Search Results - (Author, Cooperation:D. B. Dingwell)

2015-12-25

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2012-01-20

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

1365-246X

Blackwell Publishing Journal Backfiles 1879-2005

Geosciences

The present study deals with the experimental modelling of two different mechanisms of crystal-melt segregation in crustal rocks: (1) the buoyancy-driven compaction of the crystal + melt matrix and (2) melt filtering in a partially crystalline matrix due to differential stresses. These two segregation mechanisms have differing relative efficiencies in the deformation of crustal rocks and result in different texture scales depending on melt fraction, melt viscosity and tectonic stresses. A centrifuge furnace has been used in the present study for the modelling of melt migration in partially molten granitic rocks. Samples of Beauvoir granite (Massif Central, France) with a grain size of 0.16–0.5 mm and dimensions of diameter ∼5 mm, length ∼16 mm were used. These samples had been pre-fused at temperatures of 1000–1075 °C, yielding an initial average melt fraction of ∼45–50 volume per cent. The centrifuging of partially melted samples during ∼6 hr at an acceleration of 1000g (g is gravity) results in a linear vertical distribution of melt over the length of the sample without the development of a compaction layer. The gradient of the melt fraction (melt migrates to the top of samples) correlates with temperature: 1075°C ∼7 volume per cent mm-1; 1050°C ∼4 volume per cent mm-1; 1000°C ∼1.5 volume per cent mm-1. The calculated rate of melt migration varies from 3x10-5 cm s-1 (1075°C) to 2x10-6 cm s-1 (1000°C).Differential stresses of ∼0.7–1.4 MPa have been generated in the centrifuge by putting a piston (weight ∼1.02–2.05 g, diameter ∼4.5 mm) on the top of the partially melted sample, which is then centrifuged at ∼1000g. The rate of melt squeezing from the sample in this case is about two orders of magnitude higher than that observed without the piston. After centrifuging for 6 hr, a compaction layer below the piston is formed with a thickness of ∼2.5 mm and a crystal fraction of ∼70–65 volume per cent. Further centrifuging (∼15 hr) does not result in any increase of the compaction-layer thickness or volume percentage of crystals in it. The comparison of the two segregation mechanisms confirms the much greater efficiency of differential-stress-induced melt segregation and accumulation in veins and pockets than the compaction mechanism.

Electronic Resource

1365-246X

Blackwell Publishing Journal Backfiles 1879-2005

Geosciences

The present experimental study deals with the laboratory modelling of two different mechanisms of gravitational percolation in partially melted rocks: (1) diapiric percolation of heavy material and (2) the sedimentation of heavy particles. These two mechanisms of mass transport in partially melted rocks result in different, scales of the segregation process in the melt-crystal matrix. A centrifuge furnace was used to simulate the percolation of the heavy particle layer through the partially molten granite at temperatures of up to 1000 °C. Samples of Beauvoir granite (Massif Central, France, grain size 0.16–0.5 mm with an initial degree of partial melting ∼45 per cent) were used as a matrix. A layer of Pt powder suspended in a melt of the same composition as the partially melted matrix was placed on the top of the granite sample. After centrifuging for various times (up to 2 × 104 s), X-ray images of samples were obtained and the evolution of the percolation process of heavy suspension in the partially molten granite was monitored from the Pt particle distribution. The diapiric or finger regime of percolation starts when the growth rate of a Raleigh-Taylor instability of the heavy layer is faster than the Stokes sedimentation velocity of individual particles in the upper layer. This relationship is a complex function of the size and initial concentration of heavy particles, as well as the ratio of particle to crystal size, the permeability of the matrix, and the heterogeneity scale in the partially melted matrix. At small concentrations (several per cent) and at large concentrations (where close packing of heavy particles results in an anomalous viscosity increase in the upper heavy layer) Stokes sedimentation is dominant in the vertical percolation of the heavy material. The sinking velocity of the diapir decreases when the size of heavy particles in it becomes comparable with the size of crystals in the partially melted granite. In this situation the vertical sinking of the diapir is not stable and the horizontal instability of the vertical mass transport starts to become important. Mass transport via diapiric percolation results in more efficient crystal-melt segregation of partially melted rocks. The percolation of individual particles provides only local melt-crystal flow on a scale comparable with the heavy particle size. The diapiric percolation provides a much larger scale of partial melt segregation with a length-scale comparable with the diapir size.

