Behrens, Harald

In this study we present the compositional changes of clinopyroxene (cpx), plagioclase (plg), spinel (sp), and glass experimentally solidified from an Icelandic MORB melt. The starting material was cooled at Patm and fO2 of air, in the thermal range of cooling (ΔTc) between 1300 °C (superliquidus) to 800 °C (solidus) with rates (ΔT/Δt) of 1, 7, 60, 180, 1800, and 9000 °C/h. The run products obtained at 1, 7 and 60 °C/h are holocrystalline, whilst between 60 and 180 °C/h plg disappears, and texture of cpx + sp. shifts from faceted to dendritic. As cooling rate increases, we observe that Fe2O3 decreases and Al2O3 increases in sp. and Al2O3 + Fe2O3 increase and CaO + MgO decrease in cpx. These measured variations mirror changes induced by cooling rate in cation (atoms per formula unit, a.p.f.u.) and molecular abundances of these two crystalline phases. Plg composition shows clear linear trends versus cooling rate. The chemistry of sp., cpx and, to a minor extent, plg solidified from this basaltic liquid is thus strictly related to the cooling rate condition and is similar to those observed in previous investigations on alkaline and evolved basaltic systems. In particular, cpx is the only mineral phase profusely present at all the cooling rates, showing the greatest chemical variations in terms of oxides, cations, and components. The intra-crystalline glass (≤ 50 μm from crystal rims) obtained at 180–1800 °C/h shows compositional variations related to the surrounding crystal growth, evidencing strong supersaturation phenomena (such as dendritic texture) due to the establishment of a diffusion-controlled growth regime. Chemical attributes of mineral phases are also quantitatively related with the maximum (Gmax) and average (GCSD) growth rates of sp., cpx, and plg. When compared with the starting melt composition, the chemistry of cpx suggests the attainment of near-equilibrium crystallization conditions at cooling rate ≤ 60 °C/h, whereas disequilibrium effects are found at cooling rate > 60 °C/h. In contrast, plg is in disequilibrium with the initial melt chemistry in all experiments. By using thermometric models, the calculated crystallization of plg takes place at temperatures much lower than those of cpx, when the crystal content is high and the diffusion of cations in the melt is slow due to the higher (residual) melt viscosity. Under such conditions and due to the effect of cooling, the system cannot return to homogeneous concentrations and, consequently, plg more effectively records the disequilibrium partitioning of cations between the growing crystal surface. The data-set reported here captures the entire (superliquidus to solidus) and intrinsic (heterogeneous site-free silicate liquid) solidification behavior from an actual MORB melt from very rapid to extremely sluggish cooling rate. Finally, all analytical relationships found in this work enable careful reconstruction of the solidification conditions of MORB melts, providing novel geo-speedometers for them at high fO2.

The dynamics of deep sea explosive eruptions, the dispersion of the pyroclasts, and how submarine eruptions differ from the subaerial ones are still poorly known due to the limited access to sea environments. Here, we analyze two ash layers representative of the proximal and distal deposits of two submarine eruptions from a 500 to 800 m deep cones of the Marsili Seamount (Italy). Fall deposits occur at a distance of more than 1.5 km from the vent, while volcanoclastic flows are close to the flanks of the cone. Ash shows textures indicative of poor magma-water interaction and a gas-rich environment. X-ray microtomography data on ash morphology and bubbles, along with gas solubility and ash dispersion models suggest 200-400 m high eruptive columns and a sea current velocity <5 cm/s. In deep sea environments, Strombolian-like eruptions are similar to the subaerial ones provided that a gas cloud occurs around the vent.

