43,569 research outputs found
"Explosive Energy" during volcanic eruptions from fractal analysis of pyroclasts
No full textDespite recent advances by means of experiments and high-resolution surveys and the growing understanding of the physical processes
before and during volcanic eruptions, duration and type of eruptive activity still remain highly unpredictable. This uncertainty hinders
appropriate hazard and associated risk assessment tremendously. In an effort to counter this problem, experimentally generated pyroclasts
have been studied by fractal statistics with the aim of evaluating possible relationships between eruption energy and fragmentation efficiency.
Rapid decompression experiments have been performed on three differently porous sample sets of the 1990–1995 eruption of Unzen
volcano (Japan) at 850 °C and at initial pressure values above the respective fragmentation threshold [U. Kueppers, B. Scheu, O. Spieler, D.
B. Dingwell, Fragmentation efficiency of explosive volcanic eruptions: a study of experimentally generated pyroclasts. J. Volcanol.
Geotherm. Res. 153 (2006) 125–135.,O. Spieler, B. Kennedy, U. Kueppers, D.B. Dingwell, B. Scheu, J. Taddeucci, The fragmentation
threshold of pyroclastic rocks. EPSL 226 (2004) 139–148.]. The size distribution of generated pyroclasts has been studied by fractal
fragmentation theory and the fractal dimension of fragmentation (Df), a value quantifying the intensity of fragmentation, has been measured
for each sample. Results showthat size distribution of pyroclastic fragments follows a fractal law(i.e. power-law) in the investigated range of
fragment sizes, indicating that fragmentation of experimental samples reflects a scale-invariant mechanism. In addition, Df is correlated
positively with the potential energy for fragmentation (PEF) while showing a strong influence of the open porosity of the samples.
Results obtained in this work indicate that fractal fragmentation theory may allow for quantifying fragmentation processes during
explosive volcanic eruptions by calculating the fractal dimension of the size distribution of pyroclasts. It emerges fromthis study that fractal
dimension may be utilised as a proxy for estimating the explosivity of volcanic eruptions by analysing their natural pyroclastic deposits.Published800-807ope
Viscosity data for hydrous peraluminous granitic melts: comparison with a metaluminous model
No full textWe performed 27 viscosity determinations on dry and water-bearing peraluminous haplogranitic melts. The dry melt compositions cover the range of normative corundum to be expected in peraluminous granitic melts in nature. The compositions are based on addition
of Al2O3 to a haplogranitic melt (HPG8) whose composition is near that of the projection
of the 2 kbar H2O-saturated minimum melt composition into the system NaAlSi3O8-
KAlSi3O8-SiO2. The H2O contents of the hydrous melts were analyzed using Karl Fischer titration ranging from 1 to 3 wt%. The viscosity determinations were performed using a
modified micropenetration method in the viscosity range of 1010 to 1011 Pa·s, at 1 atm
pressure, and in the temperature ranges of 880–940 °C and 470–640 °C for the dry and wet melts, respectively. For the dry peraluminous melts in this high viscosity range, addition of the first few percent of normative corundum to a metaluminous granitic melt increases the viscosity, which remains nearly constant despite further addition of Al2O3.
Thus a viscosity maximum is inferred for dry slightly peraluminous granitic melts. The hydrous melt viscosity data were compared with the recent calculational model of Hess and Dingwell (1996), which was based on and designed for metaluminous melt viscosities.
That model is capable of describing the viscosities of hydrous peraluminous granitic melts within the uncertainties stated for its application in metaluminous melts
X-ray absorption study of Ti-bearing silicate glasses
No full textTi 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
The effect of iron on the rheological properties of silicate melts.
