomalies.The later tephras, Tephras 4^2, have compositions intermediate between the early units and the trachyte samples, and their REE patterns do not have significant Eu anomalies.The last erupted,Tephra 1, from a small intra-caldera structure, has a distinct tephriPhonolite composition. Trace element and isotopic chemistry as well as textural characteristics suggest a genetic relationship between the phaneritic lithics and their host phonolitic pumices.The Stage 4 tephras are not related to earlier phases of basaltic to trachytic magmatism (Stages 1^3). They have distinct isotopic compositions and cannot be reliably modelled by fractional crystallization processes.The differences between the explosive phonolitic (Stage 4) and effusive trachytic (Stage 2^ 3) eruptions are mainly due to different pre-eruptive pressures and temperatures, causing closedversus open-system degassing. Based on thermodynamic and thermobarometric modelling, the Phonolites were derived from deeper (subcrustal) magma storage and rose quickly, with volatiles trapped until eruption. By contrast, the trachytes were stored at shallower crustal levels for longer periods, allowing open-system volatile exsolution and degassing before eruption.
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final magma storage depth modulation of explosivity and trachyte Phonolite genesis at an intraplate volcano a case study from ulleung island south korea
Journal of Petrology, 2014Co-Authors: Marco Brenna, Richard C Price, Shane J Cronin, Ian E M Smith, Young Kwan Sohn, Gi Bom Kim, Roland MaasAbstract:Ulleung Island is the top of a 3000 m (from sea floor) intraplate alkalic volcanic edifice in the East Sea/Sea of Japan. The emergent 950 m consist of a basaltic lava and agglomerate succession (Stage 1, 137-097Ma), intruded and overlain by a sequence of trachytic lavas and domes, which erupted in two episodes (Stage 2, 083-077Ma; Stage 3, 073-024Ma). The youngest eruptions, post 20 ka BP, were explosive, generating thick tephra sequences of phonolitic composition (Stage 4), which also entrained phaneritic, porphyritic and cumulate accidental lithics. Major element chemistry of the evolved products shows a continuous spectrum of trachyte to Phonolite compositions, but these have discordant trace element trends and distinct isotopic characteristics, excluding a direct genetic relationship between the two end-members. Despite this, the Stage 3 trachytes and some porphyritic accidental lithics have chemical characteristics transitional between Stage 2 trachytes and Stage 4 Phonolites. Within the phonolitic Stage 4 tephras three subgroups can be distinguished. The oldest,Tephra 5, is considerably enriched in incompatible elements and chondrite-normalized rare earth element (REE) patterns display negative Eu anomalies.The later tephras, Tephras 4-2, have compositions intermediate between the early units and the trachyte samples, and their REE patterns do not have significant Eu anomalies.The last erupted,Tephra 1, from a small intra-caldera structure, has a distinct tephriPhonolite composition. Trace element and isotopic chemistry as well as textural characteristics suggest a genetic relationship between the phaneritic lithics and their host phonolitic pumices.The Stage 4 tephras are not related to earlier phases of basaltic to trachytic magmatism (Stages 1-3). They have distinct isotopic compositions and cannot be reliably modelled by fractional crystallization processes.The differences between the explosive phonolitic (Stage 4) and effusive trachytic (Stage 2-3) eruptions are mainly due to different pre-eruptive pressures and temperatures, causing closed-versus open-system degassing. Based on thermodynamic and thermobarometric modelling, the Phonolites were derived from deeper (subcrustal) magma storage and rose quickly, with volatiles trapped until eruption. By contrast, the trachytes were stored at shallower crustal levels for longer periods, allowing open-system volatile exsolution and degassing before eruption. © The Author 2014. Published by Oxford University Press. All rights reserved.
Philip R. Kyle - One of the best experts on this subject based on the ideXlab platform.
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experimental phase equilibrium constraints on the Phonolite magmatic system of erebus volcano antarctica
Journal of Petrology, 2013Co-Authors: Yves Moussallam, Bruno Scaillet, Clive Oppenheimer, Philip R. KyleAbstract:connection with the deep feeding system rooted in the mantle. Combined with recent seismological data, our results suggest that if a large phonolitic reservoir exists, then it should lie in the depth range 4^7·5 km.The tight constraints on temperature and redox conditions will be valuable for future thermodynamical and rheological modelling.