Mid-ocean ridge basalts (MORBs) are the most abundant eruptive tholeiitic products on Earth. Many experiments have been performed to investigate the solidification of basalts but under limited thermal ranges of cooling (ΔTc) and cooling rates (ΔT/Δt). We analyze the experimental charges solidified from previous studies: the BIR1A basalt from USGS was solidified using ΔT/Δt of 1, 7, 60, 180, 1800 and 9000 °C/h, in the ΔTc between 1300 and 800 °C, at atmospheric conditions. The previous studies allowed determining the glass-forming ability (GFA) of sub-alkaline silicate liquids, but do not give information on their textures. Here, we quantify the evolution of sizes, shapes, number of crystals per area (#/A), CSDs and growth rates (Gs) of plg (plagioclase), cpx (clinopyroxene) and sp. (spinel). Textures were investigated by image analysis on thousands of crystals and are one of the most complete datasets ever obtained from laboratory studies: they reflect rapid, intermediate and sluggish cooled parts of MORB from liquidus to solidus. Faceted plg grows only at ΔT/Δt ≤ 60 °C/h, while cpx and sp. became dendritic at ΔT/Δt between 60 and 180 °C/h. As ΔT/Δt increase, crystal size ranges decrease from 1000 to 10 μm at 1 °C/h to 100–1 at 60 °C/h μm for plg, from 400 to 8 μm at 1 °C/h to 25–0.5 μm at 1800 °C/h μm for cpx, and from 90 to 6 μm at 1 °C/h to 6–0.5 at 1800 °C/h μm, for sp. The #/A increases with increasing ΔT/Δt, except for cpx between 60 and 180 °C/h. As ΔT/Δt increases, CSDs of plg, cpx and sp increase their slopes (m) and population densities per size (n0), reduce the size ranges and tend to be log-linear. At low ΔT/Δt, CSDs are composed of several log-linear segments, which slopes are related to different pulses of crystal nucleation, and subsequent growth by coarsening. The CSDs parameters (slope, m, and nucleation density per size, n0) linearly scale each other and both are highly correlated with ΔT/Δt. Maximum (Gmax) and average (GCSD) growth rates are computed respectively by averaged major axis (Lmax) of the five longest crystals and by the m of CSDs. Both the Gs are a function of experimental time (t) and increase with the increasing of ΔT/Δt, changing up to two orders of magnitude. The Gmax of cpx is correlated with m and n0 and can be used in natural MORB to retrieve either ΔT/Δt and Gmax. The plg and cpx crystals with sizes between 0.1 and 1 mm are abundant in the experimental charges obtained at low ΔT/Δt. In volcanic rocks, these crystal sizes are generally considered representative of intra-telluric conditions (phenocrysts and microphenocrysts). Our data demonstrate that crystals with mm-sizes may also grow in syn-topost- depositional conditions. The continuous evolution of textures in response to ΔT/Δt variations implies that kinetic effects can fully capture the solidification of MORBs. As a result, the widely accepted assumption that phenocrysts represent the products of evolution processes in volcanic conduits or magma reservoirs could be not valid for some basaltic lavas.

been experimentally quantified via cooling-induced solidification approach. GFA is measured by the critical cooling rate Rc, the rate at which a melt solidifies ≤2 area% of crystals. Cooling rates of 9000, 1800, 180, 60, 7 and 1 °C/h have been run between 1300 °C (super-liquidus region) and 800 °C (quenching temperature), at air fO2 and ambient P for six silicate melts with compositions ranging from basalt (B) to rhyolite (R) (i.e., B100, B80R20, B60R40, B40R60, B20R80 and R100) and water contents comprised between 53 (B100) and 384 (B20R80) ppm. The ranges of cooling rates and chemical compositions used in this study are the broadest ever investigated in the Earth sciences. The phase proportions (area%) were determined by image analysis on about 500 back-scattered electron images collected over different magnifications. Phases are glass, clinopyroxene (cpx), spinel (sp) and plagioclase (plg). Sp is ubiquitous with abundance of few area% and nucleates earlier than silicate crystals. Cpx solidifies in all runs except in R100 and its abundance follows asymmetric broad Gaussian-like trends (with tails towards low rates) as a function of cooling rate. Moving from B100 to B40R60 these trends conserve their shape but shift progressively to lower cooling rates and mineral abundances. Plg crystallises only at low cooling rates and in SiO2-poor compositions. Run-products with low amounts of crystals (≤5 area%) clearly show that cpx preferentially nucleates on surfaces of sp, whereas a significant crystallisation of cpx (N5 area%) is observed with decreasing cooling rate and with changing composition from B100 to B20R80. The crystallisation of silicate crystals is related to the chemical diffusivity of components in the melt. Also the initial crystallisation of plg occurs preferentially on cpx. In general, the amount of crystals decreases as the cooling rate increases; however, in some cases, the amount of crystals remains constant or even decreases for B80R20 with decreasing cooling rate. Rc values change over 5 orders of magnitude being b1, 7, 620, 3020, 8020 and 9000 °C/h for R100, B20R80, B40R60, B60R40 and B80R20 and B100, respectively. The variation of Rc can be modelled through NBO/T (nonbridging oxygen per tetrahedron) parameter by the following equation: Rc=a / {1+e−[(NBO/T − b)/c]},where a, b and c are fitting parameters equal to 9214, 0.297 and 0.040, respectively. Similarly to other glass-forming liquids (network, metallic and molecular systems), Rc for natural sub-alkaline silicate melts is inversely related to the reduced glass transition parameter Trg (Trg=Tg / Tm) and can be quantified with the equation Rc= a × Trg−b, where a and b are 1.19 × 10−4 and 28.7, respectively. These results may be used to retrieve the solidification conditions of aphyric, degassed and oxidised lavas; in addition, our data provide general constrains on the crystallisation kinetics of natural crystal-bearing silicate melts erupted on Earth (e.g. lavas with phenocrysts). The relationship between crystal content and cooling rate suggests that the solidification path induced by degassing can be also complex and nonlinear. The growth of crystalswith size up to 1 mm from a nearly anhydrous superheated silicate melt indicates that variable cooling conditions of lavas have to be accounted to discriminate amongminerals formed before, during and after eruptions.Moreover, our results can be used to design glass-ceramics from naturally available easy to find, low-cost starting materials.

