Ternary Ca-Fe-Mg carbonates: subsolidus phase relations at 3.5GPa and a thermodynamic solid solution model including order/disorder

Subduction carries atmospheric and crustal carbon hosted in the altered oceanic crystalline basement and in pelagic sediments back into the mantle. Reactions involving complex carbonate solid solutions(s) lead to the transfer of carbon into the mantle, where it may be stored as graphite/diamond, in fluids or melts, or in carbonates. To constrain the thermodynamics and thus reactions of the ternary Ca-Mg-Fe carbonate solid solution, piston cylinder experiments have been performed in the system CaCO3-MgCO3-FeCO3 at a pressure of 3.5GPa and temperatures of 900-1,100°C. At 900°C, the system has two miscibility gaps: the solvus dolomite-calcite, which closes at X MgCO3 ~0.7, and the solvus dolomite-magnesite, which ranges from the Mg to the Fe side of the ternary. With increasing temperature, the two miscibility gaps become narrower until complete solid solutions between CaCO3-Ca0.5Mg0.5CO3 is reached at 1,100°C and between CaCO3-FeCO3 at 1,000°C. The solvi are characterized by strong compositional asymmetry and by an order-disorder mechanism. To deal with these features, a solid solution model based on the van Laar macroscopic formalism has been calculated for ternary carbonates. This thermodynamic solid solution model is able to reproduce the experimentally constrained phase relations in the system CaCO3-MgCO3-FeCO3 in a broad P-T range. To test our model, calculated phase equilibria were compared with experiments performed in carbonated mafic protolithes, demonstrating the reliability of our solid solution model at pressures up to 6GPa in complex system

Melting relations in the system FeCO3–MgCO3 and thermodynamic modelling of Fe–Mg carbonate melts

To constrain the thermodynamics and melting relations of the siderite\u2013magnesite (FeCO3\u2013MgCO3) system, 27 piston cylinder experiments were conducted at 3.5\ua0GPa and 1170\u20131575\ua0\ub0C. Fe-rich compositions were also investigated with 13 multi-anvil experiments at 10, 13.6 and 20\ua0GPa, 1500\u20131890\ua0\ub0C. At 3.5\ua0GPa, the solid solution siderite\u2013magnesite coexists with melt over a compositional range of XMg (=Mg/(Mg\ua0+\ua0Fetot))\ua0=\ua00.38\u20131.0, while at 6510\ua0GPa solid solution appears to be complete. At 3.5\ua0GPa, the system is pseudo-binary because of the limited stability of siderite or liquid FeCO3, Fe-rich carbonates decomposing at subsolidus conditions to magnetite\u2013magnesioferrite solid solution, graphite and CO2. Similar reactions also occur with liquid FeCO3 resulting in melt species with ferric iron components, but the decomposition of the liquid decreases in importance with pressure. At 3.5\ua0GPa, the metastable melting temperature of pure siderite is located at 1264\ua0\ub0C, whereas pure magnesite melts at 1629\ua0\ub0C. The melting loop is non-ideal on the Fe side where the dissociation reaction resulting in Fe3+ in the melt depresses melting temperatures and causes a minimum. Over the pressure range of 3.5\u201320\ua0GPa, this minimum is 20\u201335\ua0\ub0C lower than the (metastable) siderite melting temperature. By merging all present and previous experimental data, standard state (298.15\ua0K, 1\ua0bar) thermodynamic properties of the magnesite melt (MgCO3L) end member are calculated and the properties of (Fe,Mg)CO3 melt fit by a regular solution model with an interaction parameter of 127600\ua0J/mol. The solution model reproduces the asymmetric melting loop and predicts the thermal minimum at 1240\ua0\ub0C near the siderite side at XMg\ua0=\ua00.2 (3.5\ua0GPa). The solution model is applicable to pressures reaching to the bottom of the upper mantle and allows calculation of phase relations in the FeO\u2013MgO\u2013O2\u2013C system

Efeitos de dietas com dois níveis de leucaena leucocephala (lam.) de Wit com e sem adição de Saccharomyces cerevisiae sobre a emissão do gás metano em bovinos.

O trabalho teve como objetivo avaliar o uso de leucena e seu efeito sobre a emissão de metano ruminal em dietas controladas contendo dois níveis dietéticos na presença ou ausência de levedura (Saccharomyces cerevisiae). Foram utilizados quatro bovinos mestiços com fístulas de rúmen, mantidos em baias individuais, submetidos aos seguintes tratamentos em Quadrado Latino 4x4: 20S = 80% de feno de gramínea + 20% de feno de leucena + zero de levedura; 50S = 50% de feno de gramínea + 50% de feno de leucena sem levedura; 20L = 80% feno de gramínea + 20% de feno de leucena + 10 g de levedura adicionada no rúmen; 50L = 50% feno de graminea + 50% de feno de leucena + 10 g de levedura. Foram determinadas a curva de pH do líquido ruminai e produção de gás metano coletado através da técnica do gás traçador interno SF6. Os valores médios de pH variaram de 6,72 a 6,96, sendo que o tratamento 50S foi o que manteve os maiores e mais estáveis pH, houve efeito significativo (PO,05) para níveis de leucena na ausência de levedura. O tratamento 50L promoveu redução na emissão de metano em g/kg MSI de 17,5% em relação ao uso de 20% de leucena com levedura e também diminuição nos valores médios absolutos de 12,3% e 10,5% comparados com dietas contendo 50% e 20% de leucena sem levedura, respectivamente, demonstrando haver um efeito associativo da leucena no nível mais alto na dieta com a presença de levedura no rúmen sobre a redução na produção de metano no rúmen de bovinos