Jan Matas

Non‐adiabaticity in mantle convection

Seismic observations indicate that Earths lower mantle is homogeneous as revealed by smooth depth variations of the bulk sound velocity and Bullens inhomogeneity parameter η being close to one. Here we show with 3D spherical convection simulations that it should also be non-adiabatic because a significant proportion of mantle heat sources is internal. The computer simulations predict non-adiabaticity of 100–300 degrees for the lower mantle geotherm, with η ∼ 1.01 implying that the temperature drop across the Core Mantle Boundary exceeds 1000 degrees.

A thermodynamic model for MgSiO3-perovskite derived from pressure, temperature and volume dependence of the Raman mode frequencies

Raman spectra of MgSiO3-perovskite (Mg-pv) were recorded at simultaneous high-pressure and low-temperature conditions. This allowed to estimate characteristic frequencies (nu(i)) and other mode parameters as a function of both pressure and temperature. The cross derivatives partial derivative(2)nu(i)/partial derivative T partial derivative P were measured for the first time, thus providing new insights into the lattice dynamics of Mg-pv. These parameters are negative for the two lowest frequency modes at 250 and 255 cm(-1) (approximate to-6 X 10(-4) cm(-1) GPa(-1) K-1) and positive for the other modes (+3 X 10(-4) to +5 X 10(-4) cm(-1) GPa(-1) K-1). These measurements were combined with previously published vibrational density of states (VDoS) for deriving entropy, specific heat, thermal pressure, equation of state (EoS), and various thermoelastic parameters of Mg-pv at mantle P-T conditions. The calculations were performed using a general anharmonic formulation including the spectroscopically measured parameters, It is shown that anharmonic effects are relatively small in this compound under geophysically relevant conditions especially for the EoS. The model is used to discuss the discrepancies in the pressure and temperature calibrations in diamond-anvil cells and multianvil presses. Finally, a complete thermodynamic data set for (Mg0.9Fe0.1)SiO3-perovskite is proposed along lower mantle geotherms, It is shown that a pure perovskite lower mantle is unlikely to exist

Thermodynamic properties of carbonates at high pressures from vibrational modelling

Simplified vibrational densities of states for five different carbonates are constructed using measured IR and Raman spectra. From the spectroscopic models we calculate thermodynamic and thermoelastic properties of magnesite, calcite, aragonite, dolomite, and siderite. The effects of temperature and pressure on the vibrational frequencies are explicitly introduced into the computations. These spectroscopic models provide high level agreement with the measured values of entropy and heat capacity (within +/- 2%), with the exception of aragonite (within +/- 5% above 600 K) due to its breakdown to calcite: For the molar volumes the agreement is within +/- 0.5 %. The Gibbs free energies of each mineral are then computed in order to obtain pressure and temperature equilibrium conditions for different chemical reactions involving carbonates. Comparing the predicted phase diagrams with those experimentally determined provides an additional constraint on the validity of spectroscopic models and in the values of formation enthalpies.

Experimental investigation of silicate-carbonate system at high pressure and high temperature

Melting and subsolidus relations in the (Mg,Fe)SiO3-(M,Fe)CO3, (Mg,Fe)(2)SiO4-(Mg,Fe)CO3, and (Mg,Fe)O-(Me,Fe)CO2 systems have been investigated at 14, 15, 16 and 25 GPa, 1973 K and 2173 K, using a 1000 t uniaxial multi anvil split sphere apparatus. The iron-magnesium partition coefficients between magnesite and silicates or oxides have been measured in subsolidus assemblages. Iron is always partitioned preferentially in the silicate and oxide phases, the order of increasing partitioning being pyroxene, olivine, silicate perovskite, wadsleyite and magnesiowustite. A thermodynamic model of iron-magnesium distribution between magnesite and all these phases, based on Gibbs free energy minimization, is established. Melting of pyroxene-magnesite and olivine-magnesite pseudo binary systems is eutectic, with eutectic points close to 1973 K and 60 mol % carbonate at 15 GPa in both systems. In the more complex mantle system, it is likely that such melts would form in the transition zone by heating and homogenization of deep subducted carbonates. The melts formed in the olivine-carbonate system are characterized by high Mg+Fe/Si ratios and thus unlikely to be primary kimberlitic magmas, a conclusion in agreement with previous studies in the peridotite-CO2 system, On the other hand, the observed pyroxene-magnesite melts formed at transition zone conditions have Mg+Fe/Si ratios that are comparable to those of natural kimberlites, suggesting that melting of carbonated pyroxenites at high pressures could be a source of kimberlitic magmas.