The influence of pressure and composition on the viscosity of andesitic melts

The effect of pressure and composition on the viscosity of both anhydrous and hydrous andesitic melts was studied in the viscosity range of 10⁸ to 10^11.5 Pa · s using parallel plate viscometry. The pressure dependence of the viscosity of three synthetic, iron-free liquids (andesite analogs) containing 0.0, 1.06, and 1.96 wt.% H₂O, respectively, was measured from 100 to 300 MPa using a high-P-T viscometer. These results, combined with those from Richet et al. (1996), indicate that viscosities of anhydrous andesitic melts are independent of pressure, whereas viscosities of hydrous melts slightly increase with increasing pressure. This trend is consistent with an increased degree of depolymerization in the hydrous melts. Compositional effects on the viscosity were studied by comparing iron-free and iron-bearing compositions with similar degrees of depolymerization. During experiments at atmospheric and at elevated pressures (100 to 300 MPa), the viscosity of iron-bearing anhydrous melts preequilibrated in air continuously increased, and the samples became paramagnetic. Analysis of these samples by transmission electron microscopy showed a homogeneous distribution of crystals (probably magnetite) with sizes in the range of 10 to 50 nm. No significant difference in the volume fractions of crystals was found in samples after annealing for 170 to 830 min at temperatures ranging from 970 to 1122 K. An iron-bearing andesite containing 1.88 wt.% H₂O, which was synthesized at intrinsic fO₂ conditions in an internally heated pressure vessel, showed a similar viscosity behavior as the anhydrous melts. The continuous increase in viscosity at a constant temperature is attributed to changes of the melt structure due to exsolution of iron-rich phases. By extrapolating the time evolution of viscosity down to the time at which the run temperature was reached, for both the anhydrous (at 1055 K) and the hydrous (at 860 K) iron-bearing andesite, the viscosity is 0.7 log units lower than predicted by the model of Richet et al. (1996). This may be explained by differences in structural properties of Fe²⁺ and Fe³⁺ and their substitutes Mg²⁺, Ca²⁺, and Al³⁺, which were used in the analogue composition.The effect of iron redox state on the viscosity of anhydrous, synthetic andesite melts was studied at ambient pressure using a dilatometer. Reduced iron-bearing samples were produced by annealing melts in graphite crucibles in an Ar/CO atmosphere for different run times. In contrast to the oxidized sample, no variation of viscosity with time and no exsolution of iron oxide phases was observed for the most reduced glasses. This indicates that trivalent iron promotes the exsolution of iron oxide in supercooled melts. With decreasing Fe³⁺/ΣFe ratio from 0.58 to 0.34, the viscosity decreases by ∼1.6 log units in the investigated temperature range between 964 and 1098 K. A more reduced glass with Fe³⁺/ΣFe = 0.21 showed no additional decrease in viscosity. Our conclusion from these results is that the viscosity of natural melts may be largely overestimated when using data obtained from samples synthesized in air.     

Flow banding in basaltic pillow lavas from the Early Archean Hooggenoeg Formation, Barberton Greenstone Belt, South Africa

Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the Hooggenoeg Formation in the Barberton Greenstone Belt exhibit pronounced flow banding. The banding is defined by mm to several cm thick alternations of pale green and a dark green, conspicuously variolitic variety of aphyric metabasalt. Concentrations of relatively immobile TiO₂, Al₂O₃ and Cr in both varieties of lava are basaltic. Compositional differences between bands and variations in the lavas in general have been modified by alteration, but indicate mingling of two different basalts, one richer in TiO₂, Al₂O₃, MgO, FeO^t and probably Ni and Cr than the other, as the cause of the banding. The occurrence in certain pillows of blebs of dark metabasalt enclosed in pale green metabasalt, as well as cores of faintly banded or massive dark metabasalt, suggest that breakup into drops and slugs in the feeder channel to the lava flow initiated mingling. The inhomogeneous mixture was subsequently stretched and folded together during laminar shear flow through tubular pillows, while diffusion between bands led to partial homogenisation. The most common internal pattern defined by the flow banding in pillows is concentric. In some pillows the banding defines curious mushroom-like structures, commonly cored by dark, variolitic metabasalt, which we interpret as the result of secondary lateral flow due to counter-rotating, transverse (Dean) vortices induced by the axial flow of lava towards the flow front through bends, generally downward, in the tubular pillows. Other pillows exhibit weakly-banded or massive, dark, variolitic cores that are continuous with wedge-shaped apophyses and veins that intrude the flow banded carapace. These cores represent the flow of hotter and less viscous slugs of the dark lava type into cooled and stiffened pillows.     