Electronic Resource

1365-3121

Blackwell Publishing Journal Backfiles 1879-2005

Geosciences

Relaxation geospeedometry using differential scanning calorimetry (DSC) has been applied to quantify the cooling history across the glass transition of flow ramps at the front of the calc-alkaline rhyolite Rocche Rosse flow of Lipari, Aeolian Islands, Italy. Modelled cooling rates for the obsidian retrieved from two profiles range between 0.2 and 0.03 K min–1. Cooling at the flow front appears to be dominated by conductive heat loss of individual flow ramps forming individual cooling units. Cooling rates of tens of Kelvins per day appear to have controlled the last stage of viscous deformation before the entire flow came to rest. It is inferred that cooling rates slower than those modelled are required to sustain flow in highly viscous rhyolitic lavas.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Magma fragmentation—the process by which relatively slow-moving magma transforms into a violent gas flow carrying fragments of magma—is the defining feature of explosive volcanism. Yet of all the processes involved in explosively erupting systems, fragmentation is possibly the least ...

Electronic Resource

1432-0819

Key words Rhyolite ; Vesiculation ; Kinetic ; Water ; Diffusion ; Degassing

Springer Online Journal Archives 1860-2000

Geosciences

Abstract  The vesiculation of a peralkaline rhyolite melt (initially containing ∼0.14 wt.% H2O) has been investigated at temperatures above the rheological glass transition (T g≈530  °C) by (a) in situ optical observation of individual bubble growth or dissolution and (b) dilatometric measurements of the volume expansion due to vesiculation. The activation energy of the timescale for bubble growth equals the activation energy of viscous flow at relatively low temperatures (650–790  °C), but decreases and tends towards the value for water diffusion at high temperatures (790–925  °C). The time dependence of volume expansion follows the Avrami equation ΔV (t)∼{1–exp [–(t/τav) n ]} with the exponent n=2–2.5. The induction time of nucleation and the characteristic timescale (τav) in the Avrami equation have the same activation energy, again equal to the activation energy of viscous flow, which means that in viscous melts (Peclet number 〈1) the vesiculation (volume expansion), the bubble growth process, and, possibly, the nucleation of vesicles, are controlled by the relaxation of viscous stresses. One of the potential volcanological consequences of such behavior is the existence of a significant time lag between the attainment of a super-saturated state in volatile-bearing rhyolitic magmas and the onset of their expansion.

Electronic Resource

1432-0819

Rhyolite ; rheology ; vesiculation ; bubble growth ; shear stresses ; foaming ; two-phase suspension

Springer Online Journal Archives 1860-2000

Geosciences

Abstract The style of magma eruption depends strongly on the character of melt degassing and foaming. Depending on the kinetics of these processes the result can be either explosive or effusive volcanism. In this study the kinetics of foaming due to the internal stresses of gas expansion of two types of obsidian have been investigated in time series experiments (2 min-24 h) followed by quenching the samples. The volumetric gas-melt ratio has been estimated through the density measurements of foamed samples. The variation of gas volume (per unit or rhyolite melt volume) with time may be described by superposition of two exponentials responsible for gas generation and gas release processes respectively. An observed difference in foaming style in this study is interpreted as the result of variations in initial contents of microlites that serve as bubble nucleation centers during devolatilization of the melts. Quantitatively the values of the gas generation rate constants (k g) are more than an order of magnitude higher in microlite-rich obsidian than in microlite-free obsidian. Possible origins of differences in the degassing style of natural magmas are discussed in the light of bubble nucleation kinetics in melts during foaming. In a complementary set of experiments the mechanical response of vesicular melt to external shear stress has been determined in a concentric cylinder viscometer. The response of vesicular melt to the pulse of shear deformation depends on the volume fraction of bubbles. The obtained response function can be qualitatively described by a Burgers body model. The experimental shear stress response function for bubble-bearing melt has an overshoot due to the strain-dependent rheology of a twophase liquid with viscously deformable inclusions.