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pulsatory magma supply to a Phonolite lava lake
Earth and Planetary Science Letters, 2009Co-Authors: Clive Oppenheimer, Philip R. Kyle, Alexandra S Lomakina, N G Kingsbury, Marie BoichuAbstract:Abstract A few lava lakes, like that at Erebus volcano, Antarctica, have been continuously active for decades, reaching a steady-state. We report spectroscopic and thermal observations from Erebus that reveal remarkable, phase-locked cycles of lava lake convection and gas plume composition. We argue that the observed fluctuations in gas ratios, including the SO2/CO2 content in the plume, identify two end-member contributions to the Erebus emission: a sustained source of CO2-rich gas percolating through permeable conduit magma, and a shallower source of H2O-rich gas exsolved from magma pulses that periodically enter the lava lake. The unstable magma flow may reflect the viscosity stratification between rising and descending magma in the conduit, and the resulting oscillatory behaviour of the Phonolite lake exemplifies the relative roles of closed- and open-system degassing in persistently active volcanoes.
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geochemistry and mineralogy of the Phonolite lava lake erebus volcano antarctica 1972 2004 and comparison with older lavas
Journal of Volcanology and Geothermal Research, 2008Co-Authors: Peter J Kelly, Philip R. Kyle, Nelia W Dunbar, Kenneth W W SimsAbstract:Abstract Mount Erebus, Antarctica, is a large (3794 m) alkaline open-conduit stratovolcano that hosts a vigorously convecting and persistently degassing lake of anorthoclase Phonolite magma. The composition of the lake was investigated by analyzing glass and mineral compositions in lava bombs erupted between 1972 and 2004. Matrix glass, titanomagnetite, olivine, clinopyroxene, and fluor-apatite compositions are invariant and show that the magmatic temperature (∼ 1000°C) and oxygen fugacity (ΔlogFMQ = − 0.9) have been stable. Large temperature variations at the lake surface (~ 400–500°C) are not reflected in mineral compositions. Anorthoclase phenocrysts up to 10 cm in length feature a restricted compositional range (An10.3–22.9Ab62.8–68.1Or11.4–27.2) with complex textural and compositional zoning. Anorthoclase textures and compositions indicate crystallization occurs at low degrees of effective undercooling. We propose shallow water exsolution causes crystallization and shallow convection cycles the anorthoclase crystals through many episodes of growth resulting in their exceptional size. Minor variations in eruptive activity from 1972 to 2004 are decoupled from magma compositions. The variations probably relate to changes in conduit geometry within the volcano and/or variable input of CO2-rich volatiles into the upper-level magma chamber from deeper in the system. Eleven bulk samples of Phonolite lava from the summit plateau that range in age from 0 ± 4 ka to 17 ± 8 ka were analyzed for major and trace elements. Small compositional variations are controlled by anorthoclase content. The lavas are indistinguishable from modern bulk lava bomb compositions and demonstrate that Erebus volcano has been erupting lava and tephra from the summit region with the same bulk composition for ∼ 17 ka.
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Petrogenesis of a Phonolite-Trachyte Succession at Mount Sidley, Marie Byrd Land, Antarctica
Journal of Petrology, 1997Co-Authors: Kurt S. Panter, Philip R. Kyle, John L. SmellieAbstract:The 1.5 Ma evolution of the Late Pliocene (5.7 to 4.2 Ma) Mt Sidley volcano, Marie Byrd Land, is examined using major and trace elements, Sr, Nd, O and Pb isotopic data. A large (5 km × 5 km) breached caldera exposes lavas and tephras, deep within Mt Sidley, and allows its magmatic evolution to be elucidated. Two alkaline rock series are distinguished: (a) a strongly silica-under-saturated basanite to Phonolite series; (b) a more silica-saturated to -oversaturated alkali basalt to trachyte series. Rock compositions in both series fall within a narrow range of 77Sr/86Sri (0.7028–0.7032), 143Nd/144Ndi (0.51285–0.51290) and δ18O (5.0–6.0‰), and with 206Pb/204Pb (>19.5), suggest an asthenospheric source containing a strong mantle plume component. Partial melting models require ≤2% melting to produce primary basanite and ≤5% melting to produce alkali basalt from the same mantle source. The differentiation of the phonolitic series is modeled by fractionation of diopside, olivine, plagioclase, titaniferous magnetite, nepheline and/or apatite from basanite to derive 35% mugearite, 25% benmoreite and 20% Phonolite as residual liquids. Fractional crystallization of a similar mineral assemblage from alkali basalt is modeled for compositions in the trachyte series. However, many trachytes have variable 87Sr/86Sri (0.7033–0.7042), low 143Nd/144Ndi (0.51280–0.51283), high δ18O (6.5–8.4‰) and are silica oversaturated, suggesting they are contaminated by crust. The trachytes evolved by a two-step assimilation–fractional crystallization process (AFC). The first step involved contamination of alkali basalt by calc-alkaline granitoids within the middle crust where high assimilation to crystallization rates (high-r AFC) produced trachytic magmas characterized by depletions in Ta and Nb relative to K and Rb. The second step involved further fractionation of these magmas by low-r AFC within the upper crust to produce another suite of trachytes showing extreme incompatible element enrichment (e.g. Zr>1000 p.p.m/ and Th>100 p.p.m.).
