The rheological evolution of alkaline Vesuvius magmas and comparison with alkaline series from the Phlegrean Fields, Etna, Stromboli and Teide

Somma-Vesuvius is considered one of the highest-risk volcanic systems in the world due to its high population density and record of highly destructive explosive activity. Eruptive style at Vesuvius varies greatly, alternating between effusive and explosive activities, and is likely strongly controlled by the evolution of the physical and chemical properties of the magma. Nevertheless, with the exception of the 1631 eruption, the rheological properties of Vesuvius magmas remain largely unconstrained. Here, we investigate the Newtonian shear viscosity (η) of dry and hydrous melts from the Mercato (plinian) and 1906 (violent strombolian) eruptions. These eruptions differ in size, eruptive style and magma chemistry (from phonolite to phono-tephrite). To evaluate the dry liquid viscosity variation covered by the eruptive products of the recent activity at Vesuvius, we measured the melt viscosities of bulk rock compositions and, for highly crystalline samples, of the separated groundmasses of tephras from the Pollena and 1906 eruptions. Hydrated samples with up to 4.24 wt% dissolved water were synthesised in a piston cylinder apparatus at confining pressure up to 10 kbar. The dry high temperature and the dry and hydrous low-temperature viscosities were obtained by combining the concentric cylinder and micropenetration techniques. The measured viscosities were parameterized by a modified Vogel-Fulcher-Tammann equation, accounting for the effect of water content, and were compared with previous measurements and models. At magmatic temperatures, the viscosities of Mercato samples are about four orders of magnitude higher than that of the least viscous investigated products from the 1906 eruption. Complex numerical models to forecast eruptive scenarios and their environmental impact are extremely sensitive to the accuracy of the input parameters and constitutive equations of magma properties. As a consequence, the numerical expressions obtained here are of particular relevance in the context of hazard assessment related to the different possible eruptive scenarios at Vesuvius through numerical simulation tools. The effect of composition on the liquid viscosities is compared to other high-Na (e.g., samples from Teide and Etna) and high-K (e.g., samples from Stromboli and Phlegrean Fields) alkaline magmas.     

Temperature dependence of sulfide and sulfate solubility in olivine-saturated basaltic magmas

The sulfur concentration at pyrrhotite- and anhydrite-saturation in primitive hydrous basaltic melt of the 2001-2002 eruption of Mt. Etna was determined at 200 MPa, T = 1050-1250 °C and at log fO₂ from FMQ to FMQ+2.2 (FMQ is Fayalite-Magnetite-Quartz oxygen buffer). At 1050 °C Au sample containers were used. A double-capsule technique, using a single crystal olivine sample container closed with an olivine piston, embedded in a sealed Au₈₀Pd₂₀ capsule, was developed to perform experiments in S-bearing hydrous basaltic systems at T > 1050 °C. Pyrrhotite is found to be a stable phase coexisting with melt at FMQ-FMQ+0.3, whereas anhydrite is stable at FMQ+1.4-FMQ+2.2. The S concentration in the melt increases almost linearly from 0.12 ± 0.01 to 0.39 ± 0.02 wt.% S at FeS-saturation and from 0.74 ± 0.01 to 1.08 ± 0.04 wt.% S at anhydrite-saturation with T ranging from 1050-1250 °C. The relationships between S concentration at pyrrhotite and/or anhydrite saturation, MgO content of the olivine-saturated melt, T, and log fO₂ observed in this study and from previous data are used to develop an empirical model for estimating the magmatic T and fO₂ from the S and MgO concentrations of H₂O-bearing olivine-saturated basaltic melts. The model can also be used to determine maximum S concentrations, if fO₂ and MgO content of the melt are known. The application of the model to compositions of melt inclusions in olivines from Mt. Etna indicates that the most primitive magmas trapped in inclusions might have been stored at log fO₂ slightly higher than FMQ+1 and at T = 1100-1150 °C, whereas more evolved melts could have been trapped at T ? 1100 °C. These values are in a good agreement with the estimates obtained by other independent methods reported in the literature.     

H2S fluxes from Mt. Etna, Stromboli, and Vulcano (Italy) and implications for the sulfur budget at volcanoes

We present here new measurements of sulfur dioxide and hydrogen sulfide emissions from Vulcano, Etna, and Stromboli (Italy), made by direct sampling at vents and by filter pack and ultraviolet spectroscopy in downwind plumes. Measurements at the F0 and FA fumaroles on Vulcano yielded SO₂/H₂S molar ratios of ≈0.38 and ≈1.4, respectively, from which we estimate an H₂S flux of 6 to 9 t · d⁻¹ for the summit crater. For Mt. Etna and Stromboli, we found SO₂/H₂S molar ratios of ≈20 and ≈15, respectively, which combined with SO₂ flux measurements, suggest H₂S emission rates of 50 to 113 t · d⁻¹ and 4 to 8 t · d⁻¹, respectively. We observe that “source” and plume SO₂/H₂S ratios at Vulcano are similar, suggesting that hydrogen sulfide is essentially inert on timescales of seconds to minutes. This finding has important implications for estimates of volcanic total sulfur budget at volcanoes since most existing measurements do not account for H₂S emission.     