Dynamic cooling-induced solidification experiments were run using six silicate glasses along the basalt - rhyolite join (B100= 100 wt % of basalt, R100= 100 wt % of rhyolite), i.e. B100, B80R20, B60R40, B40R60, B20R80 and R100; the glasses directly quenched from 1300 °C after a dwell of 120 minutes (experiment E0) contain 50-400 ppm H2O, << 1 area% μm-sized bubble, and Fe2+/Fetot between 0.34 and 0.46. Experiments were performed in Pt capsules at room pressure and fO2 of air, between 1300 and 800 °C using three different cooling rates of 0.0167, 3 and 30 °C/min; these cooling rates were run two times: E1-E2 experiments at 0.0167°C/min, S1-E3 at 3 °C/min, and E4-E5 at 30 °C/min. In experiments E1 to E5, samples were annealed for 120 minutes at 1300 °C, whereas in the experiment S1 the samples were firstly heated for 30 minutes at 1400 °C followed by a dwell time of 2400 minutes at 1300°C before cooling. In the experiments a preferential crystallization was not observed at the melt/gas interface. B100, B80R20 and B60R40 run-products have a low tendency to preferentially crystallize on Pt walls, while B40R60, B20R80 and R100 are not affected by the presence of Pt substrata. All run-products show very homogeneous textures, except for B60R40 and B40R60 at 0.0167°C/min in the E1 experiment. The duplicates of B40R60 and B60R40 at 0.0167°C/min and B100 at 30 °C/min show relatively large differences in crystal content (> 4 and < 14 area%). B40R60 and B60R40 duplicated run-products have the same amount of earlycrystallized clinopyroxene and spinel, but different contents in lately-formed plagioclase. The run-products with the same starting composition from E3-S1 (3 °C/min) show a high reproducibility in terms of crystal shape, size, and amount (< 4 area%). This demonstrates that the crystallization path is not affected by the different heat treatment above the liquidus temperature, i.e. the time scale of structural re-equilibration (relaxation) and chemical rehomogenization are shorter than our experimental time scale. Possible chemicalheterogeneities on a length scale of several micrometers for R100 and several hundreds of micrometers for B100 can be removed at 1300 °C within 120 minutes. A heat treatment at 1300 °C for 120 minutes significantly reduces the amount of μm-sized bubbles, potentially responsible for the onset of nucleation and unreveals the intrinsic solidification of silicate melts. The experimental reproducibility is low when the cooling path intersects the tip of the time-temperature-transformation (TTT) curves, i.e. when the nucleation rate is near its maximum (Imax). In that case, even small thermal variations in cooling rate and local composition can have large effects on phase abundance and crystal size. Dynamic crystallization experiments can be properly interpreted and compared only if they are texturally homogeneous and the physico-chemical state of the superheated silicate liquid is known. The solidification conditions used in this study mirror those of aphyric lavas and dikes emplaced at shallower crustal levels.

Viscosities of shoshonitic and latitic melts, relevant to the Campi Flegrei caldera magmas, have been experimentally determined at atmospheric pressure and 0.5 GPa, temperatures between 840 K and 1870 K, and H2O contents from 0.02 to 3.30 wt%. The concentric cylinder technique was employed at atmospheric pressure to determine viscosity of nominally anhydrous melts in the viscosity range of 101.5 - 103 Pa·s. The micropenetration technique was used to determine the viscosity of hydrous and anhydrous melts at atmospheric pressure in the high viscosity range (1010 Pa·s). Falling sphere experiments were performed at 0.5 GPa in the low viscosity range (from 100.35 to 102.79 Pa·s) 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, 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 9 km to 4 km 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.

