V21B-2086
Sulphur Solubility in Mount Hood Andesites and CO2 Fluids: Implications for Volcanic Degassing
Despite the ubiquity of sulphur in volcanic systems, its partitioning behaviour during volatile exsolution and ascent is still relatively poorly known. Considerable attention has been paid to the solubility of the primary volatile budget of a magma i.e. H2O and CO2, and in recent years, this has broadened to include the behaviour of S as an H2O dominated fluid exsolves from a melt. However, despite the fact that CO2 is degassed from a melt some time before water reaches saturation [1], the partition coefficients for sulphur between melt and a CO2 dominated fluid have hitherto been neglected. We present experimental results based on an andesitic melt from Mt. Hood, oversaturated with varying fluids of end members CO2-H2O, at 1250-1300°C, and at pressures ranging from 200-800 MPa. Sulphur melt/fluid Kd values range from approx 200 at 800 MPa, to over 2000 at 200MPa, demonstrating that at super-liquidus conditions (i.e. prior to the crystalisation of sulphur stabilising minerals such as pyrrhotite or anhydrite), sulphur displays an extreme affinity for the fluid phase. These results have profound implications for the interpretation of the degassing patterns of active volcanoes. [1] King P. L. and Holloway J. R. (2002) CO2 solubility and speciation in intermediate (andesitic) melts: The role of H2O and composition
V21B-2087
The diffusion of water in haploanesite
Diffusive transport of water in silicate melts is a key process in magma dynamics and volcanic eruptions, including bubble growth. Previous studies demonstrate that in additional to temperature, water content and pressure, melt composition also plays an important role in determining water diffusivity. We carried out high temperature (1311-1512°C) diffusion-couple experiments and intermediate temperature (470- 600°C) dehydration experiments to investigate H2O diffusion in a melt of haploandesitic composition. The diffusion couple is composed of an anhydrous (with <0.1 wt.% H2O) and a hydrous (with 2 wt.% H2O) haploandesitic glass. A platinum capsule is used to contain the couple and then it is welded shut. Diffusion runs are carried out in a 12.7-mm piston-cylinder apparatus at 1 GPa and superliquidus temperatures of 1584-1785 K. Infrared microscopy is applied on quenched glass to measure the profile of total H2O concentration (H2Ot). The profile shape is best fit by an error function, indicating an H2O diffusivity virtually independent of H2O concentration, consistent with the results of Behrens et al. (2004) on an Fe-bearing andesite. Dehydration experiments are performed at 743-873 K in a rapid-quench cold-seal vessel, with a heated hydrous glass losing water to 0.1 GPa Ar atmosphere. Measured diffusion profiles, however, show that water diffusivity is dependent on water content. Experimental data can be explained by H2Om being the dominating diffusant or a total H2O diffusivity proportional to total H2O content. The distinction between the high-temperature experiments where H2Ot diffusivity is apparently independent of H2Ot content, and the intermediate-temperature experiments where H2Ot diffusivity depends on H2Ot can be rationalized if OH diffusion has a higher activation energy than molecular H2O diffusion, and their comparable diffusivities at high T gradually diverge as temperature is lowered. At below 1 wt.% H2O, water diffusivity increases from rhyolite to dacite to andesite at >1300°C, and this sequence is reversed at <600°C.
V21B-2088
Water Content And Behavior In Mafic Glasses And Melt Inclusions: An Approach Using microRaman Spectroscopy
Determining water content in silicate melt inclusions is key information to understand magmatic processes. μRaman spectroscopy is a powerful and non destructive technique to assess the total water content in silicate glasses, at micrometer scale (spatial resolution of 1-2 μm). Its potential for analyzing water was demonstrated for hydrous, silicic glasses [1-3]. In complement we analyzed both anhydrous and different sets of hydrous glasses, covering a large range of polymerization degree (NBO/T from 0.01 for rhyolite to 0.74 for basanite). This study allowed us to propose a general equation to determine accurately (5-7% relative error) the water content on a large compositional range [4]. We present here a systematic determination of the water contents in natural mafic melt/glass inclusions hosted in olivine using Raman spectroscopy and a comparison with FTIR measurements. We have specifically selected basanitic water-rich melt inclusions from La Sommata (Vulcano Island) to assess the possible effects of the olivine post-entrapment crystallization and diffusion on the water concentration, using μRaman. Water was analyzed in inclusions having sizes from 10 to >200 μm, a few being trapped in a single crystal. Profiles, performed in the largest, show no significant water diffusion in the shrinkage bubble and water increase while MgO decreases within the 10-20 μm layer at the contact with the host olivine. Moreover, high-T experiments performed on these samples confirm the water loss during heating, a process that is not systematic but possibly depends on the melt inclusion position in the crystal. All this investigation will allows us to understand the behavior of water after entrapment and the interface processes between melt inclusion and its host mineral. [1] Thomas, Amer. Mineral. 2000. [2] Behrens et al., Chem. Geol. 2006. [3] Di Muro et al., Chem. Geol. 2006. [4] Mercier et al., Geochim. Cosmochim. Acta. (accepted).
V21B-2089
Preliminary Results on Fractionation of H, C, S and Cl Isotopes by Thermal Diffusion in Silicate Melts
Molten silicates containing dissolved volatiles (H2O, CO2, SO2 or Cl) were placed in a thermal gradient and the resulting 'hot-to-cold' fractionation of H, C, S and Cl isotopes was characterized in the quenched samples by ion microprobe. The specific melt compositions were tailored to the volatile species of interest: natural obsidian for H-D, Fe-free haplobasalt for C, natural MORB basalt for S, and synthetic andesite for Cl. The SIMS analyses were facilitated by adding volatiles (H2O, Na2CO3, and FeS) that were markedly enriched in the (rare) heavy isotopes of interest (D, 13C, 36S). The ~1-cm long samples were run vertically in the piston-cylinder apparatus (hot end up), with the basalt and andesite contained in graphite and the other two melts in Pt. The Cl, S and C experiments were run for ~24h with a 'hot end' temperature of 1500°C (ΔT ~ 150°C) and the H-D experiment was run for ~16h with a 'hot-end' T of 1200°C (ΔT ~ 300°C). Short-duration (30-60 min) duplicates of all experiments were run as controls. Isotope ratios of Cl (37/35), S (34/32 and 36/32) and C (13/12) were characterized in the quenched glasses using the Cameca 1280 ion microprobe at WHOI; H/D was measured at ASU using the Cameca ims 6f. 'Hot-to-cold' fractionation of Cl isotopes along the quenched glass samples was not detectable above analytical uncertainty, but all other isotope ratios (and dissolved volatile concentrations) conformed in some way to the thermal field of the diffusion samples. In the case of sulfur, 34S variation across the sample was barely above analytical uncertainty, but 36S showed ~1% variation, with the heavy isotope (and total S) enriched at the 'cold' end. Carbon isotopes (13/12) are more strongly fractionated, with the 'cold' end depleted in 13C by ~20%. The obsidian sample showed enrichment in D at the 'cold' end by approximately a factor of ~2. Taken together, these preliminary results indicate that sustained temperature gradients in magmas are capable of producing substantial isotopic fractionations of volatile-forming elements.