Partitioning behavior of chlorine and fluorine in the system apatite-melt-fluid. II: Felsic silicate systems at 200 MPa

Hydrothermal experiments were conducted to determine the partitioning of Cl between rhyolitic to rhyodacitic melts, apatite, and aqueous fluid(s) and the partitioning of F between apatite and these melts at ca. 200 MPa and 900-924 °C. The number of fluid phases in our experiments is unknown; they may have involved a single fluid or vapor plus saline liquid. The partitioning behavior of Cl between apatite and melt is non-Nernstian and is a complex function of melt composition and the Cl concentration of the system. Values of D_Cl^apat/melt (wt. fraction of: Cl in apatite/Cl in melt) vary from 1 to 4.5 and are largest when the Cl concentrations of the melt are at or near the Cl-saturation value of the melt. The Cl-saturation concentrations of silicate melts are lowest in evolved, silica-rich melts, so with elevated Cl concentrations in a system and with all else equal, the maximum values of D_Cl^apat/melt occur with the most felsic melt. In contrast, values of D_F^apat/melt range from 11 to 40 for these felsic melts, and many of these are an order of magnitude greater than those applying to basaltic melts at 200 MPa and 1066-1150 °C. The Cl concentration of apatite is a simple and linear function of the concentration of Cl in fluid. Values of D_Cl^fluid/apat for these experiments range from 9 to 43, and some values are an order of magnitude greater than those determined in 200-MPa experiments involving basaltic melts at 1066-1150 °C.In order to determine the concentrations and interpret the behavior of volatile components in magmas, the experimental data have been applied to the halogen concentrations of apatite grains from chemically evolved rocks of Augustine volcano, Alaska; Krakatau volcano, Indonesia; Mt. Pinatubo, Philippines; Mt. St. Helens, Washington; Mt. Mazama, Oregon; Lascar volcano, Chile; Santorini volcano, Greece, and the Bishop Tuff, California. The F concentrations of these magmas estimated from apatite-melt equilibria range from 0.06 to 0.12 wt% and are generally equivalent to the concentrations of F determined in the melt inclusions. In contrast, the Cl concentrations of the magmas estimated from apatite-melt equilibria (e.g., ca. 0.3-0.9 wt%) greatly exceed those determined in the melt inclusions from all of these volcanic systems except for the Bishop Tuff where the agreement is good. This discrepancy in estimated Cl concentrations of melt could result from several processes, including the hypothesis that the composition of apatite represents a comparatively Cl-enriched stage of magma evolution that precedes melt inclusion entrapment prior to the sequestration of Cl by coexisting magmatic aqueous and/or saline fluid(s).     

The oxidation state of sulfur in synthetic and natural glasses determined by X-ray absorption spectroscopy

Sulfur K-edge X-ray absorption near edge structure (XANES) spectra were recorded for experimental glasses of various compositions prepared at different oxygen fugacities (fO₂) in one-atmosphere gas-mixing experiments at 1400 °C. This sample preparation method only results in measurable S concentrations under either relatively reduced (log fO₂ < −9) or oxidised (log fO₂ > −2) conditions. The XANES spectra of the reduced samples are characterised by an absorption edge crest at 2476.4 eV, typical of S²⁻. In addition, spectra of Fe-bearing compositions exhibit a pronounced absorption edge shoulder. Spectra for all the Fe-free samples are essentially identical, as are the spectra for the Fe-bearing compositions, despite significant compositional variability within each group. The presence of a sulfide phase, such as might exsolve on cooling, can be inferred from a pre-edge feature at 2470.5 eV.The XANES spectra of the oxidised samples are characterised by an intense transition at 2482.1 eV, typical of the sulfate anion SO₄²⁻. Sulfite (SO₃²⁻) has negligible solubility in silicate melts at low pressures. The previous identification of sulfite species in natural glass samples is attributed to an artefact of the analysis (photoreduction of S⁶⁺). S⁴⁺ does, however, occur unambiguously with S⁶⁺ in Fe-free and Fe-poor compositions prepared in equilibrium with CaSO₄ at 4-16 kbar, and when buffered with Re/ReO₂ at 10 kbar. Solubility of S⁴⁺ thus requires partial pressures of SO₂ considerably in excess of 1 bar. A number of experiments were undertaken in an attempt to access intermediate fO₂s more applicable to terrestrial volcanism. Although these were largely unsuccessful, S²⁻ and S⁶⁺ were found to coexist in some samples that were not in equilibrium with the imposed fO₂.The XANES spectra of natural olivine-hosted melt inclusions and submarine glasses representative of basalts at, or close to, sulfide saturation show mainly dissolved S²⁻, but with minor sulfate, and additionally a peak at 2469.5 eV, which, although presumably due to immiscible sulfide, is 1 eV lower than that typical of FeS. These sulfate and sulfide-related peaks disappear with homogenisation of the inclusions by heating to 1200 °C followed by rapid quenching, suggesting that both these features are a result of cooling under natural conditions. The presence of small amounts of sulfate in otherwise reduced basaltic magmas may be explained by the electron exchange reaction: S²⁻ + 8Fe³⁺ = S⁶⁺ + 8Fe²⁺, which is expected to proceed strongly to the right with decreasing temperature. This reaction would explain why S²⁻ and S⁶⁺ are frequently found together despite the very limited fO₂ range over which they are thermodynamically predicted to coexist. The S XANES spectra of water-rich, highly oxidised, basaltic inclusions hosted in olivine from Etna and Stromboli confirm that nearly all S is dissolved as sulfate, explaining their relatively high S contents.     