Electronic Resource

1432-0819

Key words Rhyolite ; Calc-alkaline ; Peralkaline ; Viscosity ; Water ; Activation energy

Springer Online Journal Archives 1860-2000

Geosciences

Abstract  As a major volatile in volcanic systems, water has a significant influence on the rheological properties of silicic magmas. This is especially so at minor water contents relevant to the emplacement of silicic lavas. To investigate the influence of water on the viscosity of natural rhyolitic obsidians, a novel strategy has been adopted employing parallel-plate and micropenetration techniques. Viscosities have been determined on three types of material: (a) raw water-bearing obsidians; (b) remelted (1650  °C, 1 atm) degassed glasses of the obsidians; and (c) hydrothermally hydrated (1300  °C, 3 kbar) obsidians. Ten natural rhyolitic obsidians (peraluminous, calc-alkaline and peralkaline) were employed: seven originated from lava flows and contained 〈0.2 wt.% H2O, two samples were F-rich from pyroclastic successions, and one was an obsidian cobble with 1.5 wt.% water also associated with pyroclastic units. Melt compositions and water contents were stable during viscometry. The measured decreases in activation energies of viscous flow and viscosity with small amounts of water are much greater than the Shaw calculation scheme predicts. In addition, a marked non-linear decrease in η exists with increasing water content. In contrast to the case for peralkaline rhyolites, 0.1–0.2 wt.% water decreases activation energies significantly (up to 30%) for calc-alkaline compositions. These results have important implications for the ease of near-surface degassing of silicic magmas during emplacement and permit the testing of calculational models for viscosity, largely based on synthetic systems.

Electronic Resource

1432-0819

Key words Microlites ; Rhyolite ; Obsidians ; Viscosity ; Glass transition ; Lava flow rheology

Springer Online Journal Archives 1860-2000

Geosciences

Abstract  To investigate the influence of microlites on lava flow rheology, the viscosity of natural microlite-bearing rhyolitic obsidians of calc-alkaline and peralkaline compositions containing 0.1–0.4 wt.% water was measured at volcanologically relevant temperatures (650–950  °C), stresses (103–105 Pa) and strain rates (10–5 to 10–7 s–1). The glass transition temperatures (T g ) were determined from scanning calorimetric measurements on the melts for a range of cooling/heating rates. Based on the equivalence of enthalpic (calorimetric) and shear (viscosity) relaxation, we calculated the viscosity of the melt in crystal-bearing samples from the T g data. The difference between the calculated viscosity of the melt phase and the measured viscosity for the crystal-bearing samples is interpreted to be the physical effect of microlites on the measured viscosity. The effect of 〈5 vol.% rod-like microlites on the melt rheology is negligible. Microlite-rich and microlite-poor samples from the same lava flow and with identical bulk chemistry show a difference of 0.6 log10 units viscosity (Pa s), interpreted to be due to differences in melt chemistry caused by the presence of microlites. The only major differences between measured and calculated viscosities were for two samples: a calc-alkaline rhyolite with 1 vol.% branching crystals, and a peralkaline rhyolite containing crystal-rich bands with 〉45 vol.% crystals. For both of these samples a connectivity factor is apparent, with, for the latter, a close packing framework of crystals which is interpreted to influence the apparent viscosity.

Electronic Resource

1432-0819

Key words Magma ; Mount St. Helens ; Cryptodome ; Viscosity ; Physical ; Rheological ; Experimental

Springer Online Journal Archives 1860-2000

Geosciences

Abstract  Physical properties of cryptodome and remelted samples of the Mount St. Helens grey dacite have been measured in the laboratory. The viscosity of cryptodome dacite measured by parallel–plate viscometry ranges from 10.82 to 9.94 log10 η (Pa s) (T=900–982  °C), and shrinkage effects were dilatometrically observed at T〉900  °C. The viscosity of remelted dacite samples measured by the micropenetration method is 10.60–9.25 log10 η (Pa s) (T=736–802  °C) and viscosities measured by rotational viscometry are 3.22–1.66 log10 η (Pa s) (T=1298–1594  °C). Comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresponding water-bearing melt demonstrates significant deviations between measured and calculated values. This difference reflects a combination of the effect of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis for the calculation of crystal-bearing vesicular melt viscosities at low temperature. Assuming that the cryptodome magma of the 18 May 1980 Mount St. Helens eruption was residing at 900  °C with a phenocryst content of 30 vol.%, a vesicularity of 36 vol.% and a bulk water content of 0.6 wt.%, we estimate the magma viscosity to be 1010.8 Pa s.

