Physical Properties Of Magma Research Articles

Petrogenetic insights on tephritic magmatism from Davis Bank (South Atlantic Ocean - Vitória-Trindade Ridge (VTR), Brazil): The role of a CO2-enriched mantle source

Davis Bank is one of the largest physiographic features in the Vitória-Trindade Ridge (VTR), which is located offshore of the Eastern Brazilian margin, midway between the coastline and the South Atlantic mid-ocean ridge. The VTR is related to a linear volcanic track of ca. 1200 km in length, composed of several volcanic seamounts that are lithologically characterized by nephelinitic/tephritic alkaline rocks. The Davis Bank lies between the parallels 20°29′ and 20°32′ S and the meridians 29°17′ and 29°21′ W, and is located southeast of the Abrolhos Volcanic Complex (AVC), at the middle of the volcanic ridge, far from the Brazilian coast. This work focuses on the 40Ar/39Ar age determination of the rocks recovered from the seamount and characterization of Sr–Nd isotopes, mineral chemistry and whole-rock composition, aiming to access information about physical properties of magmas, source composition, magmatic evolution of the volcanic ridge, and the regional tectonic interpretation in the context of other seamounts along the Vitória-Trindade Ridge (VTR). Davis Bank presents 40Ar/39Ar age of ca. 21 Ma and is in an oceanic lithosphere of ca. 100 km thickness, which represents a recharging area in the oceanic lithospheric mantle for CO2-metasomatism enrichment. It is associated with mantle anomalies caused by a hotspot or mantle plume that was influenced by the E-W direction of leaking transform fracture zones in the South Atlantic Ocean. Age constraints indicate decreasing ages eastward of the AVC, favoring the hotspot hypothesis for the generation of magmatic rocks in the Vitória-Trindade Ridge.

29Si solid state NMR and Ti K-edge XAFS pre-edge spectroscopy reveal complex behavior of Ti in silicate melts

An understanding of the mechanisms of Ti is incorporation into silicate glasses and melts is critical for the field of petrology. Trace-element thermobarometry, high-field-strength element partitioning, and the physical properties of magmas are all be influenced by Ti incorporation into glasses and changes therein in response to changes in composition and temperature. In this study, we combine 29Si solid state NMR and Ti K-edge XAFS spectroscopy to investigate how Ti is incorporated into quenched Na-silicate glasses, and the influence of Ti on the structure of silicate species in these glasses. 29Si NMR shows that in both Ti-bearing Na2O•4SiO2 (NS4) and Na2O•8SiO2 (NS8) glasses, increasing the amount of Ti in the melt results in a shift of Si Q4 peak in the 29Si NMR spectra reflecting Ti nearest neighbors for Si in Q4 speciation. The Ti XAFS results from NS8 glass indicate that Ti is primarily incorporated in [5]-fold coordination. At higher Ti content, there is a shift of the XAFS pre-edge feature suggesting mixing of [4]-fold Ti into the spectra. Combined, the 29Si NMR and XAFS pre-edge data are consistent with Ti incorporation as isolated [5]Ti atoms and the formation of [5]Ti clusters at relatively low Ti concentrations, with no evidence for Ti–Na interactions as suggested by previous studies. As the Ti content increases, the Ti atoms begin to occupy 4-fold coordinated sites interacting primarily with Si in Q4 speciation (no significant Na–[4] Ti bonding). The internal consistency of these two techniques provides a uniquely complete snapshot of the complexity of Ti incorporation in silicate melts and underlies the importance of understanding Ti incorporation mechanisms in natural magmatic systems.

A Physical Model of Sill Expansion to Explain the Dynamics of Unrest at Calderas with Application to Campi Flegrei

Many calderas show remarkable unrests, which often do not culminate in eruptions (non-eruptive unrest). In this context the interpretation of the geophysical data collected by the monitoring networks is difficult. When the unrest is eruptive, a vent opening process occurs, which leads to an eruption. In volcanic calderas, vent locations typically are scattered over a large area and monogenic cones form. The resulting pattern is characterized by a wide dispersion of eruptive vents, therefore, the location of the future vent in calderas is not easily predictable. We propose an interpretation of the deformation associated to unrest and vent pattern commonly observed at volcanic calderas, based on a physical model that simulates the intrusion and the expansion of a sill. The model can explain both the uplift and the subsequent subsidence, through a single geological process. In particular, we simulate the vertical displacement that occurred at the central area of Campi Flegrei caldera during the last decades, and we obtain good agreement with the data of a leveling benchmark near the center of the caldera. Considering that the stress mainly controls the vent opening process, we try to gain insight on the vent opening in calderas through the study of the stress field produced by the intrusion of an expanding sill. We find that the tensile stress in the rock above the sill is concentrated in a ring-shaped area with radius depending on the physical properties of magma and rock, the feeding rate and the magma cooling rate. This stress field is consistent with widely dispersed eruptive vents and monogenic cone formation, which are often observed in the calderas. However, considering the mechanical properties of the elastic plate and the rheology of magma, we show that remarkable deformations may be associated with low values of stress in the rock at the top of the intrusion, thereby resulting in non-eruptive unrests. Moreover, we have found that, under the assumption of isothermal conditions, the stress values decrease over time during the intrusion process. This result may explain why the long-term unrests, in general, do not culminate in an eruption.