Effects of CO<Subscript>2</Subscript> flushing on crystal textures and compositions: experimental evidence from recent K-trachybasalts erupted at Mt. Etna

Changes in magmatic assemblages and crystal stability as a response of CO₂-flushing in basaltic systems have rarely been directly addressed experimentally, making the role of CO₂ in magma dynamics still controversial and object of scientific debate. We conducted a series of experiments to understand the response of magmas from Etna volcano to CO₂ flushing. We performed a first experiment at 300 MPa to synthesize a starting material composed of crystals of some hundreds of µm and melt pools. This material is representative of an initial magmatic assemblage composed of plagioclase, clinopyroxene and a water-undersaturated melt with 1.6 wt% H₂O. In a second step, the initial assemblage was equilibrated at 300 and 100 MPa with fluids having different XCO ₂ ^fl (CO₂/(H₂O + CO₂)). At low XCO ₂ ^fl (< 0.2 to 0.4), plagioclase is completely dissolved and clinopyroxene show dissolution textures. For relatively high XCO ₂ ^fl (0.9 at 300 MPa), the flushing of a CO₂-rich fluid phase leads to an increase of the amount of clinopyroxene and a decrease of the abundance of plagioclase at 300 MPa. This decrease of plagioclase proportion is associated with a change in An content. Our experiments demonstrate that flushing basaltic systems with fluids may drastically affect crystal textures and phase equilibria depending on proportions of H₂O and CO₂ in the fluid phase. Since texture and crystal proportions are among the most important parameters governing the rheology of magmas, fluid flushing will also influence magma ascent to the Earth’s surface. The experimental results open new perspectives to decipher the textural and compositional record of minerals observed in volcanic rocks from Mt. Etna, and at the same time offer the basis for interpreting the information preserved in minerals from other basaltic volcanoes erupting magmas enriched in CO₂.     

The effects of undercooling and deformation rates on the crystallization kinetics of Stromboli and Etna basalts

We have investigated the effect of undercooling and deformation on the evolution of the texture and the crystallization kinetics of remelted basaltic material from Stromboli (pumice from the March 15, 2007 paroxysmal eruption) and Etna (1992 lava flow). Isothermal crystallization experiments were conducted at different degrees of undercooling and different applied strain rate (T = 1,157–1,187 °C and $ \dot{\gamma }_{i} $ γ · i  = 4.26 s^?1 for Stromboli; T = 1,131–1,182 °C and $ \dot{\gamma }_{i} $ γ · i  = 0.53 s^?1 for Etna). Melt viscosity increased due to the decrease in temperature and the increase in crystal content. The mineralogical assemblage comprises Sp + Plg (dominant) ± Cpx with an overall crystal fraction (?) between 0.06 and 0.27, increasing with undercooling and flow conditions. Both degree of undercooling and deformation rate deeply affect the kinetics of the crystallization process. Plagioclase nucleation incubation time strongly decreases with increasing ΔT and flow, while slow diffusion-limited growth characterizes low ΔT—low deformation rate experiments. Both Stromboli (high strain rate) and Etna (low strain rate) plagioclase growth rates (G) display relative small variations with Stromboli showing higher values (4.8 ± 1.9 × 10^?9 m s^?1) compared to Etna (2.1 ± 1.6 × 10^?9 m s^?1). Plagioclase average nucleation rates J continuously increase with undercooling from 1.4 × 10⁶ to 6.7 × 10⁶ m^?3 s^?1 for Stromboli and from 3.6 × 10⁴ to 4.0 × 10⁶ m^?3 s^?1 for Etna. The extremely low value of 3.6 × 10⁴ m^?3 s^?1 recorded at the lowest undercooling experiment for Etna (ΔT = 20 °C) indicates that the crystallization process is growth-dominated and that possible effects of textural coarsening occur. G values obtained in this paper are generally one or two orders of magnitude higher compared to those obtained in the literature for equivalent undercooling conditions. Stirring of the melt, simulating magma flow or convective conditions, facilitates nucleation and growth of crystals via mechanical transportation of matter, resulting in the higher J and G observed. Any modeling pertaining to magma dynamics in the conduit (e.g., ascent rate) and lava flow emplacement (e.g., flow rate, pāhoehoe–‘a‘ā transition) should therefore take the effects of dynamic crystallization into account.     