Validation of official erosion modelling based on high‐resolution radar rain data by aerial photo erosion classification

The universal soil loss equation (USLE) is the most frequently applied erosion prediction model and it is also implemented as an official decision‐making instrument for agricultural regulations. The USLE itself has been already validated using different approaches. Additional errors, however, arise from input data and interpolation procedures that become necessary for field‐specific predictions on a national scale for administrative purposes. In this study, predicted event soil loss using the official prediction system in Bavaria (Germany) was validated by comparison with aerial photo erosion classifications of 8100 fields. Values for the USLE factors were mainly taken from the official Bavarian high‐resolution (5 × 5 m²) erosion cadastre. As series of erosion events were examined, the cover and management factor was replaced by the soil loss ratio. The event erosivity factor was calculated from high‐resolution (1 × 1 km², 5 min), rain gauge‐adjusted radar rain data (RADOLAN). Aerial photo erosion interpretation worked sufficiently well and average erosion predictions and visual classifications correlated closely. This was also true for data broken down to individual factors and different crops. There was no reason to assume a general invalidity of the USLE and the official parametrization procedures. Event predictions mainly suffered from errors in the assumed crop stage period and tillage practices, which do not reflect interannual and farm‐specific variation. In addition, the resolution of radar data (1 km²) did not seem to be sufficient to predict short‐term erosion on individual fields given the strong spatial gradients within individual rains. The quality of the input data clearly determined prediction quality. Differences between USLE predictions and observations are most likely caused by parametrization weaknesses but not by a failure of the model itself. Copyright © 2017 John Wiley & Sons, Ltd.     

Railway irregularity measuring using Rauch–Tung–Striebel smoothed multi-sensors fusion system: quad-GNSS PPP,IMU, odometer,and track gauge

GPS Solutions - Despite the broad range of navigation, positioning, and timing services offered by the global navigation satellite system (GNSS), its signals are vulnerable to blockage and...     

Scenario-based seismic hazard analysis using spectral element method in northeastern Pakistan

Natural Hazards - Seismic hazard analysis is carried out in this study by estimating ground motion for hypothetical earthquakes in the area of Muzaffarabad, Pakistan, with the MT solution of the...     

Structural states of Mg-cordierite I: Order parameters from synchrotron X-ray and NMR data

The hexagonal to orthorhombic phase transition in synthetic Mg-cordierite has been studied by (i) measuring the spontaneous strain associated with the transition using Synchrotron X-ray powder diffraction and (ii) measuring the degree of Al, Si order in terms of the number of Al-O-Al bonds per formula unit using solid state NMR spectroscopy. This defines the two order parametersQ andQ _od respectively, and their relationship as a function of annealing temperature and time is used to define the structural states of cordierite during the ordering sequence. The formation of modulated hexagonal cordierite within which a high degree of Al, Si order can be attained, results in a strongly non-linear relationship betweenQ andQ _od .The transition from modulated to orthorhombic cordierite is strongly first-order under all temperature conditions studied and involves a large step inQ, whileQ _od changes continuously throughout the ordering sequence with no marked discontinuity at the phase transition. The lattice distortion, traditionally defined in cordierite by the Δ index provides no full information on the degree of Al, Si order in anhydrous Mg-cordierite, and both order parameters must be used to define its structural state. Transmission electron microscopy has been used to study the mechanism of the transformation from hexagonal to modulated to orthorhombic cordierite.     

