Search

Abstract: The Newtonian viscosity of molten peridotite has been determined experimentally at superliquidus and supercooled conditions. The high-temperature determinations were obtained using a concentric cylinder technique employing constant high-speed deformation. The low-temperature determinations have been obtained from the analysis of the glass transition in scanning calorimetric traces and conversion via published shift factors into viscosity data. These latter measurements were made possible by the experimental synthesis of peridotite glass using a splat-quenching device. Despite having an extremely low viscosity near its liquidus temperature (10−1 Pa s), peridotite exhibits a very high glass transition temperature, 1006 to 1018 K (depending on scanning rates), at which viscosities of 1010.13 to 1010.73Pa s were calculated. These data show that the viscosity of molten peridotite has an extremely non-Arrhenian temperature dependence and allow its viscosity to be predicted at the even higher temperatures expected to exist where molten peridotite is or was present in the mantle. [Copyright &y& Elsevier]

Presents information on a study which determined the viscosities of a series of granitic (sensu lato) melts. Methodology of the study; Results and discussion on the study; Conclusion.

Presents a study on mineralogy, investigating the shear viscosities of alkaline melts in the system Na-Fe-Si-O-F-Cl, over a wide range of temperatures (400-1200 ...C) at 1 atm pressure in air. Methodology used to conduct the study; Information on the oxidation state of Fe in the melts; Details on the procedure used for obtaining the low range viscosities; Findings of the study.

Presents an equation for the calculation of the shear viscosity of hydrous leucogranitic melts based on non-Arrhenian temperature dependence on viscosity. Sources of data; Viscosities of hydrous, metaluminous melts used to construct the model; Comparison of the non-Arrhenian temperature dependence with other methods for the calculation of hydrous granitic melt viscosities.

Abstract: Glass transition temperatures (T g) have been determined for natural multicomponent melts using differential scanning calorimetry. Trachytic, dacitic, phonolitic and basaltic base compositions have been analysed over a range of water contents up to 3.75 wt.%. For each sample T g has been obtained over a range of cooling/heating rates using the extrapolated onset and the peak temperatures in heat capacity–temperature curves. T g of all compositions are strongly reduced by increasing water content, particularly for the first 1 wt.% added. Base composition also has an effect, with the lowest T g occurring in the peralkaline phonolite suite. For all samples a clear dependence on the cooling/heating rate has been recorded. These results have been compared with rheological investigations on the same samples. On the basis of the equivalence of the shear and enthalpic relaxation process timescales we provide a method to predict the shear viscosity at the glass transition for all the melts investigated in this study, both dry and hydrous. Our determinations of T g provide a lower limit for the time–temperature envelope that gives rise to densely welded deposits and constraints on their emplacement temperature. Furthermore, by using the viscosity values predicted at the glass transition, we suggest that welding processes may occur over timescales on the order of tens of seconds to tens of minutes at T g. [Copyright &y& Elsevier]

Abstract: The Newtonian viscosity of water-plus-fluorine-bearing silicate melt of haplogranitic composition (HPG8) has been determined. Viscosities of HPG8 melt with addition of 3.11 and 4.25 wt.% of F and up to 3 wt.% H2O have been obtained using a micropenetration technique in the interval 109.74 to 1011.84 Pa · s and temperatures varying from 370 to 700°C, at ambient pressure. Determination of the temperature dependence of viscosity from this and previous studies permits the parameterization of the viscosity of melts containing water and fluorine, having similar composition, within a 0.3 log units standard error. The viscosity of water-bearing, F-rich haplogranitic samples is represented by a modified Vogel-Fulcher-Tammann (VFT) equation which provides a non-Arrhenian description of the temperature dependence of the viscosity. The results of this study indicate that, taken individually or together, both H2O and F− have a strong and similar effect on the viscosity of SiO2-rich compositions. This similarity between F2O−1 and H2O greatly simplifies the task of predicting viscosity for volatile-rich, highly silicic magmas. The low viscosities of hydrous fluorine-bearing granitic melts favour efficient crystallization-fractionaction paths for these liquids, controlling degassing paths and consequently the eruptive behaviour. Numerical simulations of eruptive events normally do not take into account the contribution of fluorine; this may introduce a significant error in the description of the fluid-dynamic properties of magma and, therefore, in the accurate prediction of eruptive scenarios, as well as in hazard assessment studies. Fluorine, unlike water, remains dissolved in the melt at high concentrations and low confining pressures. The incorporation of fluorine data and the modelling of fluorine-bearing viscosity data are therefore of fundamental importance for simulations of magma dynamics and prediction of eruptive scenarios. [Copyright &y& Elsevier]