Bruno Scaillet - One of the best experts on this subject based on the ideXlab platform.
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Toward accurate magma temperature determination: in situ direct emissivity measurements of Phonolite Teide and Erebus volcanos
2019Co-Authors: Joan Andujar, Aneta Slodczyk, Domingos De Sousa Meneses, Bruno ScailletAbstract:The temperature of magma is an important parameter to understand the activity of volcanos in consideration of minimize the volcano hazard. Recently, real time thermal remote sensing (such as camera IR or different instruments install on satellite) is largely used for the temperature measurements. This technology is based on measuring the infrared (IR) emitted flux of the body. However, these techniques have large uncertainties of ± 100 • C [1]. Our study is aimed at gaining new information on the radiative heat transfer of rocks, in order to improve the accuracy of magmatic temperature determination through laboratory optic measurements. Samples from two Phonolite volcanos (Erebus and Teide) have been selected for the measurements, their chemistry composition are very similar (except the percentage of FeO*, 3 wt% for Teide sample and 5 wt% for Erebus sample). We used high potential IR emissivity apparatus [2] allowing a direct non-contact in situ IR emittance measurements as a function of high temperature and in a wide spectral range. The Erebus and Teide samples were measured from room temperature up to 2000K with a CO 2 laser, both on heating and cooling; spectra were collected from 400 to 8000 cm-1. Our results reveal that thermal emissivity of magmatic rock is affected by changes in composition and thermal history. The Teide sample has no significant evolution of emissivity during heating. However, the Erebus sample shows an important emissivity changes (level of 50% difference between high and low emissivity) with the temperature, pointing clearly that the emissivity is not a constant value and it can be very variable during heating and cooling. Two cooling rates are applied on the sample during the test, fast cooling refers to volcanic bomb, and slow cooling is associate to lava flow. Erebus sample shows different emissivity with different rate of cooling. The emissivity measurements show also the important role of the iron on the spectral response of the Phonolite composition. Starting from 5 % the iron-based clusters are formed and change drastically radiative properties of the sample. Finally, our results allowed to discuss the accuracy of Erebus lava lack temperature presented in literature.
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experimental phase equilibrium constraints on the Phonolite magmatic system of erebus volcano antarctica
Journal of Petrology, 2013Co-Authors: Yves Moussallam, Bruno Scaillet, Clive Oppenheimer, Philip R. KyleAbstract:connection with the deep feeding system rooted in the mantle. Combined with recent seismological data, our results suggest that if a large phonolitic reservoir exists, then it should lie in the depth range 4^7·5 km.The tight constraints on temperature and redox conditions will be valuable for future thermodynamical and rheological modelling.
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relationships between pre eruptive conditions and eruptive styles of Phonolite trachyte magmas
Lithos, 2012Co-Authors: Bruno Scaillet, Joan AndujarAbstract:Abstract Phonolitic eruptions can erupt either effusively or explosively, and in some cases develop highly energetic events such as caldera-forming eruptions. However, the mechanisms that control the eruptive behaviour of such compositions are not well understood. By combining pre-eruptive data of well studied phonolitic eruptions we show that the explosive–effusive style of the phonolitic magma is controlled by the amount of volatiles, the degree of water-undersaturation and the depth of magma storage, the explosive character generally increasing with pressure depth and water contents. However, external factors, such as ingestion of external water, or latter processes occurring in the conduit, can modify the starting eruptive dynamic acquired at the levels of magma ponding.
