Chemical mass transfer in magmatic processes (original) (raw)
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Multicomponent diffusion in basaltic melts
Geochimica et Cosmochimica Acta, 1995
Experimental results are presented for eighteen experiments exploring multicomponent chemical-diffusion in basaltic liquids. Experiments were performed in Columbia River Basalt (CRB) composition doped with about 5 wt% SiOZ, Ti02, AlzOl, FeO, MgO, and CaO, under reducing conditions at 1 atm., at 1473 K, 1573 K, and 1723 K. Results indicate that diffusion behavior in CRB compositions is consistent with a simple Fick's law formulation. This Fickian behavior in CRB compositions contrasts with more complicated diffusion behavior observed in MgO-AlZ03-Si02 and CaO-MgO-Al@-Si02 melts. Results of CRB experiments are combined to calibrate a diffusion matrix (D) in CRB liquids at 1473 K, 1573 K, and 1723 K. Our D estimates indicate negative coupling between CaO and both Fe0 and Al?O? components, though diagonal elements still dominate. These general features persist across the temperature range considered. Self-and tracer-diffusion data from the literature are used with the predictive model of Richter ( 1993) to estimate a D matrix at 1573 K. The resulting matrix does not compare well with our measured 1573 K diffusion matrix. It cannot be established if this discrepancy indicates a failure of the Richter ( 1993) model, or merely reflects deficiencies in the available tracer-and self-diffusion data, or limitations in the melt activity model. Effective Binary Diffusion Coefficients (EBDC) were also estimated for TiO?, A1203, FeO, MgO, and CaO at 1473 K, 1573 K, and 1723 K. These EBDC estimates are used to constrain a polythermal Arrhenian model for the prediction of EBDC values at super-liquidus temperatures. Results of experiments on alkali diffusion in CRB compositions are included to add NazO and K20 to the polythermal EBDC model.
Multicomponent diffusion in MgO-Al2O3-SiO2 and CaO-MgO-Al2O3-SiO2 melts
Geochimica et Cosmochimica Acta, 1993
The process of multicomponent diffusion in silicate melts has been explored through a series of diffusion experiments in MgO-A1203-Si02 and CaO-MgO-A1203-Si02 melts. Experiments were performed in a variety of compositional directions in both systems at temperatures close to 1773 K. These experiments demonstrate a variety of uphill diffusion effects. The first-order features of our composition profiles can be represented using a constant n -1 dimensional diffusion matrix, however, many secondorder features cannot. A more complex model, in which the melt is postulated to be made up of a number of diffusing melt species, is more successful at reproducing flux reversal features. Unfortunately, this scheme suffers from a surplus of unconstrainable parameters. Detailed compositional maps and computer simulations suggest that convective fingering processes are not responsible for the unusual features in our profiles.
Geochemistry International, 2006
The paper summarizes the principal results obtained over the past three decades at the Vernadsky Institute and the Department of Geochemistry of the Moscow State University by the computer simulation of basaltic magma differentiation in magma chambers. The processes of diffusion-controlled mass transfer in a chamber are demonstrated to be principally limited by the heat resources of the cooling magma and cannot play any significant role during the large-scale partitioning of melt components. The leading mass-transfer mechanism is the settling of crystals from convecting magma in the form of suspension flows that are enriched and depleted in the solid phase. The physical prerequisite for the onset of this concentration convection is the existence of boundary layers, which are characterized by volume crystallization and a gradient distribution of the suspended phases. Considered in detail are the principles used in the development of algorithms with regard for the occurrence of a boundary layer and the "instantaneous" stirring of the crystallizing magma that does not hamper the settling of mineral grains forming the cumulus. The plausibility of the convection-accumulation model is illustrated by the example of the reconstructed inner structure of differentiated Siberian traps. In application to these bodies, it is demonstrated that the solutions of the forward and inverse simulation problems with the use of geochemical thermometry techniques are identical. This is a convincing argument for the predominance of convection-accumulation processes during the formation of thin tabular magmatic bodies. The further development of the computer model for the differentiation dynamics should involve the processes of compositional convection related to the migration and reactivity of the intercumulus melt.
Earth and Planetary Science Letters, 1991
We report on thermodynamic non-equilibrium crystallization calculations for a unit volume of a binary melt subject to a constant, prescribed rate of heat loss. Crystallization histories and crystal size distributions for both melt components were calculated by accounting for the nucleation and growth of crystals. The crystal sizes were found to decrease with increasing rates of heat loss. The crystallization time defined as the time to crystallize 99% of the unit volume also decreased strongly with increasing rates of heat loss up to a critical rate. The critical rate was found to be somewhat smaller than the heat loss rate for the beginning of glass formation. At larger than critical rates, crystallization time increased again and for rates larger than the glass formation rate, crystallization time became infinite. The residual melt composition was found to increasingly deviate from the equilibrium composition with increasing rates of heat loss. But as long as the loss rate was less than the critical rate the crystallization path reverted to the euteetic composition during the final crystallization. For supercritical rates, no such reversion was observed. We compared the critical rate with estimates of the rates of heat loss in magmatic intrusions based on the Stefan solution for a freezing half space. It was found that rates of heat loss should be supercritical at distances of up to 0.5 m from the margin of an intrusion. In this region, non-equilibrium effects are expected to dominate and the texture of the crystallized rock should be characterized by small crystals and by glass. The glass and the crystals should be of non-equilibrium composition. Non-equilibrium effects should be negligible only at distances of more than 5 m from the margin where the rates of heat loss are less than 10 2 times critical. At these distances, the crystallized rock should have an equigranular texture and an equilibrium composition.