During the four month-long 2018 Kilauea Lower East Rift Zone (LERZ) eruption, the bulk chemical compositions of magma ranged from basalt to andesite. This compositional variety was reflected in eruptive style, which ranged from Hawaiian fountaining to Strombolian explosions. Here, we quantified the evolution of the melt viscosity of the eruptive products through high-temperature laboratory experiments performed on a representative sample set that was collected in the field immediately after the eruptive series. This suite of 18 samples comprises all major eruptive phases (early phase I, late phase I, phase II, phase III, fissure 17). The results illustrate the significant rheological variability of the eruptive products, and appear to link to variations in eruption dynamics. We propose a new standard for the rheological study of a multi-episode effusive eruption, whereby precise, near-real-time viscosity results are obtained during ongoing eruptions will become a routine component of volcano monitoring during future eruptive events. During the 2018 eruption of Kilauea, emerging magma spanned a wider compositional range than ever previously observed during a single eruption. This compositional diversity was matched by a variety in eruptive styles, which ranged from more persistent fountaining to short-lived explosions. Immediately after the eruption ceased, we collected a representative suite of 18 samples in the field, which comprises all major eruptive phases (early phase I, late phase I, phase II, phase III, fissure 17). We measured the melt viscosity of such samples through high-temperature laboratory experiments. The results illustrate a significant variability in viscosity, which is linked to the highly variable eruption dynamics. Here we propose a new standard for the study of multi-episode effusive eruptions from a viscosity standpoint. We hope and expect that this methodology will become routine practice during future eruption. [ABSTRACT FROM AUTHOR]

Storage and transport of silicate melts in the Earth's crust and their emplacement on the planet's surface occur almost exclusively at sub-liquidus temperatures. At these conditions, the melts undergo crystallization under a wide range of cooling-rates, deformation-rates, and oxygen fugacities ( f O 2 ). Oxygen fugacity is known to influence the thermodynamics and kinetics of crystallization in magmas and lavas. Yet, its influence on sub-liquidus rheology remains largely uncharted. We present the first rheological characterization of crystallizing lavas along natural cooling paths and deformation-rates and at varying f O 2 . Specifically, we report on apparent viscosity measurements for two crystallizing magmatic suspensions 1) at log ⁡ f O 2 of −9.15 (quartz–fayalite–magnetite buffer, QFM, −2.1) and 2) in air. These fugacities span a range of reduced to oxidized conditions pertinent to magma migration and lava emplacement. We find that: 1) crystallization at constant cooling-rates results in a quasi-exponential increase in the apparent viscosity of the magmatic suspensions until they achieve their rheological cut off temperature ( T cutoff ), where the melt effectively solidifies 2) the rheological departure and T cutoff increase with increasing f O 2 and 3) increasing f O 2 results in decreased crystallization-rates. Based on the experimental results and by comparison with previous rheological isothermal studies we propose a generalisation of the effect of f O 2 on the dynamic rheological evolution of natural magmatic and volcanic suspensions. We further discuss the implications for magmatic transport in plumbing and storage systems (e.g. conduits, dikes and magma chambers) and during lava flow emplacement. [ABSTRACT FROM AUTHOR]