Electronic Resource

1432-2021

Springer Online Journal Archives 1860-2000

Chemistry and Pharmacology

Geosciences

Physics

Abstract The effect of pressure on titanium coordination in glasses, with composition K2TiSi4O11, quenched isobarically from liquids equilibrated at high pressure (5, 10, 15, 20, 25, 30 kbar respectively) and T=1600° C has been investigated by X-ray absorption spectroscopy (XAS). The XANES spectra collected at the Ti K-edge clearly show a variation with pressure that is related to changes in the geometrical environment around the Ti atoms. By comparison with spectra of standard materials, the XANES spectra of the glasses suggest a relatively low average coordination number (near 5) in samples quenched at low pressure and a higher coordination number (near 6) in samples quenched from the highest pressure. The combination of XANES data with density and compressibility measurements supports the idea that a mixture of 6- and lower coordinated (4- and/ or 5-coordinated) Ti geometries are present in the 1 bar glass, and an increasing proportion of 6-coordinated Ti occurs in the glasses synthesized at progressively higher pressures.

Electronic Resource

1432-2021

Springer Online Journal Archives 1860-2000

Chemistry and Pharmacology

Geosciences

Physics

Abstract The thermal expansivity of liquid GeO2 at temperatures just above the glass transition has been obtained using a combination of scanning calorimetry and dilatometry. The calorimetric and dilatometric curves of c p and dV/dT are normalized to the temperature derivative of fictive temperature versus temperature using the method of Webb et al. (1992). This normalization, based on the equivalence of relaxation parameters for volume and enthalpy, allows the completion of the dilatometric trace across the glass transition to yield liquid expansivity and volume. The values of liquid volume and expansivity obtained in this study are combined with high temperature densitometry determinations of the liquid volume of GeO2 by Sekiya et al. (1980) to yield a temperature-volume relation for GeO2 melt from 660 to 1400 °C. Liquid GeO2 shows a strongly temperature-dependent liquid molar expansivity, decreasing from 20.27 × 10−4 cm3 mol−1°C−1 to 1.97 × 10−4cm3 mol−1 °C−1 with increasing temperature. The coefficient of volume thermal expansion (α v ) decreases from 76.33 × 10−6 °C−1 to 2.46 × 10−6 °C−1 with increasing temperature. A qualitatively similar volume-temperature relationship, with α v decreasing from 335 × 10−6 °C−1 to 33 × 10−6 °C−1 with increasing temperature, has been observed previously in liquid B2O3. The determination of the glass transition temperature, liquid volume, liquid and glassy expansivities and heat capacities in this study, combined with compressibility data for glassy and liquid GeO2 from the literature (Soga 1969; Kurkjian et al. 1972; Scarfe et al. 1987) allows the calculation of the Prigogine-Defay ratio (Π), c p -c v and the thermal Grüneisen parameter (γ th) for GeO2. From available data on liquid SiO2 it is concluded that liquid GeO2 is not a good analog for the low pressure properties of liquid SiO2.