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quantification of water content and speciation in natural silicic glasses Phonolite dacite rhyolite by confocal microraman spectrometry
Geochimica et Cosmochimica Acta, 2006Co-Authors: A. Di Muro, Bruno Scaillet, Gilles Montagnac, Benoit Villemant, Bruno ReynardAbstract:Abstract The determination of total water content (H2OT: 0.1–10 wt%) and water speciation (H2Omolecular/OH) in volcanic products by confocal microRaman spectrometry are discussed for alkaline (Phonolite) and calcalkaline (dacite and rhyolite) silicic glasses. Shape and spectral distribution of the total water band (H2OT) at ∼3550 cm−1 show systematic evolution with glass H2OT, water speciation and NBO/T. In the studied set of silicic samples, calibrations based on internal normalization of the H2OT band to a band related to vibration of aluminosilicate network (TOT) at ∼490 cm−1 vary with glass peraluminosity. An external calibration procedure using well-characterized glass standards is less composition-dependent and provides excellent linear correlation between total dissolved water content and height or area of the H2OT Raman band. Accuracy of deconvolution procedure of the H2OT band to quantify water speciation in water-rich and depolymerized glasses depends on the strength of OH hydrogen bonding. System confocal performance, scattering from embedding medium and glass microcrystallinity have a crucial influence on accuracy of Raman analyses of water content in glass-bearing rocks and melt inclusions in crystals.
Marco Brenna - One of the best experts on this subject based on the ideXlab platform.
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final magma storage depth modulation of explosivity and trachyte Phonolite genesis at an intraplate volcano a case study from ulleung island south korea
Journal of Petrology, 2014Co-Authors: Marco Brenna, Richard C Price, Shane J Cronin, Ian E M Smith, Young Kwan Sohn, Gi Bom Kim, Roland MaasAbstract:Ulleung Island is the top of a 3000 m (from sea floor) intraplate alkalic volcanic edifice in the East Sea/Sea of Japan. The emergent 950 m consist of a basaltic lava and agglomerate succession (Stage 1, 1·37^0·97Ma), intruded and overlain by a sequence of trachytic lavas and domes, which erupted in two episodes (Stage 2, 0·83^ 0·77Ma; Stage 3, 0·73^0·24Ma). The youngest eruptions, post 20 ka BP, were explosive, generating thick tephra sequences of phonolitic composition (Stage 4), which also entrained phaneritic, porphyritic and cumulate accidental lithics. Major element chemistry of the evolved products shows a continuous spectrum of trachyte to Phonolite compositions, but these have discordant trace element trends and distinct isotopic characteristics, excluding a direct genetic relationship between the two end-members. Despite this, the Stage 3 trachytes and some porphyritic accidental lithics have chemical characteristics transitional between Stage 2 trachytes and Stage 4 Phonolites. Within the phonolitic Stage 4 tephras three subgroups can be distinguished. The oldest, Tephra 5, is considerably enriched in incompatible elements and chondrite-normalized rare earth element (REE) patterns display negative Eu anomalies.The later tephras, Tephras 4^2, have compositions intermediate between the early units and the trachyte samples, and their REE patterns do not have significant Eu anomalies.The last erupted,Tephra 1, from a small intra-caldera structure, has a distinct tephriPhonolite composition. Trace element and isotopic chemistry as well as textural characteristics suggest a genetic relationship between the phaneritic lithics and their host phonolitic pumices.The Stage 4 tephras are not related to earlier phases of basaltic to trachytic magmatism (Stages 1^3). They have distinct isotopic compositions and cannot be reliably modelled by fractional crystallization processes.The differences between the explosive phonolitic (Stage 4) and effusive trachytic (Stage 2^ 3) eruptions are mainly due to different pre-eruptive pressures and temperatures, causing closedversus open-system degassing. Based on thermodynamic and thermobarometric modelling, the Phonolites were derived from deeper (subcrustal) magma storage and rose quickly, with volatiles trapped until eruption. By contrast, the trachytes were stored at shallower crustal levels for longer periods, allowing open-system volatile exsolution and degassing before eruption.