No full textAs the viscosity is probably one of the most important properties governing the dynamics of magmas at all the scales, its investigation is fundamental to provide importantconstraints on all the magmatic processes such as crystallization, magma differentiation and eruption dynamics on terrestrial planets. Igneous provinces have been found on Earth, Moon, Venus, Io and Mars. As matter of fact, it is generally accepted, that Martian rocks have a higher Fe content than rocks on Earth. Unfortunately, existing models of the physico-chemical properties of silicate melts are not calibrated for the high Fe contents. In order to adapt these models to high Fe-content compositions, we have developed a new experimental approach from which the physico-chemical properties of both iron components (FeO and Fe2O3) will be derived. Therefore, viscosity measurements have been conducted in this study over the whole temperature liquid range. High-T viscosities (1594-1275_C) have been measured in air using the concentric cylinder (CC) method. Low-T viscosities (817-711_C) have been measured under argon using the micro-penetration method (MP) for the melts that could be quenched to glasses. The oxidation state of Fe has been determined on quenched glasses at regular T steps by wet chemistry method. Two different simple Fe-bearing systems have been studied to date: (i) anorthite-diopside eutectic composition (AnDi) with variable amount of Fe (up to 30 wt%) as described in the literature as -a basalt analogue- and (ii) sodium disilicate (NS2 up to 20 wt% of Fe). In addition, the compositional range has been extended to include a Martian mantle composition based on the model of [1]. The high T viscosity data obtained for the AnDi-eutectic in this study are in good agreement with the data provided by previous authors [2] on the same composition. Since no previous studies have dealt with the low T viscosity of the AnDi-eutectic composition, our experimental data were compared with viscosity of Etna lava [3] taken as an example of natural basaltic composition. In fact, whereas the high T viscosities are similar to each other, this is not true at low temperature (i.e., the Etna basalt shows a much lower activation energy with respect to our AnDi-eutectic composition). High-T viscosities of SNC shows a good agreement with viscosities obtained for a natural peridotite from Balmuccia (Earth mantle analogue) [Dingwell - personal communication]. Low-T viscosities of SNC could not be measured due to the occurrence of crystallization. All the materials exhibit non-Arrhenian behaviour when observed within the complete range of the experimental determinations. Therefore, a Vogel Fulcher Tammann (VFT) form of equation was used to characterize the variation of the viscosity with temperature. In addition, the present results suggest that the effect of iron on decreasing the viscosity at isothermal temperatures (more effective at low T) is linear for AnDi samples containing up to 10 wt% of Fe, while this behaviour becomes non-near for higher Fe-content.[1] Dreibus, G., Waenke, H., (1985) Mars, A Volatile-Rich Planet, Meteoritics 20, 367-381. [2] Scarfe et al., (1983) Viscosity-tempearture relationship at 1 atm in the system diopside-anortite, Am.Mineral. 68, 1083-1088. [3] Giordano, D., Dingwell, D.B., (2003) Viscosity of hydrous Etna basalt: implications for Plinian-style basaltic eruptions, Bull.Volcanol. 65, 8-14
Viscosities of granitic (sensu lato) melts: influence of the anorthite component
No full textThe viscosities of a series of granitic (sensu lato) melts have been determined in the range of 103 to 1012 Pa·s. The anhydrous melt compositions are based on the addition of 10, 20, 50, and 75 wt% of the anorthite component (CaAl2Si2O8) to a haplogranitic melt (HPG8) whose composition lies near
the 2 kbar water-saturated minimum melt composition in the system NaAlSi3O8-KAlSi3O8-SiO2. Melts with 10 and 20 wt% normative anorthite were subjected to high-pressure hydration syntheses using a piston-cylinder apparatus to generate water contents up to 2 wt%. Viscosities were determined for the anhydrous melts using the concentric cylinder method in the viscosity range of 102 to
105 Pa·s, and for both anhydrous and hydrated melts in the range of 109 to 1012 Pa·s. The results for the temperature dependence of viscosity in the anhydrous system indicate that the
influence on melt viscosity, caused by the addition of normative anorthite to the haplogranitic melt
composition, is strongly temperature-dependent. Viscosity-temperature relationships of the melts
become much more non-Arrhenian with addition of normative anorthite. The addition of water to
melts with 10 and 20 wt% normative anorthite results in strong nonlinear decreases in viscosity. In the high viscosity range, the results for hydrous melts with 10% normative anorthite are adequately reproduced using the calcalkaline melt viscosity model of Hess and Dingwell (1996), whereas those
for hydrous melts with 20 wt% normative anorthite are higher than the model predictions by amounts
that depend on the water content. It appears that, at the higher temperatures anticipated for intermediate
granitic magmatism, the calcalkaline model can adequately deal with up to 15 wt% normative anorthite in the melt composition in the range of temperatures relevant for intermediate magmas in nature
Pressure-induced coordination change of Ti in silicate glass: a XANES study
No full textThe 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
Viscosity of a Teide phonolite in the welding interval
No full textThe viscosity of a natural phonolitic composition with variable amounts of H2O has been experimentally determined. The
starting materials were crystal-free phonolitic glasses from Montana Blanca, situated within the Las Canadas caldera of Teide.
Dry phonolitic melt viscosities were determined using concentric cylinder viscometry in the low viscosity range. The glassy
quench products of these runs were then hydrated by high pressure synthesis in a piston cylinder apparatus to generate a suite of samples with water contents ranging from 0.02 to 3.75 wt%. Samples thus hydrated were quenched rapidly and prepared (cut and polished) for the determination of water contents by infrared spectroscopy before and after experimental viscometry. The
viscosities of the melts (dry and hydrated) were determined at 1 bar using a micropenetration technique. Samples were stable under the measurement conditions up to 3.75 wt% H2O. Homogeneity of water content was confirmed by infrared spectroscopy
and total water contents were calculated using absorptivity coef®cients for compositions extremely close to that investigated here. The variation of viscosity as a function of water content and temperature can be described in the high viscosity interval of relevance to many welding processes by the non-Arrhenian expression:
log10 visc = -5.900-0.286 ln (H2O)+ (10775.4 - 394.8(H2O))/(T-148.7 + 21.65 ln(H2O))
whereas the high viscosity range alone is adequately described by the Arrhenian expression
log10 visc = 10.622 - 0.738 ln(H2O) + (17114.3 - 590.4(H2O))x 1/T
where visc is the viscosity in Pa s, H2O is the water content in wt% and T is the temperature in K.