Bubble growth in slightly supersaturated albite melt at constant pressure

Bubble growth experiments were performed in a piston-cylinder by hydrating albite melt with ∼11 wt.% H₂O at 550 MPa followed by rapid decompression at 1 MPa s⁻¹ to pressures of 450 or 400 MPa. At these conditions the melt was supersaturated with ∼0.5 or ∼1.5 wt.% H₂O, respectively, which caused rapid exsolution and bubble growth. Results at 1200 °C demonstrate that portions of the initial cumulative bubble-area distributions may be characterized by a power law with an exponent near 1, but they rapidly evolve to exponential distributions and approach a unimodal distribution after 32 h of growth. This evolution occurs by the growth of larger bubbles at the expense of smaller ones. The growth rate of the average bubble radius in these experiments is described by a power law whose exponent is 0.35, close to the theoretical exponent of 1/3 for phase growth in which coalescence is dominated by Ostwald ripening of the bubbles. Over the range of pressures and water contents investigated at 1200 °C, the bubble-size distributions and growth rate are not significantly affected by changes in the amount of exsolved water or by splitting the decompression path into two steps. Similar decompression experiments at 800 °C are dominated by smaller bubbles than in the 1200 °C experiments and also demonstrate exponential cumulative size distributions, but consistently contain a small fraction of larger bubbles. The growth rate of these bubble radii cannot be fit with a power law, but a logarithmic dependence of the bubble radii on time is possible, suggesting a difference in the growth mechanisms at low and high temperatures. This difference is attributed to the orders of magnitude changes in melt viscosity and water diffusion in the melt as the temperature varies from 800 to 1200 °C. At 1200 °C the transport properties of albite melt resemble those of natural basaltic melts whereas at 800 °C the properties are similar to those of andesitic to dacitic melts. The decompression rate used in this study exceeds natural rates by one to two orders of magnitude. Thus, these results indicate that natural mafic-to-intermediate magmas supersaturated with only a small excess of water should easily nucleate bubbles during ascent and that bubble growth in mafic magmas will proceed much more rapidly than in andesitic to dacitic magmas. Intermediate composition magmas also may be capable of forming bimodal bubble-size distributions even in the case when only one nucleation event occurred. The rapid evolution of the bubble-size distribution from a power law to an exponential may be useful in constraining the time duration between bubble nucleation and the quenching of natural samples.     

Lithium isotopes in island arc geothermal systems: Guadeloupe, Martinique (French West Indies) and experimental approach

We report lithium (Li) isotopic measurements in seawater-derived waters that were discharged from geothermal wells, thermal springs, and sub-marine springs located in volcanic island arc areas in Guadeloupe (the Bouillante geothermal field) and Martinique (Lamentin plain and the Diamant areas). While Li isotopic signatures of the geothermal fluids collected from deep reservoirs were found to be homogeneous for a given site, the δ⁷Li signatures for each of these reservoirs were significantly different. The first low temperature (25-250 °C) experiments of Li isotope exchange during seawater/basalt interaction confirmed that Li isotopic exchange is strongly temperature dependent, as previously inferred from natural studies. Li isotopic fractionation ranged from +19.4‰ (Δ_{solution-solid}) at 25 °C to +6.7‰ at 250 °C. These experiments demonstrated the importance of Li isotopic fractionation during the formation of Li-bearing secondary minerals and allowed us to determine the following empirical relationship between isotopic fractionation and temperature: Δ_{solution-solid} = 7847/T − 8.093. Application of experimental results and literature data to the Bouillante area suggested that geothermal water was in equilibrium at 250-260 °C. It likely has a deep and large reservoir located in the upper sheeted dike complex of the oceanic crust, just below the transition zone between andesite volcanic flows and the basaltic dikes. The upper dike section, from which Li is extracted by hydrothermal fluids, was characterized by light Li isotopic values in the rocks, indicating retention of ⁶Li by the altered rocks. For the Lamentin and Diamant areas, the geothermal fluids appeared to be in equilibrium with reservoir volcano-sedimentary rocks at 90-120 °C and 180 °C, respectively. Further evidence for this argument is provided by the fact that only the Na/Li thermometric relationship determined for sedimentary basins yielded temperature values in agreement with those measured or estimated for the reservoir fluids. This suggests the importance of a sedimentary signature in these reservoir rocks. Altogether, this study highlights that the use of Li isotopic systematics is a powerful tool for characterizing the origin of geothermal waters as well as the nature of their reservoir rocks.     

Relationship between plagioclase crystallization and cooling rate in basaltic melts

Rock textures commonly preserve a record of the near-surface crystallization history of volcanic rocks. Under conditions of simple cooling without convection or mixing, textures will reflect sample cooling rate, the temperature at which crystallization was initiated, and the distribution of mineral phase precipitation across the crystallization interval. Compilation of plagioclase size and number density data on natural (dike, sill and lava lake) and experimental samples suggests that (1) growth and nucleation rates of plagioclase in natural basaltic samples are a predictable function of cooling rate, and (2) the observed crystallization rate dependence on cooling rate is similar to that observed in experiments initiated at subliquidus temperatures. Comparison of natural and experimental samples thus suggests that most basalts crystallize under conditions of heterogeneous nucleation, with the number density of preexisting nucleii partially controlling textural responses to cooling rate changes. Time scales of crystallization and cooling in magmatic systems are intimately linked through a balance between heat removal from the system and heat evolved through crystallization. Evaluation of textural data in the context of recent numerical models of crystallization in simple (one- and two-component systems) provides new insight into regularities in the crystallization behavior of basaltic magmas. For example, the rate of change in crystal size (and number density, as dictated by mass balance) has been used as a measure of the relative importance of time scales of crystallization and cooling in numerical models of crystallizing systems. In natural samples, plagioclase size scales with the length scale of cooling such that a logarithmic plot of grain size as a function of normalized distance across the dike has a slope that appears approximately independent of dike width (solidification time). Comparison with available textural data for other phenocryst phases suggests that the same may be true for pyroxene and magnetite crystallization, with each phase having a characteristic slope probably controlled by the thermodynamic properties of the crystallizing phase. Measured crystal size distributions are unimodal and show maximum frequencies in the smaller size classes; distributions broaden and the grain size at peak frequency increases with increasing crystallization times (decreasing cooling rates). In contrast, partially crystallized Makaopuhi lava lake samples have crystal size distributions that decrease exponentially with increasing crystal size. Measured size distributions in dikes can be explained by late stage modification of Makaopuhi-type distributions through loss of small crystals, possibly the consequence of growth without nucleation. Finally, this compilation of the textural response of basaltic magmas to changes in cooling rate suggests that empirical calibrations of crystallization rate dependence on cooling rate from natural samples provide a reasonable model for plagioclase crystallization in near-surface basaltic systems. Predicted growth rates will be slow and relatively constant (10^-10–10^-11 cm/s) for crystallization times expected in most shallow volcanic systems (<1000 years).     