V21B-2090
Permeabilities and pore characteristics of vesicular volcanic products: the relationship among permeability, porosity, and pore radius
Gas permeability of magma is one of the controlling factors of the explosivity of volcanic eruptions (Eichelberger et al., 1986). Many previous researches on permeability of volcanic products have investigated the relationship between permeability and porosity, and empirical equations relating permeability and porosity have been presented (e.g., Klug and Cashman, 1996; Mueller et al, 2005). Although permeability is correlated with pore size as well as porosity, the effect of pore size remains insufficiently investigated compared with the effect of porosity. Water expulsion method is a technique for evaluating the radius of pore throat by analyzing the way in which pore water is extruded from a water-saturated porous medium (Gelinas and Angers, 1986). We applied this method to determine the characteristic maximum pore throat radius (rch) of vesicular volcanic products. The result showed that the value of rch and porosity (φ) correlated well with permeability (k) according to the following form of the Kozeny-Carman relation: k = 0.0022*φ*(rch)2. This means that permeability is strongly dependent on pore size. Estimation of pore size is therefore essential for understanding the permeabilities of vesicular volcanic products.
V21B-2091
No more troubles with magma bubbles: numerical simulations of gas dynamics in viscous magma
Gas bubbles in magma play a crucial role in eruption dynamics, particularly in basaltic volcanism. Here we examine the dynamics and stability of bubbles during their ascent in magmatic conduits through numerical simulations in both two and three dimensions. More precisely, we investigate (1) the impact of bubble size on stability and breakup time, (2) whether bubbles filling the entire conduit have different stability properties than bubbles that are small in comparison to the conduit size, and (3) the importance of multiple bubble interactions for breakup and coalescence of bubbles. We present our results in terms of the relevant non- dimensional numbers: Reynolds number, Bond number, viscosity ratio, and either the ratio of bubble radii (in the case of multiple bubbles) or the ratio of bubble radius to conduit width (for single bubble simulations). We solve the Navier-Stokes equation by combining the projection method on a staggered grid with a ghost fluid technique, which allows us to model the gas-magma interface as a perfectly sharp discontinuity in material properties. This is particularly important in light of the drastic viscosity contrast of several orders of magnitude between gas and magma. We track the position of the gas-magma interface through level sets, since this method poses no restrictions on the deformability of the interface or the topological changes in the interface during coalescence or breakup. Our results indicate that both large gas bubbles and gas slugs are dynamically unstable and will break apart during ascent. Multiple bubble interactions can enhance breakup further.
V21B-2092
Effects of bubble coalescence and breakup on conduit dynamics
Volatiles play a central role in eruption behavior. The ability of an exsolved gas phase to move through, and escape from, ascending magma controls whether an eruption is explosive or effusive. In this study, we investigate the dynamics and interactions of gas bubbles as they rise in a conduit. We focus on the coalescence and breakup dynamics of buoyant bubbles in a shear flow (conduit flow) below the fragmentation level. We use a 3D multiphase lattice Boltzmann model to investigate the dynamics at the scale of the bubbles. The net coalescence rate and evolution of the bubble size distribution and number density depend exponentially on the capillary number, Ca (ratio of shear forces to surface tension forces). The Bond number (ratio of buoyancy force to surface tension force) also influences breakup dynamics at large Ca. The results obtained from our 3D lattice Boltzmann model are used to develop a parameterization for the evolution of both bubble size distribution and number density. This parameterization is then used as a model for subgrid scale processes in a 2D conduit model in which the bubbly fluid is treated as a continuum. Our bubble-scale dynamics model can also be used to quantify the development of permeability as bubbles coalesce and connect with the fragmentation surface. We use this multiscale approach to investigate the eruption dynamics of the mafic andesite eruptions that have recently occurred at Tungurahua volcano, Ecuador
V21B-2093
The Influence of Solid Particles on Bubble Size Distributions in Magma
The shapes and size distributions of bubbles in magma are influenced by their interactions with other bubbles and solid particles in addition to their nucleation and growth rates. In previous studies we have found that solid particles can cause a bubble to deform and split, and that small bubbles can get trapped inside layers with high solids concentrations, thereby accumulating relative to larger bubbles. This implies that the relationship between bubble number density and nucleation rate could be less direct than is often assumed during interpretations of bubble concentrations in solidified magmas. We have found that, for a single bubble in a crystal suspension, the extent of retardation of bubble rise depends on the probability of interaction with particles. At high particle concentrations, all bubbles are slowed down, and the rise rate through the suspension is determined by the pore aperture size. Here, we expand upon our previous work and examine the influence of solid particles on multiple-bubble trains with analog experiments. We focus on the high- crystallinity end member of natural magmas, where the influence of crystals is greatest. Starting from a homogeneous population of spherical bubbles, we study the change in shape and size distribution of the bubbles as they rise through a suspension of plastic beads in corn syrup (simulating magma with crystals). We focus particularly on the influence of particles on bubble coalescence.