Electronic Resource

1432-2021

Springer Online Journal Archives 1860-2000

Chemistry and Pharmacology

Geosciences

Physics

Abstract Ti K-edge XANES spectra have been collected on a series of Ti-bearing silicate glasses with metasilicate and tetrasilicate compositions. The intensity of the preedge feature in these spectra has been found to change with glass composition and varies from 29 to 58% (normalized intensity) suggesting a variation in structural environent around the absorbing atom. The pre-edge peak intensity increases for the alkali titanium tetrasilicate glasses from 35% to 58% in the order Li 〈 Na 〈 K 〈 Rb, Cs whereas for the metasilicate compositions there is a maximum for the K-bearing glass. The pre-edge peak intensity remains constant for the alkaline earth titanium metasilicate glasses, Ca and Sr (34%) but increases slightly for Ba (41%). As the intensity of this feature is inversely correlated with coordination number, a comparison of the pre-edge intensity data for the investigated glasses with those of materials of known coordination number leads us to establish a regression equation and to infer that the average coordination number of Ti in these glasses ranges from 4.8 to 5.8. Large alkali cations appear to stabilize a relatively low average coordination number for Ti in silicate melts. The Ti structural environment results appear also to vary as a function of SiO2 content within the K2O-TiO2-SiO2 system. A number of physical properties of the melts from which these glasses were quenched and of other Ti-bearing silicate melts, have been determined in recent years. Clear evidence of a variable coordination number of Ti, consistent with the interpretation of the present XANES data is available from density measurements. These and other property determinations are compared with the present spectroscopic observations in an attempt to relate structure and properties in these melts which contain a major component with variable coordination number.

Electronic Resource

1432-0967

Springer Online Journal Archives 1860-2000

Geosciences

Abstract Despite the abundant evidence for the enrichment of phosphorus during the petrogenesis of natural ferro-basalts, the effect of phosphorus on the physical properties of these melts is poorly understood. The effects of phosphorus on the viscosity, density and redox ratio of a ferro-basaltic melt have been determined experimentally. The viscosity measurements were obtained using the concentric cylinder method on a ferro-basaltic melt above its liquidus, at 1 atm, in equilibrium with air and with CO2. The density measurements were performed using the double Pt-bob Archimedean method at superliquidus conditions under 1 atm of air. The redox ratio was obtained by wet chemical analysis of samples collected during physical property measurements. Phosphorus pentoxide reduces ferric iron in ferro-basaltic melt. The reduction due to P2O5 is much larger than that for most other oxide components in basaltic melts. A coefficient for the reduction of ferric iron has been generated for inclusion in calculation schemes. The effect of P2O5 on the viscosity is shown to be complex. The initial reduction of ferric iron with the addition of P2O5 results in a relatively small change in viscosity, while further addition of P2O5 results in a strong increase. The addition of phosphorus to a ferro-basaltic melt also reduces the density. A partial molar volume of 64.5±0.7 cm3/mol for P2O5 in this melt has been obtained at 1300° C, yielding a volume of 12.9 cm3/mol per oxygen, consistent with a tetrahedral coordination for this high field strength cation. The effects of P2O5 on redox state, density and viscosity provide constraints on the structural role of phosphorus in these melts. The results suggest a complex interaction of phosphorus with the aluminosilicate network, and tetrahedral ferric iron. In light of the significant effects of phosphorus on the physical and chemical properties of ferro-basaltic liquids, and the extreme enrichments possible in these liquids in nature, the role of phosphorus in these melts should, in future, be considered more carefully.

Electronic Resource

1432-0967

Springer Online Journal Archives 1860-2000

Geosciences

Abstract  The viscosities of hydrous haplogranitic melts synthesized by hydrothermal fusion at 2 kbar pressure and 800 to 1040° C have been measured at temperatures just above the glass transition and at a pressure of 1 bar using micropenetration techniques. The micropenetration viscometry has been performed in the viscosity range of 109 Pa s to 1012 Pa s. The samples ranged in water content from 0.4 to 3.5 wt%. For samples with up to 2.5 wt% H2O, the water contents have been determined using infrared spectroscopy obtained before and after each viscometry experiment to be constant over the duration of the measurements. Above this water content a measurable loss of water occurs during the viscometry. The viscosity data illustrate an extremely nonlinear decrease in viscosity with added water. The viscosity drops drastically with the addition of 0.5 wt% of water and then shallows out at water contents of 2 wt%. An additional viscosity datum point obtained from the analysis of fluid inclusions in a water-saturated HPG8 confirms a near invariance of the viscosity with the addition of water between 2 and 6 wt%. These measurements may be compared directly with the data of Hess et al. (1995, in press) for the effects of excess alkali and alkaline earth oxides on the viscosity of HPG8 (also obtained at 1 bar). The viscosity of the melts, compared on an equivalent molar basis, increases in the order H2O〈(Li2O〈Na2O〈 K2O〈Rb2O,Cs2O〈BaO〈SrO〈CaO〈MgO〈 BeO). The extraordinary decrease in melt viscosity with added water is poorly reproduced by the calculation scheme of Shaw (1972) for the range of water contents investigated here. The speciation of water in the quenched glasses can be used to quantify the dependence of the viscosity on hydroxyl content. Considering only the hydroxyl groups as active fluidizers in the hydrous melts the nonlinearity of the viscosity decrease and the difference with the effects of the alkali oxides becomes larger. Consequences for degassing calcalkaline rhyolite are discussed.