Multicomponent diffusion in the molten system K2O-Na2O-Al2O3-SiO2-H2O
1998
We have measured multicomponent chemical diffusion coefficients in a melt near to the low pressure water-saturated eutectic granite composition in the system K 2 O-Na 2 O-Al 2 O 3 -SiO 2 -H 2 O at 1.0 GPa and temperatures of 1300 and 1600 ЊC. The measured diffusion profiles can be accounted for within the analytical error by diffusion coefficients, which are not dependent on composition within the range of compositions accessed by our experiments. The multicomponent diffusion coefficient matrix [D] has a highly degenerate set of real, positive eigenvalues that show a regular relation to melt viscosity on an Arrhenius diagram. The smallest eigenvalue is that associated predominantly with Si-Al exchange. The larger two eigenvalues are those associated with Si-Na and Si-K exchange and are effectively degenerate, with the result that exchanges of alkalis for silica or for each other can proceed in pseudo-binary fashion without inducing fluxes of other components. The eigenvalue associated with H-Si exchange is smaller than the alkali-silica eigenvalues, but analytical uncertainties make it also effectively degenerate with the alkalis. Uphill diffusion, notably of water and alkalis, was observed in several experiments, and this would lead to transient partitioning of water and alkalis across diffusion interfaces showing large Al 2 O 3 concentration gradients. Such partitioning in natural systems would persist until Al concentration gradients were erased by continued, much slower Al-Si interdiffusion.
Predicting Major Element Mineral/melt Equilibria
1979
A data set has been collected from the literature, comprising 230 synthetic mineral/melt pairs for which phase composition and run temperature are known. All phase pairs were equilibrated at I atm under anhydrous conditions. Solid phases represented are olivine, low-Ca pyroxene, high-Ca pyroxene, and plagioclase. We have developed empirical equations for calculating the mole fractions of NaOo.5, MgO, A1Ol.5, SiO2, KOo.5, CaO, TiO2, and FeO in a solid phase of initially unknown identity given only the composition of the coexisting silicate melt. The approach involves a linear multivariate regression analysis in which solid composition is expressed as a Taylor series expansion of the liquid compositions. We obtain an internally consistent precision of-•0.94; that is, we can correctly predict the nature of the liquidus phase in our input data set for approximately 94% of the entries. The composition of the liquidus phase may be calculated to better than 5 mol % absolute. An important feature of this 'generalized solid' model is its reversibility; that is, the dependent and independent variables in the linear multivariate regression may be inverted to permit prediction of the composition of a silicate liquid produced by equilibrium partial melting of a polymineralic source assemblage. We have added 14 points from a silica-olivine-anorthite pseudoternary phase diagram to the data set to deal with polymineralic source assemblages. The composition of the first partial melt can be calculated to better than 3 mol % absolute.
Stepwise magma migration and accumulation processes and their effect on extracted melt chemistry
Estonian Journal of Earth Sciences, 2009
Numerical and analogue models suggest that melt production, its segregation from the solid matrix and subsequent transport and accumulation are highly dynamic and stepwise processes exhibiting scale invariant patterns in both time and length scales, which is characteristic of self-organized critical systems. This phenomenon is also observed in migmatites at several localities, where the leucosome thickness statistics obey power laws. Stepwise melt transport and deformation-enhanced melt mobility affect melt production dynamics by determining the distribution of extracted melt batch sizes and residence times of melt pockets within the host rock, which in turn would influence the geochemistry of extracted melts. We introduce a numerical approach, which enables qualitative and quantitative assessment of the effects of stress-induced melt migration and accumulation on the chemistry of partial melts. The model suggests that apart from different sources and melting percentages, deformation can be an important factor in producing geochemical variations within and between intrusive/extrusive complexes.
Thermodynamics of minerals and melts
Rev Geophys, 1991
The thermochemical properties of earth materials and models of fundamental geodetic processes are discussed, along with the energetics of mineral-melt systems and microscopic-level processes contributing to thermodynamic quantities, in a critical review of U.S. research from the period 1987-1990. Consideration is given to compilations of thermodynamic data; computational methods; links between thermodynamics and kinetics; thermometry, bathymetry, and the estimation of intensive variables; mineral basic data, phase transitions, and solid solutions; and melt and fluid modeling, macroscopic properties, and spectroscopy. A comprehensive bibliography is provided.
Densities of melts in the CaO-MgO-Al2O3-SiO2system
American Mineralogist, 1999
Density measurements have been performed on 4 Mg-aluminosilicate melts and 4 melts in the CaO-MgO-Al 2 O 3-SiO 2 system (including stoichiometries corresponding to the mineral compositions åkermanite, diopside, enstatite, and cordierite) in the temperature ranges from their respective melting points up to 1800 ЊC, using the very precise Ir-based doublebob Archimedean method. The measured densities of the melts range from 2.67 to 2.42 g/cm 3. Together with our previous density measurements in the CaO-Al 2 O 3-SiO 2 system, the present results were analyzed using a regression equation, including a non-ideal mixing term between CaO and SiO 2 , from which the partial molar volume of each oxide liquid component was obtained by the method of least squares. This procedure yields partial molar volumes of 12.66, 20.66, 36.67, 27.30 cm 3 /mol at 1873 K for MgO, CaO, Al 2 O 3 , and SiO 2 , respectively. A calculation scheme for melts in the CaO-MgO-Al 2 O 3-SiO 2 system is proposed, involving an excess volume term between SiO 2 and CaO that is capable of reproducing the liquid molar volumes to within 1% except for extremely Ca-, Mg-, and Al-rich compositions. Better constraints on melt volumes in those extreme composition ranges requires new very high-temperature data. The volume of fusion of various minerals was calculated and implications for the structure of their respective melts are also discussed. In addition, the volumes of fusion from this study are also compared to that obtained using the Clausius-Clapeyron equation.