V21B-2094
Magma Permeability Estimates From X-ray Tomography Scans
Volatile pressure build-up and degassing rates within volcano conduit magmas appear to be linked to volcanic explosivity. Magma permeability, established by bubble and fracture networks, in turn, determines volatile degassing rates. Therefore, exploring bubble network and fracture permeabilities within magma contributes to an overall understanding of volcanic eruption processes. While the most accurate representations of magmatic bubble networks are typically found in quenched volcanic ejecta, they unlikely form representative elementary volumes that reflect conduit-scale permeabilities. This difficulty may be alleviated by determining the statistical spatial relationships of bubble inclusions and networks in pumice. Lattice-Boltzmann simulations allow us to then calculate pumice permeability of these numerical bubble networks based on three-dimensional, high-resolution, X-ray tomography scans. However, bubble walls, separating inclusions, may be thinner than the X-ray tomography resolution. In such cases, tomographic scans will omit microscopic characteristics, resulting in a misrepresentation of bubble connectivity and macroscopic permeability. Therefore, it is imperative to accurately define these thin bubble walls during image post-processing. Here we present how pumice permeability is affected when thin bubble walls are re-introduced during post-processing. This approach allows improved inferences of large-scale magma permeabilities.
V21B-2095
Vesiculation Characteristics in Pyroclasts of the 3.1 ka Oneraki Eruption, Raoul Island, Kermadec Arc
Raoul Island is the emergent 30 square km portion of a > 200 cubic km volcanic edifice which rises 900 m from the sea floor along the Kermadec ridge. Although the island is composed mainly of basalt and basaltic andesite, the last 4000 years has seen several dacitic explosive eruptions associated with caldera formation [Lloyd & Nathan, N.Z. Geol. Surv. Bull., 1981; Smith et al., J.Volc. Geotherm. Res. v. 156, 2006]. Fall deposits of the 3.1 ka Oneraki eruption, of possible plinian dispersal, were sampled at five stratigraphic levels. The 16-32 mm size pumice clasts of the lower four levels display narrow, unimodal density ranges. The upper level fall deposit shows a bimodal density distribution, reflecting a change in eruption characteristics as dense, degassed fragments were also ejected, but without other signs of any interaction with external water. For this study, qualitative and quantitative vesicularity data have been collected from 16- 32 mm clasts from three of the stratigraphic levels to provide insights to the various processes involved in vesiculation and fragmentation of this magma. Future work will include comparisons of vesicle textures in this eruption to other dry and wet subaerially erupted Raoul deposits, and to submarine deposits of similar composition at Macauley and Healy volcanoes. By characterizing eruption products from these volcanoes and using constraints provided by the different degrees of interaction with water (and at different water depths in submarine examples) we hope to better understand the dynamics of the violent degassing processes driving these eruptions.
V21B-2096
Insights into Explosive Eruption Processes: Density Studies of Subaerial and Submarine Pyroclastic Deposits, Kermadec Arc
Explosive volcanism involving crystal-poor dacite to rhyolite magmas is common in the young records of many volcanoes along the intraoceanic Kermadec arc. Such volcanism occurs at both submarine and subaerial volcanoes, and is often of a size that caldera collapse occurs. Three volcanoes present unique circumstances that can provide insights into the processes involved in explosive volcanism. Healy, Macauley and Raoul volcanoes have erupted similar silicic magmas within the last 10 kyr in deep marine, shallow marine and subaerial settings, respectively. To investigate eruption processes from these three volcanoes we have characterized the density spectrum for juvenile pumice clasts in the 16-32 mm size fraction using water immersion techniques [Houghton & Wilson, Bull Volc. 51, 1989]. At Raoul, we have data from five eruption deposits of widely contrasting dispersal (strombolian to plinian). Four of these eruptions show no evidence for involvement of external water: all samples show narrowly-defined peaks of density despite wide differences in eruption size. Only one subaerial eruption shows evidence for interaction with external water, consistent with the large density range observed. Subaerial deposits from Macauley Island show a narrow peak in density, but display a slightly wider range caused by a subtle tail-off to denser clasts. Co-eruptive submarine deposits show large variations in density, with multiple peaks identified. Although from the same eruption (as shown by continuity on seismic profiles), these submarine deposits display a significant contrast in density spectra, which reflects the syn-eruptive redistribution of clasts with diverse densities. Material dredged from 1320 to 2110 m water depth at Healy also shows a large density range, but displays a unimodal low-density peak and lacks syn-eruptive selective redistribution of clast populations. Density spectra will be used to constrain the choice of clasts for imaging of vesicle textures and geochemical studies.
V21B-2097
Magma Degassing and Evolution Processes of the 2000 Eruption of Miyakejima Volcano, Japan, Deduced From of Olivine-Hosted Melt Inclusion Analyses
Chemical analyses of melt inclusions in Mg-poor (Mg#68-73) and Mg-rich (Mg#76-84)@olivines from a bomb and lapilli from the 18 August 2000 eruption of Miyakejima volcano, Japan, were carried out in order to investigate degassing and evolution process ƒÍf the magma. Analyses of major elements, S and Cl of the melt inclusions were made by EPMA, and H2O and CO2 by FTIR and SIMS (Miyagi et al., 1995; Hauri et al., 2002). Major element composition of Mg-poor olivine-hosted melt inclusions (Mg-poor Ol MIs) is similar to that of groundmass in the bomb, indicating the melt entrapment just before the eruption. The Mg-poor Ol MIs have volatile contents of 0.7-2.5 wt.% H2O, 0.005-0.02 wt.% CO2, 0.05-0.17 wt. % S, and 0.06-0.1 wt. % Cl, that are roughly similar to those of plagioclase-hosted melt inclusions (Saito et al., 2005). Gas saturation pressure of the magma is calculated to be 20-100 MPa on the basis of the H2O and CO2 contents of Mg-poor Ol MIs, corresponds to the depth of 1-4 km. On the other hand, the Mg-rich olivine-hosted melt inclusions (Mg-rich Ol MIs) have SiO2 and K2O-poor but Al2O3-rich composition than the whole rock composition of the bomb and lapilli. They have volatile contents of 1.9-3.5 wt.% H2O, 0.003-0.025 wt.% CO2, 0.06-0.21 wt.% S, and 0.04-0.07 wt.% Cl, that are a little higher H2O and S and lower Cl contents than those of Mg-rich Ol MIs. Gas saturation pressure of the magma is calculated to be 50-150 MPa on the basis of the H2O and CO2 contents of Mg-rich Ol MIs. Ratios of H2O and S contents of both the Mg-poor and Mg-rich Ol MIs are similar to that of volcanic gas emitted from the summit after the 2000 eruption, while their ratios of CO2 and H2O contents are lower than that of volcanic gas. Existence of the Al2O3-rich less-evolved melt with high H2O content is consistent with the petrological and experimental studies that low-MgO high-alumina basalt is derived from primary magma with high H2O content (Uto, 1986; Sisson and Grove, 1993). The volatile contents and major element composition of the Mg-rich Ol MIs suggest degassing of the less-evolved magma occurred together with fractional crystallization at the depth of 2-6 km. The Mg-poor Ol MIs having lower H2O and higher CO2 contents than those of the Mg-rich Ol MIs suggest addition of CO2-rich gas to the degassed and evolved magma. It is necessary to investigate volatile evolution process of the magma at the shallow depth (<4 km) in more detail.