These results are particularly useful for the scaling of conditions extant during the welding of phonolitic products of Montana
Blanca. The welding of glassy phonolitic rocks is enhanced by the lower viscosity of these melts with respect to calcalkaline rhyolites. The ratio of viscosities of phonolitic to calcalkaline rhyolitic melts is a complex function of temperature and water content and reaches up to 104.5 at 0.1 wt% H2O and 500°C. Abundant evidence of welding and remobilisation of pyroclastic and spatter products of Teide system volcanism are consistent with these experimental observations
The 2002 January 17th Nyragongo eruption: insights from textural and rheological investigation
No full textExistence of molten peridotite in the early history of the Earth has long been the subject of debate and conjecture. Interest in the physical properties stems from a number of sources but was re-focussed in the wake of the proposal for the existence of a ̧Smagma ocean ¡T in the evolution of the moon, the Earth and other terrestrial planets. The application of phase equilibrium, buoyancy, thermodynamic and fluid dynamic constraints on the behaviour of molten mantle all rely on adequate characterisation ofthe properties of molten peridotite, largely lacking to date. The viscosity in particular, has received too little attention. A big experimental effort has been provided to obtain the dependence on temperature (T) of viscosity at ambient pressure (P) for the natural peridotite collected at Balmuccia, Italy. High-T measurements were performed by using concentric cylinder (CC). The high-T viscometry was started at 1600_C and proceeded at 10_C intervals, separated by cooling stages at 5_C/min, each one held for 1 hour. No measurements were possible below 1570 _C, because crystallization had occurred. All standard attempts to obtain a homogeneous glass failed. A new technique was therefore used. Small 1-2 mm chips were hung in Pt loops suspended from a long Pt wire and the loops lowered by hand into the high-T viscosity furnace until the chips fused into a bead of liquid held in the loop by surface tension. These samples were then left to quench and placed aside to be used in the splat-quenching device (SQD) (which allows quench rates on the order of 10exp4 _C/s) to finallyobtain a supercooled liquids by squeezing and rapidly quenching a falling liquid drop, through a joint action of a complex photoelectric-driven electromagnetic device. Electron microprobe analysis revealed that only a few vol% of the obtained glasses crystallized in isochemical crystals, whereas the homogeneity of the glassy matrix composition was found to be excellent. As the amount of glass obtained was too small to be used in the micropenetration technique we used differential scanning calorimetry (DSC) to derive the viscosity at low-T. DSC allowed us to unequivocally determine glass transition temperatures (Tg) for cooling/heating rates of 20, 15, 10, 8 and 5 K/min, as the peak of the Cp curves. At this point we used a recent method developed by [1] that, on the basis of the equivalence of the shear stress and the enthalpic relaxation time, allow to predict the low-T viscosity. The combined results obtained by using the different techniques above mentioned were fit by VFT equation with the high-T limiting value (viscosity value at infinite temperature) being fixed at a value of 10exp-4.31 Pas [2]. A comparison between the data obtained here with the recent model from [3] (calibrated with melts as basic as basanite), have shown that in the range 900 to 1600 _C, the viscosity calculated according to [3] is very similar to those measured or calculated by the VFT fit, if A = -4.31; the discrepancy becoming significant at T<900 _C. The very low T dependence of viscosity at superliquidus conditions obtained from the fitting here, indicates that at putative temperatures of the core-mantle boundary, near 5000 _C, the viscosity will decrease up to 10exp-3.5 Pa s. [1] J. Gottsmann et al. (2002), EPSL, 198, 417;. [2] J.K. Russell et al. (2003), Am. Mineral., 88, 1390;. [3] D. Giordano & D.B. Dingwell (2003), EPSL, 208, 337 and Errata Corrige, in press
An expanded non Arrhenian model for silicate melt viscosity: A treatment for metaluminous, peraluminous and peralkaline liquids
No full textWe 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)
The effect of P2O5 on the viscosity of haplogranitic liquid
No full textThe effect of P2O5 on the viscosity of a haplogranitic (K2O-Na2O-Al2O3-SiO2) liquid has been determined at 1 atm pressure in the temperature interval of 700 - 1650°C. Viscosity measurements of a haplogranite, haplogranite + 5.1 wt.% P2O5 and haplogranite + 9.5 wt.% P2O5 have been performed using the concentric cylinder and micropenetration methods. The viscosity of haplogranite liquid decreases with the addition of P2O5 at all temperatures investigated. The viscosity decrease is nonlinear, with the strongest decrease exhibited at low P2O5 concentration. The temperature-dependence of the viscosity of all the investigated liquids is Arrhenian, as is the case for P2O5 liquid. The Arrhenian activation energy is slightly lower in the P2O5-bearing liquids than in the P2O5-free haplogranite with the result that the effect of P2O5 on viscosity is a (weak) function of temperature. At temperatures corresponding to the crystallization of phosphorus-rich granitic and pegmatitic systems the addition of 1 wt.% of P2O5 decreases the viscosity 0.2 log10 units. The effect of P2O5 on haplogranitic melt viscosity is much less than that for B2O3, F2O−1 on the same melt composition (Dingwell et al., 1992 and this study). This implies that P2O5 concentration gradients in high-silica melts during, for example, phosphate mineral growth or dissolution in granitic magmas, will not significantly influence melt viscosity
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