Anorthositic Rocks of the Duluth Complex: Examples of Rocks Formed from Plagioclase Crystal Mush

Anorthositic rocks compose 35–40% of the Middle Proterozoic(Keweenawan; 1?1 Ga) Duluth Complex—a large, compositemafic body in northeastern Minnesota that was intruded beneatha comagmatic volcanic edifice during the formation of the Midcontinentrift system. Anorthositic rocks, of which six general lithologictypes occur in one area of the complex, are common in an earlyseries of intrusions. They are characterized on a local scale(meters to kilometers) by nonstratiform distribution of rocktypes, variably oriented plagioclase lamination, and compositeintrusive relationships. Variably zoned, subhedral plagioclaseof nearly constant average An (60) makes up 82–98% ofthe anorthositic rocks. Other phases include granular to poikiliticolivine (Fo_66–38), poikilitic clinopyrox-ene (En'_73–37),subpoikilitic Fe-Ti oxides, and various late-stage and secondaryminerals. Whole-rock compositions of anorthositic rocks are modelled bymass balance to consist of three components: cumulus plagioclase(70–95 wt.%), minor cumulus olivine (0–5%), anda gabbroic postcumulus assemblage (5–27%) representinga trapped liquid. The postcumulus assemblage has textural andcompositional characteristics which are consistent with crystallizationfrom basaltic magma ranging from moderately evolved olivinetholeiite to highly evolved tholeiite (mg=60–25). Sympatheticvariations of mg in plagioclase and in mafic minerals suggestthat cumulus plagioclase, though constant in An, was in approximateequilibrium with the variety of basaltic magma compositionswhich produced the postcumulus assemblages. Standard models of mafic cumulate formation by fractional crystallizationof basaltic magmas in Duluth Complex chambers, although ableto explain the petrogenesis of younger stratiform troctoliticto gabbroic intrusions, are inadequate to account for the field,petrographic, and geochemical characteristics of the anorthositicrocks. Rather, we suggest an origin by multiple intrusions ofplagioclase crystal mushes—basaltic magmas charged withas much as 60% intratelluric plagioclase. The high concentrationsof cumulus plagioclase (70–95%) estimated to compose theanorthositic rocks may reflect expulsion of some of the transportingmagma during emplacement or early postcumulus crystallizationof only plagioclase from evolved hyperfeldspathic magma. Althoughthe evolved compositions of anorthositic rocks require significantfractionation of mafic minerals, geophysical evidence indicatesthat ultramafic rocks are, as exposure implies, rare in theDuluth Complex and implies that plagioclase crystal mushes werederived from deeper staging chambers. This is consistent withinterpretations of olivine habit and plagioclase zoning. Moreover,plagioclase could have been segregated from coprecipitatingmafic phases in such lower crustal chambers because of the buoyancyof plagioclase in basaltic magmas at high pressure. The geochemicaleffects of plagioclase suspension in basaltic magmas are consistentwith observed compositions of cumulus plagioclase in the anorthositicrocks and with the geochemical characteristics of many comagmaticbasalts. The petrogenesis of the anorthositic rocks and theoverall evolution of Keweenawan magmas can be related to thedynamics of intracontinental rift formation.     

Geochemical constraints on the origin of mafic and silicic magmas at Cordón El Guadal, Tatara-San Pedro Complex, central Chile

The aim of this study is to quantify the crustal differentiation processes and sources responsible for the origin of basaltic to dacitic volcanic rocks present on Cordón El Guadal in the Tatara-San Pedro Complex (TSPC). This suite is important for understanding the origin of evolved magmas in the southern Andes because it exhibits the widest compositional range of any unconformity-bound sequence of lavas in the TSPC. Major element, trace element, and Sr-isotopic data for the Guadal volcanic rocks provide evidence for complex crustal magmatic histories involving up to six differentiation mechanisms. The petrogenetic processes for andesitic and dacitic lavas containing undercooled inclusions of basaltic andesitic and andesitic magma include: (1) assimilation of garnet-bearing, possibly mafic lower continental crust by primary mantle-derived basaltic magmas; (2) fractionation of olivine + clinopyroxene + Ca-rich plagioclase + Fe-oxides in present non-modal proportions from basaltic magmas at ∼4–8 kbar to produce high-Al basalt and basaltic andesitic magmas; (3) vapor-undersaturated (i.e., P _H2O<P _TOTAL) partial melting of gabbroic crustal rocks at ∼3–7 kbar to produce dacitic magmas; (4) crystallization of plagioclase-rich phenocryst assemblages from dacitic magmas in shallow reservoirs; (5) intrusion of basaltic andesitic magmas into shallow reservoirs containing crystal-rich dacitic magmas and subsequent mixing to produce hybrid basaltic andesitic and andesitic magmas; and (6)␣formation and disaggregation of undercooled basaltic andesitic and andesitic inclusions during eruption from shallow chambers to form commingled, mafic inclusion-bearing andesitic and dacitic lavas flows. Collectively, the geochemical and petrographic features of the Guadal volcanic rocks are interpreted to reflect the development of shallow silicic reservoirs within a region characterized by high crustal temperatures due to focused basaltic activity and high magma supply rates. On the periphery of the silicic system where magma supply rates and crustal temperatures were lower, cooling and crystallization were more important than bulk crustal melting or assimilation. Received: 2 July 1997 / Accepted: 25 November 1997     