V21B-2098
First Assessment Of Volatiles Dissolved In Magma Feeding Yasur Activity (Vanuatu Arc)
Yasur is the active volcano of Tanna island, located in the south part of the arc where the convergence rate achieves 12 cm per year. Yasur is known, since its discovery in 1774 by Cook, for its continuous strombolian to vulcanian activity. Proximal pyroclastic deposits are constituted by alternating cm to dm thick fallout layers of highly vesicular scoria and ash, most likely representative of the Yasur early activity, although not dated. The scoriae are basaltic-trachyandesites, with relatively low abundances in Th and Nb (2.1 and 1.0 ppm, respectively), intermediate Nb/Y and La/Yb ratios (0.05 and 5.2, respectively) and high Th/Ta and Ba/La ratios (23 and 37, respectively). They display an unusual low crystal content (~14% in mass), possibly suggesting a high thermal flux. The plagioclase-phyric bombs presently thrown out at the crater, during strombolian activity, have more evolved trace and major element compositions that requires ~25% crystal fractionation. The crystal textures in scoria testify to rapid crystallization. In particular, olivine Fo75-71 contains typical hopper to closed-hopper, melt/glassy inclusions (M.I.) indicative of high cooling rates that prevented significant interactions with their hosts. Their major element compositions cover a wide range encompassing that of the bulk rocks and glassy matrices. There is no evidence of boundary layer effect in M.I. due to high olivine crystallization rate. The very first dataset on dissolved volatiles indicates that these melt inclusions are rather poor in water (H2O <1.3 wt% and CO2 (<0.03 wt%) but rich in Cl (up to 0.3-0.4 wt%) and S (up to 0.25 wt%). The glassy matrices of scoria are strongly degassed (in wt% Cl = 0.07-0.09; S <0.01 to 0.03). Thus both S (>90%) and Cl (up to 75-80%) are extensively degassed from the erupting Yasur magma. The expected S/Cl wt ratio in gas emissions should be close to 1, a factor 2- 3 lower than actually measured [1]. It implies that only one third of S emissions should be provided by shallow magma degassing whereas two thirds are deeper derived and supplied by gas bubble differential transfer. Similarly, the measured flux of water (2.1×104 tons per day [1]) would imply significantly higher dissolved water content. An accurate estimate of the volume of degassing magma, combining gas emission [1], and M.I. data, is our objective but requires additional analyses of dissolved volatiles, a rather difficult task owing to the small size of the inclusions. Finally, Yasur bulk rocks and primitive M.I. are remarkably similar, geochemically, to the less evolved magma that refilled the long-lived reservoir associated with the Siwi ignimbrite [2], that generated the caldera where Yasur cone has been growing. Yasur volcano possibly represents the current open conduit degassing vent of a large magma reservoir underlying the caldera. [1] Allard et al., (2008) IAVCEI General Assembly, August 2008, Reykjavik, Iceland. [2] Robin et al., (1994) Bull. Volcanol. 56,10-22
V21B-2099
Volatile Concentrations in Calc-Alkaline Andesites from Mount Hood Volcano, Oregon
Oregon's Mount Hood has erupted compositionally homogeneous andesitic lava domes and flows over the last ~0.5 m.y., and has erupted with anomalously low explosivity relative to most arc volcanoes of similar composition. Pyroclastic deposits are comparatively rare at Hood and those that are present are mostly block-and-ash flows produced by the collapse of lava domes. The two possible causes for Hood's low explosivity are low intrinsic magmatic volatile concentrations or shallow level degassing prior to eruption. Previous plagioclase hygrometer estimates of pre-eruptive water contents suggest that Hood lavas are volatile-rich and contain up to 6 wt% H2O. Newly analyzed plagioclase compositions from the Timberline eruptive period (1.5 ka) predict water contents from 0.9-3.7 wt% based on plagioclase from An42-An45 and temperature estimates from 900-950 °C. This relatively large range of predicted water contents within a narrow compositional and temperature span demonstrates the challenge of predicting the effects of volatiles on eruptive behavior without direct measurement of volatile concentrations. Melt inclusions within plagioclase from Timberline pumice were analyzed for H2O, CO2, S, and Cl by FTIR and EMPA. Inclusions analyzed to date contain ~1.5-4.5 wt% H2O, and 40-100 μg/g S, with higher S associated with higher H2O. Cl contents are generally ~2000 μg/g regardless of H2O content, and CO2 is only detectable (~20 μg/g) in the most H2O-rich inclusion. Hydrous basalts typically lose S by degassing prior to losing Cl, which suggests that the decreasing S/Cl ratio (and the absence of CO2) in Hood's low-H2O inclusions may result from various extents of shallow degassing before entrapment. Our preliminary data suggest that Mount Hood's magmas have intrinsic volatile concentrations comparable to other Cascade volcanoes, but concentrations may decrease by shallow crustal degassing prior to eruption, thereby removing the volatiles that drive explosive eruptive activity.
