The thermodynamic properties of 154 mineral end-members, 13 silicate liquid end-members and 22 aqueous fluid species are presented in a revised and updated data set. The use of a temperature-dependent thermal expansion and bulk modulus, and the use of high-pressure equations of state for solids and fluids, allows calculation of mineral–fluid equilibria to 100 kbar pressure or higher. A pressure-dependent Landau model for order–disorder permits extension of disordering transitions to high pressures, and, in particular, allows the alpha–beta quartz transition to be handled more satisfactorily. Several melt end-members have been included to enable calculation of simple phase equilibria and as a first stage in developing melt mixing models in NCKFMASH. The simple aqueous species density model has been extended to enable speciation calculations and mineral solubility determination involving minerals and aqueous species at high temperatures and pressures. The data set has also been improved by incorporation of many new phase equilibrium constraints, calorimetric studies and new measurements of molar volume, thermal expansion and compressibility. This has led to a significant improvement in the level of agreement with the available experimental phase equilibria, and to greater flexibility in calculation of complex mineral equilibria. It is also shown that there is very good agreement between the data set and the most recent available calorimetric data.

An internally consistent thermodynamic data set for phases of petrological interest

Two methods of obtaining kinetic parameters from derivative thermoanalytical curves are proposed The methods are based on the general form of kinetic formulae and are applicable to general types of reactions governed by a single activation energy One method utilizes the linear relation between peak temperature and heating rate in order to estimate the activation energy, and only the information of the rate of conversion versus the temperature is necessary The other method needs the information of both the conversion and the rate of conversion versus the temperature, and the Arrhenius plot is made for an assumed kinetic mechanism

Kinetic analysis of derivative curves in thermal analysis

The thermodynamic properties of 254 end-members, including 210 mineral end-members, 18 silicate liquid end-members and 26 aqueous fluid species are presented in a revised and updated internally consistent thermodynamic data set. The PVT properties of the data set phases are now based on a modified Tait equation of state (EOS) for the solids and the Pitzer & Sterner (1995) equation for gaseous components. Thermal expansion and compressibility are linked within the modified Tait EOS (TEOS) by a thermal pressure formulation using an Einstein temperature to model the temperature dependence of both the thermal expansion and bulk modulus in a consistent way. The new EOS has led to improved fitting of the phase equilibrium experiments. Many new end-members have been added, including several deep mantle phases and, for the first time, sulphur-bearing minerals. Silicate liquid end-members are in good agreement with both phase equilibrium experiments and measured heat of melting. The new dataset considerably enhances the capabilities for thermodynamic calculation on rocks, melts and aqueous fluids under crustal to deep mantle conditions. Implementations are already available in thermocalc to take advantage of the new data set and its methodologies, as illustrated by example calculations on sapphirine-bearing equilibria, sulphur-bearing equilibria and calculations to 300 kbar and 2000 °C to extend to lower mantle conditions.

An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids

Results of high-pressure experiments on samples of hydrated mantle rocks show that the serpentine mineral antigorite is stable to approximately 720 degrees C at 2 gigapascals, to approximately 690 degrees C at 3 gigapascals, and to approximately 620 degrees C at 5 gigapascals. The breakdown of antigorite to forsterite plus enstatite under these conditions produces 13 percent H(2)O by weight to depths of 150 to 200 kilometers in subduction zones. This H(2)O is in an ideal position for ascent into the hotter, overlying mantle where it can cause partial melting in the source region for calc-alkaline magmas at a depth of 100 to 130 kilometers and a temperature of approximately 1300 degrees C. The breakdown of antigorite in hydrated mantle produces an order of magnitude more H(2)O than does the dehydration of altered oceanic crust.

https://science.sciencemag.org/content/sci/268/5212/858.full.pdf

Serpentine Stability to Mantle Depths and Subduction-Related Magmatism

Chemical characteristics of fluid phase released from a subducted lithosphere and origin of arc magmas: Evidence from high-pressure experiments and natural rocks