The temporal evolution of chemical and physical properties of magmatic systems

Abstract Exactly 100 years ago the great Canadian-born petrologist N. L. Bowen published two seminal works on the chemical differentiation of magmas in which he posed the basis for a physico-chemical understanding of the fractionation of crystals from melts in molten rock. A subsequent century of research and technological advances has enhanced our understanding of the physics and chemistry of magmatic systems and their temporal evolution. The image of sub-volcanic magmatic systems has evolved greatly in that time, from a simple ‘boiling vat’ concept of molten rock in which bubbles, crystals and melt separate gravitationally to a recognition that magma vats are relatively rare and that most magmatic systems spend much of their lifetime in a partially molten, or mushy, state. Real magmatic systems appear to be organized into a series of storage regions periodically connected by feeding structures transferring magma (and heat) at different fluxes. Magma fluxes between the different portions of this plumbing system, and the variation of the chemical and physical properties of magma as it rises through the crust, exert essential controls on the eruptive modalities of volcanoes and the geochemistry of their products. This book presents a collection of contributions that use petrology, geochemistry, geochronology and numerical modelling to identify the processes operating at different depths within magmatic systems and to characterize the fluxes of magma between them.

Stable isotope fractionation by thermal diffusion through partially molten wet and dry silicate rocks

Water plays a fundamental role in affecting geochemical transport and physical properties of magmas. Here we show the previously undocumented behavior of water within partially molten silicate resting in a temperature gradient, producing O, Li and H isotope redistribution by thermal diffusion leading to enrichment of light isotopes at the hot end of the gradient. After weeks to months, fully molten as well as mostly crystalline portions of water-bearing experiments develop remarkably large isotope and chemical redistributions: up to 28‰ for δ18O, 144‰ for δD, and 18‰ for δ7Li. In contrast, long-term dry experiments develop smaller (∼5‰ δ18O) isotopic fractionations only in the hotter end where it is molten or partially molten. Isotope fractionation of oxygen is linearly related to temperature, and the magnitude of isotopic separation per °C is ∼2× larger for wet experiments than dry ones. We explain this by water de-polymerizing the silicate structure leading to a smaller size of diffusing SiOx fragments. The magnitude of isotope separation between the hot and cold ends for Li, Mg, Fe, O, and H isotopes increases linearly with ΔMheavy–light/Mlight. These relationships provide predictive tests for natural rocks and highlight the role of water in isotope and compositional redistribution during temperature gradient mediated processes. We discuss the implications to natural environments in which the lightest stable isotopes (H, Li, O) with the greatest ΔMheavy–light/Mlight and fastest diffusion coefficients are capable of achieving mass-dependent redistribution in a transient temperature gradient. These experiments underscore the importance of solution–reprecipitation in wet subsolidus systems and demonstrate that isotopic redistribution can be established ∼6 orders of magnitude quicker than by diffusion through a traditional silicate melt at higher temperature. This has important implications for timescales of natural isotope and chemical redistribution by thermal diffusion.

Magma heating by decompression-driven crystallization beneath andesite volcanoes

Explosive volcanic eruptions are driven by exsolution of H2O-rich vapour from silicic magma. Eruption dynamics involve a complex interplay between nucleation and growth of vapour bubbles and crystallization, generating highly nonlinear variation in the physical properties of magma as it ascends beneath a volcano. This makes explosive volcanism difficult to model and, ultimately, to predict. A key unknown is the temperature variation in magma rising through the sub-volcanic system, as it loses gas and crystallizes en route. Thermodynamic modelling of magma that degasses, but does not crystallize, indicates that both cooling and heating are possible. Hitherto it has not been possible to evaluate such alternatives because of the difficulty of tracking temperature variations in moving magma several kilometres below the surface. Here we extend recent work on glassy melt inclusions trapped in plagioclase crystals to develop a method for tracking pressure-temperature-crystallinity paths in magma beneath two active andesite volcanoes. We use dissolved H2O in melt inclusions to constrain the pressure of H2O at the time an inclusion became sealed, incompatible trace element concentrations to calculate the corresponding magma crystallinity and plagioclase-melt geothermometry to determine the temperature. These data are allied to ilmenite-magnetite geothermometry to show that the temperature of ascending magma increases by up to 100 degrees C, owing to the release of latent heat of crystallization. This heating can account for several common textural features of andesitic magmas, which might otherwise be erroneously attributed to pre-eruptive magma mixing.