V21B-2100
The feeding system of Agnano-Monte Spina eruption (Campi Flegrei, Italy): dragging the past into present activity and future scenarios
The Agnano Monte Spina (Campi Flegrei, Italy – 4100 years BP) eruption is a reference scenario for a next large scale eruption at Campi Flegrei caldera, and is here selected to investigate the physico-chemical conditions of the pre-eruptive magmatic system as well as to gain insights into the source processes responsible of the huge hydrothermal-magmatic activity observed at surface nowadays. Isotope data on whole rocks and glasses and melt inclusions studies suggest that two chemically and isotopically distinct magmas, with different volatile signature mixed before the eruption. Our new data reveal that one of the magmas involved in the mixing process is similar to the less differentiated shoshonitic magma erupted at around 10 ka BP, whereas the second represents a residual of the magma discharged during the Neapolitan Yellow Tuff caldera forming eruption. Hence, the mixing process is driven by an abundant gas phase which sustains the ascent of magma blobs of deep provenance. The H2O and CO2 contents in melt inclusions give entrapment pressures between 60 and 150 MPa, corresponding to depths between 2.5 and 6 km. Degassing trends show the presence of two extreme patterns, one likely to represent the volatile signature of magma ascending from depth > 7 km; the other one related to a gas-dominated magma residing at shallow depth and developed upon flushing by deep CO2-rich gas. We suggest that volatile- rich blobs of deep shoshonitic magma periodically ascended and mixed with trachy-phonolitic magma at shallower depths. Our model is consistent with the bulk of geophysical and petrological observations at Campi Flegrei, and allows us to outline the role of magma mixing as a primary feature at Campi Flegrei caldera, as supported by the results of previous investigation of other eruptions in the area A major outcome of this study is the conceptual frame it deserves for recent unrest crises at Campi Flegrei, including the 1982-84 bradyseism. Uplift phases associated to bradyseismic crises are related to major episodes of closed-system ascent of magma blobs from depth > 7 km, followed by single-step volatile release upon their emplacement at shallow levels (3-4 km). This leads both the shallow magmatic and geothermal systems to store and progressively release important amounts of gas, hence energy. In this view, eruptive episodes are strongly conditioned by the critical achievement of an upper limit of gas storage, and by the crustal stress state and the fracturing state of the overlying cap of rocks.
V21B-2101
Magmatic Processes in Monogenetic Eruptions, Procida Island, Campi Flegrei, Italy: Geochemical Evidence From Melt Inclusions
Campi Flegrei is an active volcanic complex located in the greater Naples area, which has produced more than 50 eruptions over the past 60,000 years. These have ranged from small eruptions such as Monte Nuovo eruption of 1538 CE to extremely large eruptions such as the Campanian Ignimbrite (150-200 DRE; Barbieri et al., 1978). The volcanic field includes the mainland area located to the west of Naples and also the two islands of Ischia and Procida. The volcanic products range from basalts to shoshonitic phonolites and trachytes, with the more evolved magmas being more abundant. Three eruptive units from Procida Island have been studied to observe geochemical trends over time within a small area and to better understand magmatic processes between monogenetic eruptions. Juvenile samples from Pozzo Vecchio, Breccia Museo, and Solchiara were collected to examine the geochemistry of the mineral phases present and melt inclusions (MIs) found within the phenocrysts. Solchiara contained phenocrysts of olivine and clinopyroxene, whereas Breccia Museo and Pozzo Vecchio samples contained clinopyroxene and sanidine as the dominant phenocryst phases. Melt inclusions from Solchiara have narrow compositional ranges in major and trace elements (i.e., CaO, TiO2, Zr, Dy, La) over a large range in SiO2 contents (47 to 55 wt%) while MI from the Breccia Museo have a limited range of SiO2 contents (57 to 61 wt%) with a wider range for major and trace elements (i.e., FeO, Al2O3, CaO, La, Th, Rb). Pozzo Vecchio MI from clinopyroxene and sanidine define different chemical compositions, but petrographic evidence does not suggest a xenocrystic origin for either mineral phase. This suggests that Pozzo Vecchio is the result of magma mixing. Modeling of fractional crystallization of olivine, clinopyroxene, and sanidine are capable of producing most of the trends in major and trace elements between the most primitive samples to the most evolved samples. Volatile concentrations between the Breccia Museo and Solchiara MI are not significantly different for CO2, and S, but Cl, H2O, and F are slightly higher in Breccia Museo MIs compared to those from Solchiara. Water contents vary significantly, with many displaying statistically zero water up to approximately 2 wt% H2O. The wide range in water contents is due to reequilibration of the MIs during laboratory heating that was conducted to homogenize devitrified inclusions.
V21B-2102
Volatiles, major and trace element in quartz-host melt inclusions from ignimbrites in the Central Andes
Studies of ignimbrites provide valuable insight into the evolution processes occurring within silicic magma bodies. Two groups of eruptions producing these ignimbrites have been identified in the Central Andes. Small to moderate volume eruptions (<50 km3) associated with arc volcanism (ex. Cerro Blanco), and supereruptions of 102 to 103 km3 in systems operating in flare-up mode (ex. Altiplano Puna). In this work, we investigate these two modes of eruptions focusing on the pre-eruptive volatile contents and the characterization of the conditions and physical processes involved in the pre-eruptive magmatic differentiation. Volatiles, major and trace elements in quartz-hosted melt inclusions and matrix glass of pumice clasts from the Cerro Blanco caldera complex (CBCC), Argentina are compared with the massive ignimbrites of the Altiplano-Puna Volcanic Complex (APVC). Trace element compositions of matrix glasses of the CBCC are similar to or more evolved than quartz-hosted melt inclusions consistent with in situ crystallization of the quartz. The trace element content of the melt inclusions and the matrix glass shows enrichment in light rare earth elements and Rb, and strong Ba and Sr depletion. This depletion in is not seen in the APVC. Moreover, comparison of the trace elements variations show that CBCC rhyolites are less contaminated fractionates from andesitic magmas that typically erupt from the arc, while APVC dacites are much more crustal in origin. Infrared spectroscopy analyses of the melt inclusions trapped in quartz phenocrysts from CBCC have dissolved H2O contents varying between 2.6 and 8.6 wt% and dissolved CO2 ranging from below detection limit up to 173 ppm (saturation pressure of 39 to 325 MPa). These values are in marked contrast with data from the APVC where dissolved volatile content in quartz-hosted melt inclusions have a narrower range of H2O content (3.0 to 4.5 wt%) for a much higher range of CO2 content (from below detection limit up to 400 ppm). In a closed system environment, these high water concentrations can be produce by high degree of plagioclase and feldspars fractionation as suggest by the depletion of Ba, Sr, and Eu in the melt inclusions. Our data are consistent with the previous hypothesis that the smaller volumes eruptions in the Central Andes can be triggered by volatile oversaturation within the chamber in a closed system environment, while those from the supereruptions evolve in an open system environment where passive degassing may take place and require an external trigger.