Fluorine speciation in topsoils of three active volcanoes of Sicily (Italy)

Fluorine is one of the many environmental harmful elements released by volcanic activity. The content of total oxalate-extractable and water-extractable fluorine was determined in 96 topsoils of three active volcanic systems of southern Italy (Mt Etna, Stromboli and Vulcano). Total fluorine (F) content (F _TOT) ranges from 112 to 7,430 mg kg⁻¹, F extracted with oxalate (F _OX) ranges from 16 to 2,320 mg kg⁻¹ (2–93% of F _TOT) and F extracted with distilled water ( ) ranges from 1.7 to 159 mg kg⁻¹ (0.2–40 % of F _TOT). Fluorine in the sampled topsoils derives both from the weathering of volcanic rocks and ashes and from the enhanced deposition due to volcanic gas emissions either from open-conduit passive degassing (Mt Etna and Stromboli) or from a fumarolic field (Vulcano). Fluorine accumulation in the studied soils does not generally present particular environmental issues except for a few anomalous sites at Vulcano, where measured contents could be dangerous both for vegetation and for grazing animals.     

CO2 solubility in Martian basalts and Martian atmospheric evolution

To understand possible volcanogenic fluxes of CO₂ to the Martian atmosphere, we investigated experimentally carbonate solubility in a synthetic melt based on the Adirondack-class Humphrey basalt at 1-2.5 GPa and 1400-1625 °C. Starting materials included both oxidized and reduced compositions, allowing a test of the effect of iron oxidation state on CO₂ solubility. CO₂ contents in experimental glasses were determined using Fourier transform infrared spectroscopy (FTIR) and Fe³⁺/Fe^T was measured by Mössbauer spectroscopy. The CO₂ contents of glasses show no dependence on Fe³⁺/Fe^T and range from 0.34 to 2.12 wt.%. For Humphrey basalt, analysis of glasses with gravimetrically-determined CO₂ contents allowed calibration of an integrated molar absorptivity of 81,500 ± 1500 L mol⁻¹ cm⁻² for the integrated area under the carbonate doublet at 1430 and 1520 cm⁻¹. The experimentally determined CO₂ solubilities allow calibration of the thermodynamic parameters governing dissolution of CO₂ vapor as carbonate in silicate melt, K_II, (Stolper and Holloway, 1988) as follows: , ΔV⁰ = 20.85 ± 0.91 cm³ mol⁻¹, and ΔH⁰ = −17.96 ± 10.2 kJ mol⁻¹. This relation, combined with the known thermodynamics of graphite oxidation, facilitates calculation of the CO₂ dissolved in magmas derived from graphite-saturated Martian basalt source regions as a function of P, T, and f_O2. For the source region for Humphrey, constrained by phase equilibria to be near 1350 °C and 1.2 GPa, the resulting CO₂ contents are 51 ppm at the iron-wüstite buffer (IW), and 510 ppm at one order of magnitude above IW (IW + 1). However, solubilities are expected to be greater for depolymerized partial melts similar to primitive shergottite Yamato 980459 (Y 980459). This, combined with hotter source temperatures (1540 °C and 1.2 GPa) could allow hot plume-like magmas similar to Y 980459 to dissolve 240 ppm CO₂ at IW and 0.24 wt.% of CO₂ at IW + 1. For expected magmatic fluxes over the last 4.5 Ga of Martian history, magmas similar to Humphrey would only produce 0.03 and 0.26 bars from sources at IW and IW + 1, respectively. On the other hand, more primitive magmas like Y 980459 could plausibly produce 0.12 and 1.2 bars at IW and IW + 1, respectively. Thus, if typical Martian volcanic activity was reduced and the melting conditions cool, then degassing of CO₂ to the atmosphere may not be sufficient to create greenhouse conditions required by observations of liquid surface water. However, if a significant fraction of Martian magmas derive from hot and primitive sources, as may have been true during the formation of Tharsis in the late Noachian, that are also slightly oxidized (IW + 1.2), then significant contribution of volcanogenic CO₂ to an early Martian greenhouse is plausible.     

