Ab-initio investigation of the thermodynamic stability of the magnesio-wüstite solid solution under Earth's lower mantle conditions (Abstract/Poster in atti di convegno)

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  • Ab-initio investigation of the thermodynamic stability of the magnesio-wüstite solid solution under Earth's lower mantle conditions (Abstract/Poster in atti di convegno) (literal)
Anno
  • 2014-01-01T00:00:00+01:00 (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#doi
  • 10.3301/ROL.2014.140 (literal)
Alternative label
  • Merli M Sciascia L Pavese A Diella V (2014)
    Ab-initio investigation of the thermodynamic stability of the magnesio-wüstite solid solution under Earth's lower mantle conditions
    in 87° Congresso della Società Geologica Italiana e 90° Congresso della Società Italiana di Mineralogia e Petrologia, Milano, 10-12 09 2014
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • Merli M Sciascia L Pavese A Diella V (literal)
Pagina inizio
  • 292 (literal)
Pagina fine
  • 292 (literal)
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  • The Future of the Italian Geosciences - The Italian Geosciences of the Future (literal)
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  • 31 (literal)
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  • 1 (literal)
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  • 1 (literal)
Note
  • Poster (literal)
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  • Dipartimento di Scienze della Terra e del Mare- Università degli Studi di Palermo Dipartimento di Scienze della Terra e del Mare- Università degli Studi di Palermo Dipartimento Scienze della Terra ''A. Desio'', Università degli Studi di Milano Consiglio Nazionale delle Ricerche, CNR-IDPA, Sezione di Milano (literal)
Titolo
  • Ab-initio investigation of the thermodynamic stability of the magnesio-wüstite solid solution under Earth's lower mantle conditions (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#curatoriVolume
  • B. Cesare, E. Erba, B. Carmina, L. Fascio, F.M. Petti, A. Zuccari (literal)
Abstract
  • Mg-wüstite, (Mg,Fe)O, is the second most abundant phase in the Earth's lower mantle. A profound understanding of the thermodynamic stability of this phase under deep mantle conditions is thus crucial for developing accurate models of the Earth's interior and the importance of drawing a complete picture of the stability fields of the Mg-wustite solid solution, especially at high pressure/temperature regimes, is straightforward. In the light of the above considerations, the present work has been undertaken where the thermo-chemical properties of the (Mg,Fe)O solid solution, over a wide PT range, have been modelled using mixing Helmholtz energy, ?F T, x mixing. Calculations have been performed by means of cluster expansion, quantum mechanical and semiempirical techniques. Both high-spin (HS) and low-spin (LS) configurations of iron have been explored as a function of composition (x: molar fraction of FeO) over the MgO-FeO binary. Only the HS-model provides physically sound results at room pressure, yielding a correct trend of cell edge versus composition, whereas LS's issues are at variance with observations. Mixing Helmholtz energy has been parametrized by the following relationship ?F T, x mixing = x×y×[(U T ! + U T ! × x - y + U T ! ± (x - y)!, where y = 1 - x and U!(T) are polynomials in T of the third order. ?F T, x mixing exhibits a quasi-symmetric behaviour and allows one to build the T-X phase relations diagram over the MgO-FeO join. On the basis of the HS-model including vibrational contribution to Helmholtz energy, a solid solution's critical temperature of some 950 K has been predicted, remarkably lower than olivine's and Mg-Fe-garnet's. All this points to a more difficult Mg-Fe mixing in periclase-like structure than olivine and garnet, which, in turn, provide more degrees of freedom for atomic relaxation. From ?F T, x mixing, the values of ?H T, x mixing and ?S T, x mixing have been derived, both exhibiting so modest a dependence on T as to be negligible. ?H T, x mixing and ?S T, x mixing exhibit quasi-regularity; either has been parametrized as W×x×(1 - x), obtaining W!,!\"!!\" and W!,!\"!!\" of 17.7 kJ/mol and 26.8 J/mol/K, respectively. ?S T, x excess has been estimated to be smaller than 0.5 J/mol/K in absolute Moreover, it has been observed that the HS-configuration is stable and promote Mg-Fe solid solution up to >>15 GPa better than LS does. At higher pressures, the LS-model becomes favourite and increasingly stabilizes (Mg,Fe)O upon rising P, whereas HS predicts ex-solving into the end members above >>40 GPa (literal)
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