V21B-2103
The Magmatic Evolution of Dabbahu Volcano, Afar, Ethiopia
Dabbahu is situated in the western region of Afar, Ethiopia, at the northern end of the Manda Hararo rift
segment. This volcano came back to life in 2005 with a small rhyolitic eruption from the Da'Ure vent, the first
such eruption in Africa for a century. This coincided with the start of a major rifting event which has been
modelled as a basalt dyke injection (Wright et al 2006). The aim of this research is to provide an insight into
the history and evolution of a silicic magmatic centre in the rift, and to contribute to the wider aims of the
NERC Afar Consortium to track the creation, migration, evolution and emplacement of magma from the
asthenosphere to the crust.
Here we report the results of recent fieldwork in the northern, ESE and summit areas of the volcano, the first
geological expedition to the area for over 30 years (Barberi et al, 1975). The volcano is characterised by a
wide range of magma types from alkali-basalts, through trachytes to pantellerites. Initial mapping has
revealed that the volcano has not evolved through eruptions from a central vent but mainly through a series
of N-S trending fissures located across the volcano, sub-parallel to the current rift axis. At least four
generations of rifting have been identified, each associated with obsidian flows and pyroclastic deposits,
some of which contain pumices and obsidian-pumice bombs ranging from ~0.08 to 2 m in length. Dabbahu
shows several signs of rejuvenation, including substantial fumaroles activity. Geodetic surveys reveal
subsidence of Dabbahu and nearby Gabho following the 2005 event, and subsequent inflation, consistent
with emplacement of a shallow magma body.
Our new SIMS data from feldspar hosted melt inclusions, suggests crystallisation occurs from depths of
~12 km. However, many inclusions are trapped between 3.5 and 6 km, suggesting magma is typically
stored in this region prior to eruption.
Whole rock and micro-analytical data of our samples will allow us to comprehensively characterise the
magma which will provide information on the relationship between Dabbahu's sub-volcanic system and the
magmas involved in the dyking events. Many of these erupted units will be dated using 40Ar-39Ar
techniques.
http://www.see.leeds.ac.uk/afar/
V21B-2104
Degassing of Volatiles and Transport of Semi-Volatile Trace Metals at Piton de la Fournaise - 2007.
Piton de la Fournaise volcano, Reunion, has been in a state of high eruptive activity since 1998 which culminated in 2007 in its largest eruption since 1931. Olivine-hosted melt inclusions and matrix glasses from this eruption have been analysed for volatiles and trace elements by electron microprobe (S, Cl and F), SIMS (CO2 and H2O) and LA-ICP-MS (Cu, Zn, Pb, Sn, Zr, La, Nb, Ba). The main motivation of this work is to track the degassing processes at this volcano during the large eruption of 2007, and also to investigate further the processes of vapour-melt exchange with respect to other semi volatile elements. The products of this most recent eruption were picrites of transitional (between alkalic and tholeiitic) composition. The major element chemistry is similar to that which has been noted by earlier authors. H2O contents range from 0.08 to 0.81 wt% in melt inclusions with matrix glass representing the degassed portion of melt at 0.09 to 0.16 wt%. S behaves in a similar fashion to H2O ranging from 80- 1107 ppm within melt inclusions and 20-102 ppm in matrix glass. Cl is similar to other measurements at Reunion with 103-306 ppm in MI and a similar 151-262 ppm in the matrix glass showing that little Cl has been lost by degassing. CO2 is high (1117 ppm) in the high water melt inclusions and degasses to lower concentrations (271 ppm) towards low water contents. However, in the matrix glass where the water contents remain low the CO2 contents still range up to 1105 ppm. These elevated CO2 concentrations, in tandem with low concentrations of both S and H2O, require either that magma has been degassed at low pressures and then recirculated to depth (as suggested for Hawaii, for example); or that melts have been degassed by flushing of the magma reservoir with a CO2-rich gas (as proposed on Etna, for example). The behaviour of semi-volatile trace metals in magmatic systems has important implications for both economic and environmental reasons. A CO2-rich fluid has the potential to transport these semi-volatile trace elements by providing a vapour phase into which they partition. As we shall show, the measured variability in semi-volatile trace elements (Pb, Cu, Zn and Sn) compared to the involatile trace elements (Nb, Ba, Zr, La) in Reunion glasses and melt inclusions can best be explained by gas-phase transport processes.
V21B-2105
Submarine Explosive Eruptions at Lo'ihi Seamount Hawai'i- the Result of Perfectly-Closed System Degassing
We examine the relative roles of magma degassing and magma-water interaction that permit basalt to erupt explosively at ~1000 mbsl on Lo'ihi Seamount, Hawai'i. The ~10 MPa hydrostatic pressure at the summit of Lo'ihi is a particularly interesting pressure regime, since CO2 has limited solubility in basalt at these pressures, but the solubility of magmatic H2O (the primary volatile component that drives explosive subaerial eruptions) varies as a function of initial volatile content and degassing history. The vesicularity of submarine pyroclasts, combined with residual volatile contents in quenched matrix glass and olivine-hosted melt inclusions (MIs), measured by FTIR and SIMS, provide a nearly complete record of the degassing history of the magma. We present observations from two submarine pyroclastic deposits erupted at ~1 km depth on Lo'ihi. The first is a cone-forming transitional basalt with modal vesiculary of ~40%, matrix glass CO2 = 24±10 ppm and H2O = 0.45±0.07 wt.%, and MI volatiles ranging up to 743 ppm CO2 and 0.86 wt.% H2O. The second is a tholeiitic basalt with modal vesicularity of ~50%, matrix glass CO2 = 22±6 ppm and H2O = 0.55±0.06 wt.%, and MI volatiles ranging up to 588 ppm CO2 and 1.49 wt.% H2O. Solubility modeling indicates that for both deposits, the matrix glass volatile contents can be achieved by near-perfect closed system degassing from a melt represented by the most CO2 -rich MIs measured. Comparison of this data set with published, partially open degassing models for Lo'ihi pillow lavas suggests that for a range of magma compositions, near-perfect closed system degassing of CO2 (complete) and H2O (partial) is directly associated with explosive eruptions of magma at ocean depths of ~1 km. In a closed-system, exsolved volatiles remain mechanically coupled to the melt in bubbles that impart buoyancy to the magma. This buoyancy translates into high eruption velocities that are required to initiate hydromagmatic fragmentation by metastable fuel-coolant interactions.