Experimental determination of the role of diffusion on Li isotope fractionation during basaltic glass weathering

In order to use lithium isotopes as tracers of silicate weathering, it is of primary importance to determine the processes responsible for Li isotope fractionation and to constrain the isotope fractionation factors caused by each process as a function of environmental parameters (e.g. temperature, pH). The aim of this study is to assess Li isotope fractionation during the dissolution of basalt and particularly during leaching of Li into solution by diffusion or ion exchange. To this end, we performed dissolution experiments on a Li-enriched synthetic basaltic glass at low ratios of mineral surface area/volume of solution (S/V), over short timescales, at various temperatures (50 and 90 °C) and pH (3, 7, and 10). Analyses of the Li isotope composition of the resulting solutions show that the leachates are enriched in ⁶Li (δ⁷Li = +4.9 to +10.5‰) compared to the fresh basaltic glass (δ⁷Li = +10.3 ± 0.4‰). The δ⁷Li value of the leachate is lower during the early stages of the leaching process, increasing to values close to the fresh basaltic glass as leaching progresses. These low δ⁷Li values can be explained in terms of diffusion-driven isotope fractionation. In order to quantify the fractionation caused by diffusion, we have developed a model that couples Li diffusion with dissolution of the glassy silicate network. This model calculates the ratio of the diffusion coefficients of both isotopes (a = D₇/D₆), as well as its dependence on temperature, pH, and S/V. a is mainly dependent on temperature, which can be explained by a small difference in activation energy (0.10 ± 0.02 kJ/mol) between ⁶Li⁺ and ⁷Li⁺. This temperature dependence reveals that Li isotope fractionation during diffusion is low at low temperatures (T < 20 °C), but can be significant at high temperatures. However, concerning hydrothermal fluids (T > 120 °C), the dissolution rate of basaltic glass is also high and masks the effects of diffusion. These results indicate that the high δ⁷Li values of river waters, in particular in basaltic catchments, and the fractionated values of hydrothermal fluids are mainly controlled by precipitation of secondary phases.     

Bromine cycle in subduction zones through in situ Br monitoring in diamond anvil cells

The geochemical partitioning of bromine between hydrous haplogranitic melts, initially enriched with respect to Br and aqueous fluids, has been continuously monitored in situ during decompression. Experiments were carried out in diamond anvil cells from 890 °C to room temperature and from 1.7 GPa to room pressure, typically from high P, T conditions corresponding to total miscibility (presence of a supercritical fluid). Br contents were measured in aqueous fluids, hydrous melts and supercritical fluids. Partition coefficients of bromine were characterized at pressure and temperature between fluids, hydrous melts and/or glasses, as appropriate: D^Br_fluid/melt = (Br)_fluid/(Br)_melt, ranges from 2.18 to 9.2 ± 0.5 for conditions within the ranges 0.66-1.7 GPa, 590-890 °C; and D^Br_fluid/glass = (Br)_fluid/(Br)_glass ranges from 60 to 375 at room conditions. The results suggest that because high pressure melts and fluids are capable of accepting high concentrations of bromine, this element may be efficiently removed from the slab to the mantle source of arc magmas. We show that Br may be highly concentrated in subduction zone magmas and strongly enriched in subduction-related volcanic gases, because its mobility is strongly correlated with that of water during magma degassing. Furthermore, our experimental results suggest that a non negligible part of Br present in the subducted slab may remain in the down-going slab, being transported toward the transition zone. This indicates that the Br cycle in subduction zones is in fact divided in two related but independent parts: (1) a shallower one where recycled Br may leave the slab with a water and silica-bearing “fluid” leading to enriched arc magmas that return Br to the atmosphere. (2) A deeper cycle where Br may be recycled back to the mantle maybe to the transition zone, where it may be present in high pressure water-rich metasomatic fluids.     

The Pb-Pb age of Angrite SAH99555 revisited

Representing a suite of well-preserved basaltic meteorites with reported ages from 4566.18 ± 0.14 Ma to 4557.65 ± 0.13 Ma, angrites have been recurring targets for cross-calibrating extinct and absolute chronometers. However, inconsistencies exist in the available chronological data set, including a 4566.18 ± 0.14 Ma Pb-Pb age reported by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature436, 1127-1131] for Sahara 99555 (herein SAH99555) that is significantly older than a Pb-Pb age for D’Orbigny, despite the two meteorites yielding indistinguishable Hf-W and Mn-Cr ages. We re-evaluate the Pb-Pb age of SAH99555 using a stepwise dissolution procedure on a whole rock fragment and a pyroxene separate. The combined data set yields a linear array that reflects a mixture of radiogenic Pb and terrestrial contamination and corresponds to an age of 4564.58 ± 0.14 Ma, which is 1.60 ± 0.20 Ma younger than that reported by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature436, 1127-1131]. Our conclusion that SAH99555 crystallized at 4564.58 ± 0.14 Ma requires that all initial Pb was removed in the first progressive dissolution steps, an assertion supported by linearity of data generated by stepwise dissolution of a single fragment and the removal of an obvious highly-radiogenic component early in the dissolution process. We infer that the linear array defined by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature436, 1127-1131] and their older age reflects a ternary mixture of Pb with constant relative proportions of highly-radiogenic initial Pb and radiogenic Pb with varying amounts of a terrestrial contamination. This requires that the phase harboring the initial Pb is insoluble in 2 M HCl, the only acid applied to the samples by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature436, 1127-1131] prior to dissolution.     