A NUMBER of articles have been published dealing with the differential thermal analysis (DTA) of chemical reaction kinetics1–5. Only one of these, however, gives a direct method for determining the activation energy (E) of the chemical reaction4. In this method, DTA curves are recorded at several different rates of heating. The results are then plotted as ln b/Tm2 against 1/Tm, where Tm is the peak temperature in ° K (the temperature of point B, Fig. 1), and b is the rate of heating. The magnitude of E is calculated from the angle between this straight line and the abscissa. This procedure has two principal shortcomings: (1) it is necessary to record several DTA curves with various rates of heating; and (2) it is necessary to use a special programming device to control the temperature. This device must be capable of providing linear heating at a number of rates of heating. The latter condition imposes serious technical difficulties, because all the DTA apparatuses at present available are adjusted for only one rate of heating.

Determination of activation energies of chemical reactions by differential thermal analysis

Pyroxene-carbonate reactions in the upper mantle

The phase diagram for lherzolite–CO_2–H_2O provides a framework for interpreting the distribution of phase assemblages in the upper mantle with various thermal structures, in different tectonic settings. Experiments show that at depths >80 km, the near-solidus partial melts from lherzolite–CO_2–H_2O are dolomitic, changing through carbonate–silicate liquids with rising temperatures to mafic liquids; vapor, if it coexists, is aqueous. Experimental data from simple systems suggest that a critical end-point (K) occurs on the mantle solidus at an undetermined depth. Isobaric (T–X) phase diagrams for volatile-bearing systems with K elucidate the contrasting phase relationships for lherzolite–CO_2–H_2O at depths below and above a critical end-point, arbitrarily placed at 250 km. At levels deeper than K, lherzolite can exist with dolomitic melt, aqueous vapor, or with critical fluids varying continuously between these end-members. Analyses of fluids in microinclusions of fibrous diamonds reveal this same range of compositions, supporting the occurrence of a critical end-point. Other evidence from diamonds indicates that the minimum depth for this end-point is 125 km; maximum depth is not constrained. Constructed cross-sections showing diagrammatically the phase fields intersected by upwelling mantle indicate how rising trace melts may influence trace element concentrations within a mantle plume.

/pdf/volatile-components-magmas-and-critical-fluids-in-upwelling-2lnkbtwypl.pdf

Volatile Components, Magmas, and Critical Fluids in Upwelling Mantle

Experimental-evidence at high-pressure for potassic metasomatism in the mantle of the earth

Solubility experiments were performed at 30 kbars in the system Mg2SiO4-SiO2-H2O, and at 20 and 30 kbars on omphacitic pyroxene-water mixtures. They confirm that the solubility of the forsterite component in aqueous fluids remains rather low (up to 5 wt.%), whereas the solubility of the SiO2 component from solids of appropriate SiO2-rich compositions in the system Mg2SiO4-SiO2-H2O increases with temperature up to some 75% at 1,100° C. At this temperature a simplified harzburgite consisting of forsterite and enstatite coexists with a fluid containing about 35% (MgO+SiO2). Hydrous fluids coexisting with omphacitic clinopyroxenes leach sodium silicate component from the solid leaving less jadeitic pyroxenes behind. Most interestingly, the amount of sodium leached at constant temperature increases with decreasing pressure. Comparison of the results with previous solubility studies in the system K2O-MgO-Al2O3-SiO2-H2O indicates that hydrous fluids in the mantle must be alkaline rather than silicanormative. Alkali metasomatism caused by such fluids would lead to potassium enrichment in deeper portions of the upper mantle and to sodium enrichment at shallower levels, where amphiboles become stable. This K/Na fractionation in the upper mantle may explain the generation of K-rich or of Na-rich magmas through partial melting at different depths.

I. D. Ryabchikov

Papers

Determination of activation energies of chemical reactions by differential thermal analysis

Pyroxene-carbonate reactions in the upper mantle

Volatile Components, Magmas, and Critical Fluids in Upwelling Mantle

Experimental-evidence at high-pressure for potassic metasomatism in the mantle of the earth

Compositions of aqueous fluids in equilibrium with pyroxenes and olivines at mantle pressures and temperatures