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final magma storage depth modulation of explosivity and trachyte Phonolite genesis at an intraplate volcano a case study from ulleung island south korea
Journal of Petrology, 2014Co-Authors: Marco Brenna, Richard C Price, Shane J Cronin, Ian E M Smith, Young Kwan Sohn, Gi Bom Kim, Roland MaasAbstract:Ulleung Island is the top of a 3000 m (from sea floor) intraplate alkalic volcanic edifice in the East Sea/Sea of Japan. The emergent 950 m consist of a basaltic lava and agglomerate succession (Stage 1, 137-097Ma), intruded and overlain by a sequence of trachytic lavas and domes, which erupted in two episodes (Stage 2, 083-077Ma; Stage 3, 073-024Ma). The youngest eruptions, post 20 ka BP, were explosive, generating thick tephra sequences of phonolitic composition (Stage 4), which also entrained phaneritic, porphyritic and cumulate accidental lithics. Major element chemistry of the evolved products shows a continuous spectrum of trachyte to Phonolite compositions, but these have discordant trace element trends and distinct isotopic characteristics, excluding a direct genetic relationship between the two end-members. Despite this, the Stage 3 trachytes and some porphyritic accidental lithics have chemical characteristics transitional between Stage 2 trachytes and Stage 4 Phonolites. Within the phonolitic Stage 4 tephras three subgroups can be distinguished. The oldest,Tephra 5, is considerably enriched in incompatible elements and chondrite-normalized rare earth element (REE) patterns display negative Eu anomalies.The later tephras, Tephras 4-2, have compositions intermediate between the early units and the trachyte samples, and their REE patterns do not have significant Eu anomalies.The last erupted,Tephra 1, from a small intra-caldera structure, has a distinct tephriPhonolite composition. Trace element and isotopic chemistry as well as textural characteristics suggest a genetic relationship between the phaneritic lithics and their host phonolitic pumices.The Stage 4 tephras are not related to earlier phases of basaltic to trachytic magmatism (Stages 1-3). They have distinct isotopic compositions and cannot be reliably modelled by fractional crystallization processes.The differences between the explosive phonolitic (Stage 4) and effusive trachytic (Stage 2-3) eruptions are mainly due to different pre-eruptive pressures and temperatures, causing closed-versus open-system degassing. Based on thermodynamic and thermobarometric modelling, the Phonolites were derived from deeper (subcrustal) magma storage and rose quickly, with volatiles trapped until eruption. By contrast, the trachytes were stored at shallower crustal levels for longer periods, allowing open-system volatile exsolution and degassing before eruption. © The Author 2014. Published by Oxford University Press. All rights reserved.
J F Larsen - One of the best experts on this subject based on the ideXlab platform.
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heterogeneous bubble nucleation and disequilibrium h2o exsolution in vesuvius k Phonolite melts
Journal of Volcanology and Geothermal Research, 2008Co-Authors: J F LarsenAbstract:Abstract This study focuses on constraining bubble nucleation and H 2 O exsolution processes in alkalic K-Phonolite melts, using “white pumice” of the 79 AD eruption of Vesuvius as starting material. The first set of experiments consisted of H 2 O solubility runs at 1153 to 1250 K and pressures between 50 and 200 MPa, to constrain equilibrium water concentrations along the decompression pathways. The decompression experiments were equilibrated with H 2 O at 150 MPa and 1173 and 1223 K, and then decompressed at 3 to 17 MPa/s before rapid quenching. Experiments nucleated bubbles within the first 50 MPa pressure drop, producing maximum bubble number densities (N V ), corrected to melt volume, of 3.8 × 10 14 m − 3 at 1173 K and 4.3 × 10 13 m − 3 at 1223 K. Most bubbles were not visibly attached to crystals, except for a subset attached to pyroxenes primarily in the 1173 K experiments. When compared with prior bubble nucleation studies, the reduced nucleation ΔP and relatively low N V observed indicate predominantly a heterogeneous nucleation mechanism. Melt–vapor–crystal wetting angles measured in 1173 K experiments from bubbles attached to pyroxene crystals are 36 to 69°, which are similar to those measured on titanomagnetite crystals in calc-alkaline dacite melts. The 1223 K experiments have porosities and water concentrations that largely track equilibrium, despite the rapid decompression rate. The 1173 K experiments deviate strongly from equilibrium trends in both porosity and water concentration, and slower H 2 O diffusion rates are likely the cause of the inhibited bubble growth. Bubble number densities from 79 AD Vesuvius natural EU2 pumice are relatively high (2 to 4 × 10 15 m − 3 ; [Gurioli, L., Houghton, B.F., Cashman, K.V., Cioni, R., 2005. Complex changes in eruption dynamics during the 79 AD eruption of Vesuvius. Bull. Volcanol. 67: 144–159.]) when corrected to vesicularity. In comparison, corrected N V 's from homogeneous and heterogeneous bubble nucleation experiments from this study and prior work are at least factor of 5 lower, indicating perhaps that the natural magmas initially nucleated bubbles in the presence of CO 2 . The disequilibrium H 2 O exsolution seen in the 1173 K experiments indicates that inhibited bubble growth could lead to delayed exsolution in the conduit in cooler K-Phonolite magmas.