Abstract: In planetary sciences, the emplacement of lava flows is commonly modelled using a single rheological parameter (apparent viscosity or apparent yield strength) calculated from morphological dimensions using Jeffreysʼ and Hulmeʼs equations. The rheological parameter is then typically further interpreted in terms of the nature and chemical composition of the lava (e.g., mafic or felsic). Without the possibility of direct sampling of the erupted material, the validity of this approach has remained largely untested. In modern volcanology, the complex rheological behaviour of lavas is measured and modelled as a function of chemical composition of the liquid phase, fractions of crystals and bubbles, temperature and strain rate. Here, we test the planetary approach using a terrestrial basaltic lava flow from the Western Volcanic Zone in Iceland. The geometric parameters required to employ Jeffreysʼ and Hulmeʼs equations are accurately estimated from high-resolution HRSC-AX Digital Elevation Models. Samples collected along the lava flow are used to constrain a detailed model of the transient rheology as a function of cooling, crystallisation, and compositional evolution of the residual melt during emplacement. We observe that the viscosity derived from the morphology corresponds to the value estimated when significant crystallisation inhibits viscous deformation, causing the flow to halt. As a consequence, the inferred viscosity is highly dependent on the details of the crystallisation sequence and crystal shapes, and as such, is neither uniquely nor simply related to the bulk chemical composition of the erupted material. This conclusion, drawn for a mafic lava flow where crystallisation is the primary process responsible for the increase of the viscosity during emplacement, should apply to most of martian, lunar, or mercurian volcanic landforms, which are dominated by basaltic compositions. However, it may not apply to felsic lavas where vitrification resulting from degassing and cooling may ultimately cause lava flows to halt. [Copyright &y& Elsevier]

Abstract: Rheological variations have been postulated as the cause of transitions from effusive to explosive volcanic eruption style. Rheology is integrally linked to the composition and textural state (porosity, crystallinity) of magma as well as the stress, temperature and strain rate operative during flow. This study characterises the rheological behaviour and, importantly, the evolution of physical properties of two magmas (with different crystallinity and porosity) from Volcán de Colima (Mexico) — a volcanic system known for its rapid fluctuations in eruption style. Magma samples deformed in a uniaxial press at a constant stress of 2.8, 12 or 24MPa, a constant temperature of 940–945°C (comparable to upper conduit or lava dome conditions) to strains of 20 or 30% displayed different mechanical behaviour and significant differences in measured strain rates (10−2–10−5 s−1). The evolution of porosity, permeability, dynamic Young's modulus and dynamic Poisson's ratio illustrate a complex evolution of the samples manifested as strain-hardening, visco-elastic, constant-rate and strain-weakening deformation. Both magmas behave as shear-thinning non-Newtonian liquids and viscosity decreases as a function of strain. We find that strain localisation during deformation leads to the rearrangement and closure of void space (a combination of pores and cracks) followed by preferentially aligned fracturing (in the direction of the maximum principal stress) to form damage zones as well as densification of other areas. In a dome setting, highly viscous, low permeability magmas carry the potential to block volcanic conduits with a magma plug, resulting in the build-up of pressures in the conduit. Above a certain threshold of strain (dependent upon stress/strain rate), the initiation, propagation and coalescence of fractures leads to mechanical degradation of the magma samples, which then supersedes magmatic flow and crystal rearrangement as the dominant form of deformation. This results in lower apparent viscosities than those anticipated for magma of such crystallinity, especially at high strain rates. In a lava dome, this could result in dome collapse and the concomitant depressurisation could trigger an explosive eruption. [Copyright &y& Elsevier]