Simulation of moisture content in alpine rockwalls during freeze–thaw events

Rock moisture during freeze–thaw events is a key factor for frost weathering. Data on moisture levels of natural rockwalls are scarce and difficult to obtain. To close this gap, we can benefit from the extensive knowledge of moisture‐related phenomena in building materials, which is incorporated into simulation software, for example the WUFI® package of the Fraunhofer Institute of Building Physics. In this paper we applied and adapted this type of simulation to natural rockwalls to gain new insights on which moisture‐related weathering mechanisms may be important under which conditions. We collected the required input data on physical rock properties and local climate for two study areas in the eastern European Alps with different elevation [Sonnblick, 3106 m above sea level (a.s.l.) and Johnsbach, 700 m a.s.l.] and different lithologies (gneiss and dolomite, respectively). From this data, moisture profiles with depth and fluctuations in the course of a typical year were calculated. The results were cross‐checked with different thermal conditions for frost weathering reported in the literature (volumetric expansion and ice segregation theories). The analyses show that in both study areas the thresholds for frost cracking by volumetric expansion of ice (90% pore saturation, temperature < ?1 °C) are hardly ever reached (in one year only 0.07% of the time in Johnsbach and 0.4% at Sonnblick, mostly in north‐exposed walls). The preconditions for weathering by ice segregation (?3 to ?8 °C, > 60% saturation) prevail over much longer periods; the time spent within this ‘frost cracking window‘ is also higher for north‐facing sites. The influence of current climate warming will reduce effective frost events towards 2100; however the increase of liquid precipitation and rock moisture will promote weathering processes like ice segregation at least at the Sonnblick site. Copyright © 2016 John Wiley & Sons, Ltd.     

Impact of climatic change on the biological production in the Barents Sea

The Barents Sea is a high latitude ecosystem and is an important nursery and feeding area for commercial fish stocks such as cod, capelin and herring. There is a large inter-annual variability both in physical and biological conditions in the Barents Sea. Understanding and predicting changes in the system requires insight into the coupled nature of the physical and biological interactions. A coupled physical and biological ocean model is used to study the impact of postulated future atmospheric changes on the physical and biological conditions in the Barents Sea. Results from this simulation not only show that there is a large variability in the physical conditions on a wide range of time scales, but also that the temperature in the Barents Sea is increasing. The corresponding ice cover decrease is most noticeable in the summer months. The changes in physical properties will most likely have an impact on the biotope. On average, the primary production increases slightly over a 65 year long period, about 8%, partly due to an increased production in the northern Barents Sea. The model further simulates that the production of Atlantic zooplankton species increases approximately 20% and becomes more abundant in the east. The Arctic zooplankton biomass decreases significantly (50%) causing the total simulated production to decrease.     

High-Latitude Springtime Photochemistry. Part I: NOx, PAN and Ozone Relationships

Springtime measurements of NO_x, ozone, PAN,J(NO₂), and other compounds were made near Ny-Ålesund,Svalbard (78°54N, 11°53E), in 1994 and Poker Flat,Alaska (65°08N, 147°29W), in 1995. At Svalbard medianmixing ratios for PAN and NO_x of 237 and 23.7 pptv,respectively, were observed. The median mixing ratios at Poker Flat for PANand NO_x were 79.5 and 85.9 pptv, respectively. These data areused to estimate thermal PAN decomposition using several differentapproaches. At Svalbard PAN decomposition was very small, while at PokerFlat up to 30 pptv/h PAN decomposed. At both sites the NO_x/PANratio increased with temperature between –10 and 20°C implyingthat PAN decomposition is an important NO_x source. In-situozone production was calculated from the measured NO, NO₂,O₃, J(NO₂), and temperature data, using thesteady state assumption Median ozone production was 605 pptv/h at PokerFlat, and one order of magnitude smaller at Svalbard during the daytime.Only at Poker Flat could a direct influence on the diurnal ozone cycle beobserved from in-situ production. These results imply that PAN decompositionis a major source of NO_x in the high latitude troposphere, andthat this contributes to the observed spring maximum in surface ozone.