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rapid water exsolution degassing and bubble collapse observed experimentally in k Phonolite melts
Journal of Volcanology and Geothermal Research, 2008Co-Authors: J Mongrain, J F Larsen, P L KingAbstract:Abstract We performed three sets of decompression experiments on a hydrated K-Phonolite melt at 880 °C in order to study the exsolution and degassing behavior of this low viscosity melt. The range of decompression rates studied was 0.01 MPa/s to 0.25 MPa/s. The pressure range was 200 MPa to 10 MPa. We determined that the melt exsolved water in equilibrium at all pressures at the decompression rates studied using micro-reflectance FTIR, a new method which is particularly well suited to determining the dissolved water concentration in high porosity experimental samples. Below 40–50 MPa at all decompression rates, the samples exsolved water in equilibrium, but the sample porosities deviated from values derived from equilibrium calculations/experiments and instead porosities remained constant or even decreased at lower pressures. The bubbles in all samples were predominantly deformed with features characteristic of partially collapsed bubble textures apparent at low pressures. Analysis of the bubble size distributions and deformation parameters indicate that melt shearing due to bubble growth and attendant sample expansion may cause the bubbles to deform, leading to the formation of bubble chains. The deformation and alignment of bubbles increases connectivity and lowers the porosity at which extensive permeability develops. In this situation, many of the connected bubbles are in contact with external vapor leading to significant degassing at 40–50 MPa. This degassing reduces the sample porosity and creates collapsed bubble textures at low pressures. This behavior differs significantly from that of hydrated rhyolite melts.
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experimental study of water degassing from Phonolite melts implications for volatile oversaturation during magmatic ascent
Journal of Volcanology and Geothermal Research, 2004Co-Authors: J F Larsen, James E GardnerAbstract:Abstract We have experimentally studied degassing of Phonolite melts between 50 and 200 MPa and 825 and 850 °C. Our results indicate that decompressed Phonolite melts contain equilibrium water contents regardless of decompression rate or final pressure. In addition, the total porosities of our experiments agree with predicted equilibrium values, regardless of final pressure or decompression rate. Combined observations of mean bubble diameters, porosities, and arrangement of bubbles in our experiments indicate that degassing occurs preferentially in the large bubble fraction of the population. Because of this, retardation of growth in the small bubble population occurs. This may lead to bimodal bubble size distributions in completely degassed Phonolites. Our results indicate that water saturated Phonolites at temperatures >800 °C can degas in equilibrium, while rhyolites follow a disequilibrium trend, at decompression rates of ∼0.5–1.0 MPa/s. Thus, hotter, water-saturated phonolitic magmas are likely to maintain equilibrium magma water contents during ascent at rates up to 20 m/s, which are typical of highly explosive eruptions. These results may apply to eruptions such as the 79 AD eruption of Vesuvius, which is the type location for Plinian-style, explosive eruptions. However, caution must be used when applying these results to lower temperature, more differentiated, and water-undersaturated Phonolite magmas. If fragmentation can occur at depths of ∼2 km or greater in the conduit, our results indicate that highly explosive eruptions of phonolitic magma may occur in the absence of water oversaturation in the magma during ascent. Fast ascent rates and high shear stresses in the conduit may provide a better explanation for fragmentation in this case.