Electronic Resource

1432-0967

Springer Online Journal Archives 1860-2000

Geosciences

Abstract The temperature-dependent thermal expansivities of glasses and liquids in the ternary albite-anorthite-diopside have been determined using a combination of calorimetry, dilatometry and Pt and Ir double bob Archimedean densitometry. Supercooled liquid volumes and molar thermal expansivities were determined across the glass transition using a combination of scanning calorimetry and dilatometry, based upon the equivalence of relaxation of volume and enthalpy in the vicinity of the glass transition. Superliquidus volumes were determined using double Pt bob Archimedean densitometry at temperatures up to 1,650°C and double Ir bob densitometry at 1,800°C. Experimental access to liquid volumes near the glass transition temperatures (680–920°C) and at superliquidus temperatures (1,400–1,800°C) for these compositions results in the observation of a nonlinear temperature dependence of molar volume, i.e., temperature-dependent thermal expansivities. The diopside composition wxhibits the largest temperature dependence of thermal expansivity, decreasing by ∼50% between 800 and 1,500°C. Linear extrapolation of the high-temperature volume data of diopside to 810°C would result in a 3% overestimation of the molar voltime. The temperature dependence of the molar volume of anorthite is approximately linear. The thermal expansivities of the liquids in the albite-anorthite-diopside system appear to converge at high temperature. This study uses a combination of methods that allows interpolation rather than extrapolation of the extant melt-volume data into the petrologically meaningful (subliquidus) temperature range.

Electronic Resource

1432-0967

Springer Online Journal Archives 1860-2000

Geosciences

Abstract  The speciation of water dissolved in glasses along the join NaAlSi3O8-KAlSi3O8 has been investigated using infrared spectroscopy. Hydrous melts have been hydrothermally synthesized by chemical equilibration of cylinders of bubble-free anhydrous start glasses with water at 1040° C and 2 kbar. These melts have been isobarically and rapidly (200° C/s) “drop”-quenched to room temperature and then subsequently depressurized. The speciation of water in the quenched glasses reflects the state of water speciation at a temperature (the so-called fictive temperature) where the quenched-in structure of the glasses closely corresponds to the melt structure at equilibrium. This fictive temperature is detectable as the macroscopically measureable glass transition temperature of these melt compositions. A separate set of experiments using vesicular samples of the same chemistry has precisely defined the glass transition temperature of these melts (±5° C) on the basis of homogenization temperatures for water-filled fluid inclusions (Romano et al. 1994). The spectroscopic data on the speciation of water in these quenched glasses has been quantified using experimentally determined absorptivities for OH and H2O for each individual melt composition. The knowledge of glass transition temperatures, together with quantitative speciation data permits an analysis of the temperature dependence of the water speciation over the 113° C range of fictive temperatures obtained for these water-saturated melts. The variation of water speciation, cast as the equilibrium constant K where K = [H2O] [O m ]/[OH]2 is plotted versus the fictive temperature of the melt to obtain the temperature dependence of speciation. Such a plot describes a single linear trend of the logarithm of the equilibrium constant versus reciprocal temperature, implying that the exchange of K for Na has little influence on melt speciation of water. The enthalpy derived from temperature dependence is 36.5(±5) kJ/mol. The results indicate a large variation in speciation with temperature and an insensitivity of the speciation to the K–Na exchange.

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