Echo-resonance and hydraulic perturbations in magma cavities: application to the volcanic tremor of Etna (Italy) in relation to its eruptive activity

A study is presented of spectral features of volcanic tremor recorded at Mount Etna (Sicily, Italy) following the methods of analysis suggested by the resonant scattering formalism of Gaunaurd and Uberall (1978, 1979a, 1979b) and the model for hydraulic origin of Seidl et al. (1981). The periods investigated include summit and flank eruptions that occurred between 1984 and 1993. Recordings from a permanent station located near the top of the volcano were used, and the temporal patterns associated with (a) the average spacing (\(\bar \Delta \)) between consecutive spectral peaks in the frequency range 1–6 Hz, (b) the spectral shape and (c) the overall spectral amplitude were analyzed. \(\bar \Delta \) values are thought to depend on the physical properties of magma, such as its density, which, in turn, is controlled by the degree of gas exsolution. Variations in the spectral shape are tentatively attributed to changes in the geometrical scattering from the boundary of resonant conduits and magma batches. Finally, the overall amplitude at the station should essentially reflect the state of turbulence of magma within the superficial ascending path. A limit in the application of the resonant scattering formalism to the study of volcanic tremor is given by the fact that the fundamental modes and integer harmonics are difficult to identify in the frequency spectra, as tremor sources are likely within cavities of very complex geometry, rather than in spherical or cylindrical chambers, as expected by theory. This study gives evidence of some correlations between the analyzed temporal patterns and the major events in the volcanic activity, related to both lava flow and explosions at the summit vents. In particular, relatively high values of \(\bar \Delta \) have been attained during the SE crater eruption of 1984, the complex eruptive phases of September–October 1989 and the 1991–1993 flank eruption, suggesting the presence of a relatively dense magma for all of these events. Conversely, very low values have been recorded in coincidence with the December 1985 activity and the paroxysmal explosions at the summit craters of early 1990, which are interpreted here as fed by fluid-vesiculated magma. Appreciable modifications in the spectral shape have been observed in relation to changes of the volcanic activity that probably preceded the opening and disactivation of shallow dykes or magma batches. Finally, the overall amplitude seems to be a sensitive indicator of the state of gas turbulence within the shallow conduits, as is suggested by the high values attained during phases of intense volcanic activity.

The Effect of Water on the Solubility and Speciation of CO2 in Aluminosilicate Glasses Along the Join SiO2-NaAlO2

C O 2 and H20 are the most important volatiles in the majority of magmatic systems and although the C02 is usually subordinate in concentration, it is often the first component to reach saturation due to the much higher solubility of water in the melt. C02 vesiculation can therefore be the driving force in many eruptive situations and it is important to investigate quantitatively the effect that varying amounts of water have on CO2 saturation. A number of previous studies have suggested that the solubility of CO2 is greater in hydrous melts than in the anhydrous equivalents. This effect has been observed for a variety of polymerized and depolymerized compostions at high pressures (e.g. Brey and Green, 1975; Mysen, 1976). For albite and other simple sodium aluminosilicates, Mysen (1976) suggested that there is a maximum in the solubility as a function of the molar CO2/CO2 + H20 ratio in the capsule. In contrast, Blank et al. (1993), studying the solubility of CO2 in rhyolite at much lower pressures and temperatures found that the solubility of CO2 was not enhanced by the presence of water. The aim of this study is to examine the effect of water on the solubility and speciation of CO2 in albite and jadeite melts. The speciation information is important if changes in the physical properties of magmas under such conditions are to be predicted. Albite is the main composition studied as there is considerable data available on the solubility and speciation of the pure components (e.g. Stolper et al., 1987; Kohn et al., 1989; Silver and Stolper, 1989) as well as theoretical predictions of solubility mechanisms. In addition, our data will provide a direct test of the conclusions of Mysen (1976).

特集 地球科学 高圧下におけるケイ酸塩融体の構造

Knowledge of the structural changes of silicate melts under high pressure and temperature is extremely important to understand the chemical and physical properties of magmas in nature. In-situ x-ray measurements are possible for silicate melts under high pressure by means of synchrotron radiations and multi-anvil apparatus (MAX80). It is, however, difficult to obtain corrected x-ray intensity profiles because of the source intensity profile, absorption, incoherent scattering, etc. Observed intensity of melts can be corrected reasonably by means of a Mote Carlo simulation technique.