The effect of pressure on melt viscosity was investigated for five compositions along the join An(CaAl2Si2O8)–Di(CaMgSi2O6) and four alkali silicates containing lithium, sodium, and potassium in constant ratio of ∼ 1:1:1, but alkali-silica ratios are varying. The experiments were performed in an internally heated gas pressure vessel at pressures from 50 to 400 MPa in the viscosity range from 108 to 1011.5 Pa⋅s using parallel plate viscometry. The polymerized An composition shows a negative pressure dependence of viscosity while the other, more depolymerized compositions of the join An–Di have neutral to positive pressure coefficients. The alkali silicates display neutral to slightly positive pressure coefficients for melt viscosity. These findings in the high viscosity range of 108–1011 Pa⋅s, where pressure appears to be more efficient than in low viscous melts at high temperature, are consistent with previous results on the viscosity of polymerized to depolymerized melts in the system NaAlSi3O8–CaMgSi2O6 by Behrens and Schulze [ H. Behrens and F. Schulze, Am. Mineral. 88, 1351 (2003) ]. Thus we confirm that the sign of the pressure coefficient for viscosity is mainly related to the degree of melt polymerization in silicate and aluminosilicate melts.

Viscosity of silicate melts is a critical property for understanding volcanic and igneous processes in the Earth. We investigate the pressure effect on the viscosity of rhyolitic melts using two methods: indirect viscosity inference from hydrous species reaction in melts using a piston cylinder at pressures up to 2.8 GPa and direct viscosity measurement by parallel-plate creep viscometer in an internally-heated pressure vessel at pressures up to 0.4 GPa. Comparison of viscosities of a rhyolitic melt with 0.8 wt% water at 0.4 GPa shows that both methods give consistent results. In the indirect method, viscosities of hydrous rhyolitic melts were inferred based on the kinetics of hydrous species reaction in the melt upon cooling (i.e., the equivalence of rheologically defined glass transition temperature and chemically defined apparent equilibrium temperature). The cooling experiments were carried out in a piston-cylinder apparatus using hydrous rhyolitic samples with 0.8–4 wt% water. Cooling rates of the kinetic experiments varied from 0.1 K/s to 100 K/s; hence the range of viscosity inferred from this method covers 3 orders of magnitude. The data from this method show that viscosity increases with increasing pressure from 1 GPa to 3 GPa for hydrous rhyolitic melts with water content 0.8 wt% in the high viscosity range. We also measured viscosity of rhyolitic melt with 0.13 wt% water using the parallel-plate viscometer at pressures 0.2 and 0.4 GPa in an internally-heated pressure vessel. The data show that viscosity of rhyolitic melt with 0.13 wt% water decreases with increasing pressure. Combining our new data with literature data, we develop a viscosity model of rhyolitic melts as a function of temperature, pressure and water content.

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 openwindowon 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.

The solubility of H2O-CO2 fluids in a synthetic analogue of a phono-tephritic lava composition from Alban Hills (Central Italy) was experimentally determined from 50 to 500 MPa and 1200 and 1250°C. H2O and CO2 contents in experimental glasses were determined by bulk analytical methods and FTIR spectroscopy. For the quantification of volatile concentrations by IR spectroscopy we have calibrated the absorption coefficients of water-related and carbon-related bands for phono-tephritic compositions. The determined absorption coefficients are 0.62 ± 0.06 L mol-1cm-1 for the band at ~4500 cm-1 (OH groups) and 1.02 ± 0.03 L mol-1cm-1 for the band at ~5200 cm-1 (H2O molecules). The coefficient for the fundamental OH stretching vibration at 3550 cm-1 is 63.9 ± 5.4 L mol-1cm-1. CO2 is bound in the phono-tephritic glass as CO32- exclusively; its concentration was quantified by the peak height of the doublet near the 1500 cm-1 band with the calibrated absorption coefficient of 308 ± 110 L mol-1cm-1. Quench crystals were observed in glasses with water contents exceeding 6 wt% even when using a rapid quench device, limiting the application of IR spectroscopy for water-rich glasses. H2O solubility in the ultrapotassic melts (7.52 wt% K2O) as a function of pressure is similar to the solubility in basaltic melts up to 400 MPa (~8 wt%) but is higher at 500 MPa (up to 10.71 wt%). At 500 MPa and 1200°C, the CO2 capacity of the phono-tephritic melt is about 0.82 wt%. The high CO2 capacity is probably related to the high K2O content of the melt. At both 200 and 500 MPa, the H2O solubility shows a non linear dependence on XfH2O in the whole XfH2O range. The variation of CO2 solubility with XfCO2 displays a pronounced convex shape in particular at 500 MPa, implying that dissolved H2O promotes the solubility of CO2. Our experimental data on CO2 solubility indicate that the interaction between phono-tephritic magma and carbonate rocks occurring in the Alban Hills magmatic system may result in partial dissolution of CO2 from limestone into the magma. However, although the CO2 solubility in phono-tephritic melts is relatively high compared to that in silicic to basaltic melts, the capacity for assimilation of limestone without degassing is nevertheless limited to < 1 wt% at the P-T conditions of the magma chamber below Alban Hills.