Abstract: The viscosity of pantelleritic melts from the Khaggiar lava flow (5.5ka — Pantelleria Island) was investigated as a function of temperature and water content. High (1673–1323K) and low (973–613K) temperature dry and hydrous liquid viscosities were determined by a combination of concentric cylinder (102.44 to 104.56 Pas) and micropenetration (108.67 to 1011.37) viscometric techniques. The effect of water, from 0.02 to 3.55wt.% H2O, was explored in the temperature range of 973–613K. Liquid viscosities have been parameterized by means of a modified Vogel–Fulcher–Tammann equation (VFT) which describes viscosities and derivative properties (glass transition temperature Tg, fragility m) of silicic liquids as a function of T–X (H2O). The results yield the expected strong decrease of viscosity with temperature and water content. Fragility, as a measure of the deviation from Arrhenian behavior, increases with H2O content from m =23 to m =28. The peralkalinity (molar alkali excess over alumina), which characterizes the pantelleritic magmas, exerts a primary control over the rheological behavior of these melts. The excess of alkalies over alumina content is responsible for the peculiarly low viscosities of pantelleritic liquids compared to their metaluminous counterpart in the high temperature range, and leads to an even more dramatic decrease in the viscosities of these rhyolites in the low temperature range. Comparison with available models shows substantial differences between the measured and calculated viscosities. Based on our new viscosity results combined with existing literature data, we propose a new parameterization based on a modified Vogel–Fulcher–Tammann equation, accounting for the effect of water and composition, which can be used to determine the viscosity of pantelleritic melts. The derived relationship reproduces the experimental data (58 in total) in the viscosity range from 102.44 to 1011.37 Pas and in the temperature range from 613 to 1673K within a root-mean-square-error (RMSE) of 0.12 log units. Application of this calculation model for high temperature low viscosity hydrous melt is limited by the lack of experimental data and needs verification by additional measurements. Volcanological implications for welding and rheomorphic processes as well as conduit dynamic modeling are also discussed in light of the results presented in this work. [Copyright &y& Elsevier]

Abstract: Viscosities of shoshonitic and latitic melts, relevant to the Campi Flegrei caldera magmas, have been experimentally determined at atmospheric pressure and 0.5GPa, temperatures between 840K and 1870K, and H2O contents from 0.02 to 3.30wt.%. The concentric cylinder technique was employed at atmospheric pressure to determine viscosity of nominally anhydrous melts in the viscosity range of 101.5 −103 Pas. The micropenetration technique was used to determine the viscosity of hydrous and anhydrous melts at atmospheric pressure in the high viscosity range (1010 Pas). Falling sphere experiments were performed at 0.5GPa in the low viscosity range (from 100.35 to 102.79 Pas) in order to obtain viscosity data of anhydrous and hydrous melts. The combination of data obtained from the three different techniques adopted permits a general description of viscosity as a function of temperature and water content using the following modified VFT equation: where η is the viscosity in Pa·s, T the temperature in K, w the H2O content in wt.%, and a, b, c, d, e, and g are the VFT parameters. This model reproduces the experimental data (95 measurements) with a 1σ standard deviation of 0.19 and 0.22 log units for shoshonite and latite, respectively. The proposed model has been applied also to a more evolved composition (trachyte) from the same area in order to create a general model applicable to the whole compositional range of Campi Flegrei products. Moreover, speed data have been used to constrain the ascent velocity of latitic, shoshonitic, and trachytic melts within dikes. Using petrological data and volcanological information (geometrical parameters of the eruptive fissure and depth of magma storage), we estimate a time scale for the ascent of melt from 9km to 4km depth (where deep and shallow reservoirs, respectively, are located) in the order of few minutes. Such a rapid ascent should be taken into account for the hazard assessment in the Campi Flegrei area. [Copyright &y& Elsevier]