V21B-2106
Observations of CO2 Exsolution in Submarine Lava Flows: The 2005-6 Eruption on the East Pacific Rise, 9°50"N
The 2005-2006 eruption along a well-surveyed part of the East Pacific Rise (EPR) at 9°50"N, and the subsequent observations and collection of the flow (Soule et al., 2007) provide a rare opportunity to study how submarine basalts vesiculate as they ascend beneath the mid-ocean ridge (MOR) and are emplaced at the surface. We have measured dissolved CO2 and H2O contents and volume % vesicles in fresh natural glasses. Only the abundant tiny vesicles (5-140μm diameter) that define a good vesicle size distribution trend are considered in our study not the rare, large vesicles up to 1mm that probably existed in the magma chamber prior to eruption. There is a very good negative correlation between dissolved CO2 and the size and volume % vesicles. Glasses from the highest point of the flow, near the presumed eruptive vent, all contain 280-300 ppm dissolved CO2, and have roughly 0.2% vesicles, all <30μm diameter. Most of the glasses from deeper, distal locations (up to 12 km south or 7 km north) contain lower dissolved CO2 contents (as low as 140 ppm) and have up to 1% vesicles as large as 140μμm. The equilibrium CO2 content at the depth of eruption (2500 m) is 114 ppm. For all samples, the total CO2 content (dissolved + amount calculated in vesicles) is fairly constant at about 350 ppm, which would be in solution equilibrium at a depth of 1.6 km beneath the seafloor. This is very close to the seismically determined axial magma chamber (AMC) reflector at 1.43 km (Kent et al., 1993). The small size and volume of vesicles in the near- vent glasses suggests that the time spent by magmas ascending from the AMC in conduits was very short compared to the time spent flowing at or near the seafloor. For the distally emplaced lavas, most vesicle growth took place while the lava flowed down the axis at (or slightly below) the seafloor. The fairly constant total CO2 contents overall suggest that time was sufficient for vesicle formation but not loss. Modeling of vesicle growth will provide growth times that will be compared with eruption ages from U-series disequilibria (Rubin et al. in prep.) and with volcanic earthquake duration and location. We can use these relationships of CO2 oversaturation versus time and flow distance to determine the emplacement and flow characteristics for MORB from other regions, where eruptions are rarely documented and it is not possible to map flows. Soule S.A. et al., 2007, Geology v.35, 1070-1082. Kent, G et al., 1993, JGR, 13945-13970.
V21B-2107
Noble Gases Investigation on Etnean Volcanic Gases and on the Erupted Products During the 2001-2006 Period
Data acquired during the last 20 years of geochemical monitoring of volcanic gases lead us to better understand how volcanoes work. According to theoretical and experimental investigations, both the chemical and isotopic changes in sampled volcanic gases have been interpreted in terms of magma ascent using the models proposed by Nuccio & Paonita, (2001) and Caracausi et al. (2003). On the basis of numerical simulations of volatile degassing, we have been able to recognize episodes of magma migration from deeper reservoirs of Mount Etna to the shallower storage volume, until magma is erupted. The 3He/4He isotope ratios of gas emitted at the periphery of Mount Etna volcanic edifice exhibit synchronous variations, typically ranging between 7.5 Ra and 5.9 Ra, but displaying different average values characterizing each single sampling site. These isotope ratios normally show decreasing trends following outgassing processes, in contrast, increasing values indicate injection of new volatile-rich magma. The highest values are surprisingly similar to those measured in peridotite xenoliths from the neighbour Hyblean Plateau (Sapienza et al., 2005), suggesting it probably is the value which characterizes the upper mantle of the region. Since 2001, Mount Etna has been characterized by an intense eruptive activity with the emission of petrologically different products from various vents at the same time. Fluid inclusions trapped in olivines and pyroxenes of the erupted products have been investigated by analysing He and Ar isotope composition and abundance, and compared with those recorded by volcano monitoring of the same eruptive period. Our results confirm that olivine has the most efficient crystalline structure for preserving the pristine composition of entrapped gases, while pyroxene can suffer diffusive He loss. Significant differences were also observed among olivines of the same parental magma erupted during the 2001-2004 period, with 3He/4He isotope ratios moving from about 7.0 Ra in 2001 volcanites to 6.6 Ra in 2004-2005 products. Both abundances and isotope ratios of helium were attributed to protracted degassing of the same magma bodies from the 2001 to the 2004-2005 eruptive events. The general decrease in 3He/4He ratios measured in fluid inclusions is similar to that recorded in etnean peripheral gases by volcano monitoring during the same period, providing strong evidence of the real-time feeding of peripheral emissions by magmatic degassing.