Silicon Coordination and Speciation Changes in a Silicate Liquid at High Pressures

Coordination and local geometry around Si cations in silicate liquids are of primary importance in controlling the chemical and physical properties of magmas. Pressure-induced changes from fourfold to sixfold coordination of Si in silicate glass samples quenched from liquids has been detected with (29)Si magic-angle spinning nuclear magnetic resonance spectrometry. Samples of Na(2)Si(2)O(5) glass quenched from 8 gigapascals and 1500 degrees C contained about 1.5 percent octahedral Si, which was demonstrably part of a homogeneous, amorphous phase. The dominant tetrahedral Si speciation in these glasses became disproportionated to a more random distribution of bridging and nonbridging oxygens with increasing pressure.

Physical Properties of Magmas at High Pressure

Physical Properties of Magmas at High Pressure

Rheological Properties of Magmas

Recent studies of igneous rocks have taken a refreshing new direction, mainly as the result of a greater awareness of the important role played by physical properties of magmas in determining the eruptive behavior and compositional variations of volcanic rocks. This rcview summarizes the present state of knowledge, along with some of the recent rheological studies having a direct bearing on interpretations of volcanic phenomena and processes of crystallization and differentiation of shallow magmatic intrusions. The rheological properties of magmas reflect the inherent structures of molten silicates and will not be thoroughly understood until more is known about the basic nature of silicate liquids. Recent progress in this field has been ably reviewed by Hess (1980) and needs no elaboration here aside from mention of the obvious need for better laboratory measurements in order to place greater constraints on interpretations of silicate melts. For petro­ genetic and volcanological processes, the most conspicuous need is for improved data on rheological properties, chiefly viscosity and yield strength, and for more precise equations for predicting these properties under a wide range of natural conditions. Viscosity, 1'/, is usually defined as the ratio of shear stress to strain rate and is expressed in units of poise (g cm 1 S 1) or Pascal seconds ( 10 poise = 1.0 Pa s). In more general terms, it is the coefficient for transfer of

The peculiarities of magma rise in presence of water

The viscosity of basalts (quartz and olivine tholeiite) was studied under pressure in dry conditions and in the presence of water. In dry conditions at 1400°C when pressure increases to 20 kbars the viscosity reduces by a factor of 2. In conditions of water saturation of basalt melts at 5 kbar the viscosity is smaller by a factor of ∼ 50 than that in dry conditions. In the water undersaturated conditions when water content is fixed (3.3% H2O) in melt the viscosity considerably decreases with pressure and takes intermediate value between those in dry and water saturated conditions. Experimental data recently obtained permit us to consider the peculiarities of physical properties of magma in the presence of water on a new base. Ascending magma can reach critical velocities of transition to the turbulent regime under negligible pressure drop, as a result of low viscosity. It is known at present that water influences on the viscosity of acidic melt under pressure of 1–8 kbars and at temperatures between 800–1200°C. Various authors gave physico-chemical evaluation of the dynamics of granite melts on the basis of these data. The viscosity of basalt melts and their dynamics under normal pressure is also well-known. The known new experimental data of basaltic melt viscosity under pressure in dry conditions (Kushiro et al., 1976;Khitarov et al., 1978) and in the presence of water (Khitarov et al., 1976) embrace broader intervals of physico-chemical conditions as on the pressure (up to 20–30 kbar) as well on the content of water (from 3% up to 12 %). These data permitted to evaluate on a new base the dynamics of magmatic melts under pressure.

Effects of H 2 O and CO 2 on magma generation in the crust and mantle

The solution of H 2 O, CO 2 , and H 2 O–CO 2 mixtures in silicate magmas has many effects. Fusion temperatures are lowered, and the fusion and crystallization sequences may be modified. Hydrous minerals and carbonates may become stable within the fusion interval. Liquid compositions are changed as a function of dissolved volatile content, and as a function of H 2 O/CO 2 . The physical properties of magmas may be influenced markedly by dissolved volatile components. The presence of vapour bubbles arising from exsolution of volatile components during uprise or crystallization of magma may be responsible for physical and geochemical effects. These effects are outlined by examination of the phase relationships in several synthetic systems and rock–H 2 O–CO 2 systems. The results demonstrate that H 2 O is more influential than CO 2 in magma generation at crustal pressures. The presence of CO 2 , however, may influence the evolutionary history of a crustal magma body. In the mantle at depths of 75–80 km, the influence of CO 2 becomes as significant as that of H 2 O because CO 2 reacts with peridotite stabilizing calcic dolomite, and the solubility of CO 2 increases dramatically. The compositions of near-solidus mantle magmas are strongly influenced by H 2 O/CO 2 and by the distribution of carbonate, amphibole and phlogopite in the peridotite. The normal product of crustal anatexis is H 2 O-under-saturated granitic liquid. Tonalite liquids can-not be produced by crustal anatexis during normal regional metamorphism.