Abstract: Transitions between effusive and explosive styles of lava dome eruptions are likely accompanied by changes in lava rheology. The common presence of crystals in dome lavas produces a complex non-Newtonian rheology. Thus models of such complex rheology are essential for volcanic eruption models. Here, we have measured the rheology of natural Unzen lavas with a compressive uniaxial press operating at stresses between 1 and 70 MPa and temperatures between 940 and 1010 °C. Crystal-rich Unzen lavas are characterised by two essential rheological features which produce non-Newtonian effects. The first is an instantaneous response of the apparent viscosity to applied stress which requires that the magma be described as a visco-elastic fluid that exhibits shear-thinning. The second effect takes the form of a time-dependence of the viscosity at moderate to high stress (≥10 MPa). In this regime, the apparent viscosity slowly decreases as increasing fracturing of the phenocrysts and the groundmass occurs. Fragmentation of crystals and alignment of crystal fragments are observed to produce flow banding-effects which in turn lower the apparent viscosity of natural dome lavas. Ultimately, deformation may lead to complete rupture of the lava if the stress is sufficient. Cracking thus stands as an important process in natural dome lava rheology. The ubiquitous non-Newtonian rheology of dome lavas, observed experimentally here, needs to be adequately treated in order to generate appropriate eruption models. [Copyright &y& Elsevier]

Abstract: The 5th April 2003 paroxysmal event was the strongest explosion that has occurred at Stromboli in the last 50 years. This event lasted only few minutes and was characterised by two violent explosions, followed by gas and pyroclast emission. In order to constrain models of the dynamics of the paroxystic event the viscosity of anhydrous and hydrous Stromboli high potassium (HK)-basaltic melts have been measured. Viscosity has been investigated in the low viscosity range with the falling sphere method at superliquidus temperatures (1423 to 1673 K) and 0.5 GPa and in the high viscosity range with micropenetration near the glass transition temperature (723 to 1035 K) at atmospheric pressure. Falling sphere experiments were performed in a piston cylinder apparatus with melts whose water content varies from nominally anhydrous (0.02 wt.% H2O) to 4.16 wt.% H2O. The combination of high- and low-viscosity data permits a general description of the viscosity as a function of temperature and water contentusing a modified Tamman–Vogel–Fulcher equation. Using these new viscosity data, an estimation of the flow regime and magma velocity is performed. Our data suggest that the ascent of magma from the 7–8 km deep reservoir to a shallower reservoir located at about 3 km of depth, may occur within minutes. Moreover, we infer a turbulent flow regime. Finally, our estimates of the ascent velocity agree qualitatively with results from petrological studies (e.g. [Bertagnini, A., Métrich, N., Landi, P., Rosi, M., 2003. Stromboli volcano (Aeolian Archipelago, Italy): an open window on the deep-feeding system of a steady state basaltic volcano. Journal of Geophysical Research 108, 2336–2350.]), which indicate a turbulent flow regime and rapid ascent velocities such to inhibit volatile-loss-induced crystallization. We conclude that hazard evaluation at Stromboli Island should incorporate the likelihood of very rapid ascent of less-evolved melts from depth. [Copyright &y& Elsevier]

Abstract: The Newtonian shear viscosity and glass transition temperature (T g) of hydrous melts in the system Anorthite (An)–Diopside (Di)–H2O have been experimentally obtained. Viscosity data on hydrous samples with up to ca. 3 wt.% H2O have been measured using a micropenetration technique in the interval between 108.3 and 1013.1 Pa s and temperatures up to 880 °C at ambient pressure. Measurements of the calorimetric T g values were performed by using differential scanning calorimetry (DSC). For each sample the cooling rate dependence of T g was characterised at three different temperatures corresponding to the onset, the peak and the stable liquid regions of the heat capacity curves. These results show strong correlations between these temperatures that can probably be extrapolated to other unequivocally defined metrics of the glass transition interval. Comparison with viscosity data obtained on the same samples shows that glass transition temperatures at each single heating/cooling rate reflect constant viscosity values for these hydrous liquids. Thus observed relationship between calorimetric T g and viscosity is independent of composition and water content (c.f., [Giordano, D., Nichols, A.R., Dingwell, D.B. (2005) Glass transition temperatures of natural hydrous melts: a relationship with shear viscosity and implications for the welding process. J. Volcanol. Geotherm. Res. 142, 105–118.]). Measured and calculated viscosities and glass transition temperatures for melts in the An–Di–H2O system show substantial differences with those of basaltic composition, suggesting that, despite what commonly assumed, An–Di is not a good rheological proxy for basaltic compositions. The observed differences are reduced at high temperature in the low viscosity range and are significantly more pronounced at low temperature. We infer that such an effect is due to the different contributions to the configurational entropy provided by the simple melts in the An–Di–H2O system compared to the multicomponent basaltic melt investigated. Some implications about the role of water in influencing melt properties are discussed. The results provided here demonstrated that, in some instances, extrapolating the physical properties of simple systems to those of natural multicomponent melts is not appropriate and may result in erroneous evaluation of petrological and volcanological scenarios which require knowledge of those properties. [Copyright &y& Elsevier]