V21B-2108
Na2O and Trace Elements Behavior in Trachytes and Phonolites at Suswa Volcano, Kenya: the Result of Combined Magma Mixing and Volatile-rich Na–Trace Element Fluids
The evolution of Suswa, a Quaternary volcano in the Kenya Rift, was dominated by the eruption of two rock suites, separated by a caldera event. Suswa is part of the Central Kenya Peralkaline Province (CKPP), which includes the Greater Olkaria Volcanic Complex (GOVC) and inter-center mafic fields, e.g. Tandamara and Elmenteita, whose compositions range from basalt to basaltic trachy-andesite (BTA). Both suites at Suswa range from trachyte to phonolite, but are distinguished by the amount of SiO2: pre- and syn-caldera rocks have 60-62%, and post-caldera rocks 57-59%. Trachyte to phonolite trends within each suite result from increasing Na2O, which is accompanied by increases in a number of trace elements (Be, Hf, Nb, Rb, Th, Y, Zn, Zr, and REE, except Eu). Magmatic processes included magma mixing, in which BTA magma similar to those of Tandamara and Elmenteita intruded the pre-caldera Suswa trachytic chamber, and fluid complexing, which was responsible for the enrichment in Na2O and trace elements. The importance of magma mixing in the CKPP has been recently documented at the GOVC by Macdonald et al. (2008, J Pet 49, 1515-1547), for which mafic-intermediate magmatic inclusions within comendites and disequilibrium phenocryst assemblages are part of the evidence. Evidence for mixing at Suswa includes: 1) mixed feldspar assemblages, e.g. syn-caldera ignimbrite samples contain both alkali feldspar (An2Ab62Or36), and xenocrystic plagioclase (An45Ab52Or3), and 2) heterogeneous matrix glass compositions. Glass in pre-caldera rocks is trachytic, similar to whole-rock compositions. Syn-caldera rocks have glass compositions both trachytic and intermediate between trachyte and BTA, while Tandamara BTA rocks contain trachytic glass. Glass in post-caldera rocks is mostly phonolitic. Glass inclusions in plagioclase xenocrysts are basaltic, similar to flows in the area. X-Y elemental plots do not show linear trends, as would be predicted from a mixing process. We attribute this to the short time scale between mixing and eruption. Experimental studies by De Campos et al. (2008, Chem Geol) and Perugini et al. (2008, Chem Geol) show that short time scales of mixing result in insufficient time for diffusion of elements to distribute linearly. The second important process at Suswa is halogen complexing, evidence for which includes: 1) High concentrations of F in matrix glass. For instance, syn-caldera matrix glass F varies from 0.5% for an early phreatomagmatic group to 1 - 2% for a later ring feeder and fissure eruptions, and post-caldera rocks have 0.5%, increasing to 1% from early to late eruptions; 2) Precipitation of LREE-rich fluorapatite and fluorite in the groundmass in syn-caldera rocks, and fluorapatite as a daughter mineral in melt inclusions in post-caldera rocks; 3) A positive correlation between Na2O and F in melt inclusions and matrix glasses. Macdonald et al. (1993, CMP 114, 276-287) documented immiscible carbonatite in syn-caldera trachytes from Suswa. Buhn and Rankin (1999, GCA 63, 3781-3797) showed that interaction with Na –F –REE - trace element-bearing carbonatitic fluids in Namibia resulted in enrichment of magmas with Na and trace elements similar to those observed at Suswa.
V21B-2109
WATER CONTENTS OF PYROXENES FROM ETNA RECENT ERUPTIONS
Volatiles (i.e. water) play a fundamental role by influencing the physical-chemical properties of magmas, and they are extremely useful to understand the mechanism of explosive eruptions, thus to forecast volcanic events. Nominally anhydrous minerals, that have been discovered to incorporate H2O, can be a new tool to better understand magma volatile contents. Since pyroxenes are very common in volcanic rocks, they can be used to add new data on the magmas volatiles budget. We measured H2O contents on selected cpx crystals from the Etna 3930 BP summit picritic eruption, 2001 and 2002 flank eruptions. These Etnean eruptions are between the most explosive events in the recent history of the volcano and the products are largely known for their primitive compositions and high volatile content (Kamenetsky et al. 2007, Geology 35, 255-258 and ref. therein). Cpx crystals more than some hundred microns in size were oriented by single- crystal XRD and the three main α , β, γ optical directions were measured on (100) and (010) orientations by polarized IR spectra. All the IR spectra showed vibrational bands in the water region at 3630, 3530 and 3460 cm-1, and the water content was calculated by the Libowitzky and Rossman (Am. Mineral. 82, 1111,1997) calibration. On the same crystal FTIR, SC-XRD and EPM analysis were performed and to account for possible H loss by Fe oxidation, Mössbauer spectra were measured on selected samples. Etna cpxs have a quite high H2O content, suggesting a water rich magmatic system and show only minor variations for different eruptions: 254 ppm H2O for 3930 BP picritic eruption; 214 ppm H2O for 2001 eruption; 161-254 ppm H2O for 2002 eruption. Zoned cpx from the eruption 2001 was analysed in the core and rim portion by obtaining 214 ppm H2O content for the core and 138 ppm H2O for the rim. The Mössbauer spectra measured on the same crystal show an increase in Fe3+ in the rim (Fe3+3/Fe2+ ratio: rim = 0.66, core= 0.79). Although Fe oxidation could have been responsible for partial water loss, different physico-chemical magma conditions during the rim growth cannot be excluded. The water content measured for the Etna cpx is well compared with that measured in the cpx from Aeolian volcanic rocks (Italy) that are water rich as well and show different water contents during their volcanic evolution: i.e. Salina 75-97ppm H2O 286-390ppm H2O and 171-199ppm H2O for the three main volcanic stages. The values of the H2O content of Cpxs from Etna and Aeolian volcanoes (up to 390ppm H2O) have so far been reported only for HP mantle Cpxs.
V21B-2110
Evidence of Two-Component Iblean-Like Mantle From 2001-2006 Igneous Products of Mount Etna
Mount Etna, the largest volcano in Europe, displays the peculiar condition of lying on continental crust and close to the subduction-related Aeolian volcanic arc, while its products shows typical affinities with ocean- island basalts (OIB). The finding of subduction-related geochemical tracers in the volcanic products of the last thousand years, along with an increased explosivity of eruptions, led to develop a model for Etnean magmas that states that the mantle source is progressively undergoing metasomatic influx by slab-derived fluids released from the Ionian slab[1]. We show that the trace element signatures of melt inclusions hosted in olivine phenocrysts erupted through the 2001-2006 period reveal the contemporaneous presence of two magma types in Mt. Etna's plumbing system. The two magma types are produced by partial melting of two distinct mantle sources respectively, which have amazing geochemical affinities with deeper and shallower mantle portions underlying the adjacent Iblean crust. This suggests that a layered, Iblean-like mantle is present beneath Mount Etna. Whereas the typical Etnean magma derives from sources located in the deeper layer, at depths of 50-90 km, the less common magma displays the signature of the shallower layer, coupled with a significant enrichment in fluid mobile elements. We provide evidence that an Iblean-like lithospheric peridotite mantle, re-fertilised by slab-derived fluids, represents the most plausible source for this magma and explains its enriched mantle (EM2) signature. Mingling of the two magma types is documented at melt inclusion scale, whereas complete mixing is usually observed at bulk rock scale. Our findings strongly support the view that the evolution of Etnean magmatism is ruled by the variable contribution of the two distinct mantle sources, rather than by the progressive transition towards an arc-type signature of the pristine mantle source[2]. Ref. 1. Tonarini et al., 2001 2. Schiano et al., 2001