Volatiles in pillows of the Mid-Ocean-Ridge-Basalt (MORB) and vitreous basaltic rims

Temperature-resolved analyses of volatiles from Mid-Ocean-Ridge-Basalt (MORB) and vitreous basaltic rims were carried out to investigate the total volatile contents of basaltic melts and the influence of magma contamination on the degassing behaviour of volcanic rocks.With respect to the sources of methane evolution from the MORB the investigations are taken into consideration, the hydrocarbon (HC) release especially from the melt.The current paper presents data for H₂O, CO₂, SO₂, He, H₂, HF, HCl, CO, N₂, O₂, and HC degassing profiles of samples from the MORB sampling cruise 02.10.1983-11.11.1983 with FS Sonne 28 during the GEMINO-1 project near the Carlsberg Ridge (CR) and the Mid-Indian-Ocean-Ridge (MIOR).It aims to estimate the magnitude and nature of source magma volatiles and contamination (crustal material, seawater, atmospheric gases).The degassing of H₂O, CO₂, HCs as well as sulphur and chlorine species, or O₂ from vitreous specimens shows characteristic differences associated with sample position with respect to the lava surface.From the water release by bubbling and diffusion above 700 °C it must be concluded that any assimilation of sea water in vitreous rim is very low. The water content in the vitreous rim is about 0.1-0.2 wt%. The low interaction of melt with sea water is supported by the missing of a significant release of chlorine species during the heat treatment of the sample up to 1450 °C.Mixed H₂O/CO₂ bubbles escape between 700 and 800 °C from the vitreous rim. The CO₂ release in the temperature range of 1060-1170 °C from the basalt and the vitreous rim is interpreted as an indication for the primary carbon-dioxide content in the melt.Above 1100 °C CO₂ and SO₂ are evolved by both diffusion and small bubbles. The quantities of CO₂ in the vitreous rim and the basalt are similar (between 0.05 and 0.15 wt%), whereas the quantities of SO₂ escaping both from the vitreous rim and the crystalline basalt are between 0.013 and 0.024 wt%.Simultaneous with the CO₂ release by bubbling, HC species, especially CH fragments, were observed. The fact that the temperature of release maxima are above 1050 °C in both the vitreous rim and in the basalt is an indication for a geogenetic origin of HCs, e.g. methane.A low temperature of release for methane, which is consistent with biogenetic HC, was observed from the gas-release profiles of the basalts only. The maxima of the low-temperature gas releases are between 80 and 200 °C with a high correlation between the fragments m/z 13 and m/z 15. This correlation is a significant indication for a methane release.The oxygen release profiles of vitreous and crystalline basalts give significant indications for oxygen fugacity below the (QMF) of basaltic magma.Secondary minerals, generated by alteration of basaltic rocks, can be characterized by gas release profiles (GRPs) due to their decomposition in the temperature range below 800 °C. Only in the basalt were there observed indications of alteration processes. Small traces of carbonates (<0.0001 wt%) were detected by the gas release during the decomposition.Processes of degassing at temperatures higher than 800 °C are correlated to volatiles in the melt and to fluid inclusions of the minerals. There are no obvious correlations in the degassing characteristics between H₂O, CO₂ and SO₂. The different maxima of the degassing velocity, especially of CO₂, and SO₂, are indications of the different bonding forces of the site occupancy of the volatiles in the melt and in the glass. A micelle model for bonding sites in the basaltic glass for dissolved volatiles is discussed.     

The Occurrence of Forsterite and Highly Oxidizing Conditions in Basaltic Lavas from Stromboli Volcano, Italy

We report the occurrence of unusual, high-magnesium (Fo₉₆) olivinephenocrysts in a basaltic lava and an ejected lithic block fromthe Upper Vancori period (13 ka) and the recent activity (2002–2003)of Stromboli volcano, Italy. The samples that contain this distinctivemineral chemistry are a shoshonitic basalt and a basaltic andesitewith anomalous bulk-rock chemical characteristics in which theiron is highly oxidized (6–8 wt % Fe₂O₃ and <1 wt %FeO). In other respects these samples are similar to the majorityof Stromboli basalts, characterized by the coexistence of olivine,clinopyroxene, plagioclase and Fe–Ti oxides as phenocrysts,and clinopyroxene, plagioclase and Fe–Ti oxides in thegroundmass. In the high-magnesium olivine samples, Fe–Tioxides (pseudobrookite) typically occur as symplectitic intergrowthswith the olivine phenocrysts, indicating simultaneous growthof the two phases. We propose, as a paragenetic model, thatthe Fo₉₆ olivine phenocrysts crystallized from a highly oxidizedbasaltic magma in which most of the iron was in the ferric state;hence, only magnesium was available to form olivine. The highlyoxidized state of the magma reflects sudden degassing of volatilephases associated with instantaneous, irreversible, transientdegassing of the magma chamber; this is postulated to occurduring periods of sudden decompression induced by fracturingof the volcanic edifice associated with paroxysmic activityand edifice collapse. KEY WORDS: Stromboli; Mg-rich olivine; oxygen fugacity; redox state of magmas; irreversible processes     

Delayed fractionation of basaltic lavas

The phenocryst cores of the basaltic lavas from Jan Mayen and Hawaii display a range in compositions. The textural features of the phenocrysts also vary, both euhedral and skeletal phenocrysts are present in the same thin section. Apparently the basaltic magmas underwent crystallization within a temperature interval of 50–200° C before they became fractionated. The fractionates of basaltic lavas are therefore average compositions of the phenocryst assemblages rather than liquidus compositions. This type of fractionation is called delayed fractionation. It is considered that most tholeiitic and alkalic basaltic lavas undergo delayed fractionation.