Abstract: We present new viscosity measurements for melts spanning a wide range of anhydrous compositions including: rhyolite, trachyte, moldavite, andesite, latite, pantellerite, basalt and basanite. Micropenetration and concentric cylinder viscometry measurements cover a viscosity range of 10−1 to 1012 Pas and a temperature range from 700 to 1650 °C. These new measurements, combined with other published data, provide a high-quality database comprising ∼800 experimental data on 44 well-characterized melt compositions. This database is used to recalibrate the model proposed by Giordano and Dingwell [Giordano, D., Dingwell, D.B., 2003a. Non-Arrhenian multicomponent melt viscosity: a model. Earth Planet. Sci. Lett. 208, 337–349] for predicting the viscosity of natural silicate melts. The present contribution clearly shows that: (1) the viscosity (η)–temperature relationship of natural silicate liquids is very well represented by the VFT equation [log η = A + B /(T − C)] over the full range of viscosity considered here, (2) the use of a constant high-T limiting value of melt viscosity (e.g., A) is fully consistent with the experimental data, (3) there are 3 different compositional suites (peralkaline, metaluminous and peraluminous) that exhibit different patterns in viscosity, (4) the viscosity of metaluminous liquids is well described by a simple mathematical expression involving the compositional parameter (SM) but the compositional dependence of viscosity for peralkaline and peraluminous melts is not fully controlled by SM. For these extreme compositions we refitted the model using a temperature-dependent parameter based on the excess of alkalies relative to alumina (e.g., AE/SM). The recalibrated model reproduces the entire database to within 5% relative error (e.g., RMSE of 0.45 logunits). [Copyright &y& Elsevier]

Abstract: The viscosity of natural liquids representative of the glassy portion of pumice collected from the deposits of the Campanian Ignimbrite (IGC) and Monte Nuovo (MNV) eruption of Phlegrean Fields has been measured in the temperature range from 1770 K down to the glass transition, and for a dissolved water content range from dry to nearly 4 wt.%. Measurements were performed by a combination of techniques involving concentric cylinder and micropenetration apparatuses, depending on the specific viscosity range. These measurements, together with those made on samples from the Agnano Monte Spina (AMS) eruption of Phlegrean Fields presented in a companion paper, represent the first viscosity determinations for natural trachytic liquids. Liquid viscosities have been parameterized by means of a modified VFT equation that allows the calculation of viscosity as a function of temperature and water content. Calculated viscosities are compared with those pertaining to natural liquids of phonolitic, rhyolitic, and trachybasaltic composition, showing that trachytes are intermediate between rhyolites and phonolites, consistent with the dominant eruptive style associated with the different magma compositions (mainly explosive for rhyolites and trachytes, either explosive or effusive for phonolites, mainly effusive for basalts). Compositional diversity among the analyzed trachytes corresponds to liquid viscosity differences of one to two orders of magnitude, with higher viscosities approaching that of rhyolite at the same temperature–water content conditions. All hydrous natural trachytes and phonolites become indistinguishable when isokom temperatures (i.e., temperatures corresponding to the same viscosity) are plotted versus a compositional parameter given by the molar ratio on an element basis (Si+Al)/(Na+K+H). In contrast, rhyolitic and basaltic liquids display distinct trends, with the more fragile basaltic liquids crossing the curves pertaining to all other compositions. [Copyright &y& Elsevier]