Intrinsic and extrinsic stability of the (Mg,Fe)O solid mixture in the Fe-Mg-Si-O system at high P, T conditions relevant tothe Earth’s mantle is investigated by the combination of quantum mechanical calculations (Hartree- 26 Fock/DFT hybridscheme), cluster expansion techniques and statistical thermodynamics. Iron in the (Mg,Fe)O binary mixture is assumed tobe either in the low spin (LS) or in the high spin (HS) state. Un-mixing at solid state is observed only for the LS conditionin the 23–42 GPa pressure range, whereas HS does not give rise to un-mixing. LS (Mg,Fe)O un-mixings are shown to be ableto incorporate iron by subsolidus reactions with a reservoir of a virtual bridgmanite composition, for a maximum total enrichmentof 0.22 FeO. At very high P (up to 130/3150 GPa/K), a predominant (0.7 phase proportion), iron-rich Fe-periclasemixture (Mg0.50Fe0.50)O is formed, and it coexists, at constrained phase composition conditions, with two iron-poor assemblages[(Mg0.90Fe0.10)O and (Mg0.825Fe0.175)O]. These theoretical results agree with the compositional variability and frequencyof occurrence observed in lower mantle Fe-periclase from diamond inclusions and from HP-HT synthesisproducts. The density difference among the Fe-periclase phases increases up to 10%, between 24 and 130 GPa. The calculatedbulk Fe/Mg partitioning coefficient between the bridgmanite reservoir and Fe-periclase, Kd, is 0.64 at 24 GPa; it thendrops to 0.19 at 80 GPa, and becomes quasi-invariant (0.18–0.16) in the lowermost portion of the Earth’s mantle (80–130 GPa). These Kd-values represent an approximate estimate for the Fe/Mg-partitioning between actual bridgmanite andFe-periclase. Consequently, our Kd-values agree with experimental measurements and theoretical determinations, hintingthat iron preferentially dissolves in periclase with respect to all the other iron-bearing phases of the lower mantle. The continuouschange up to 80 GPa (2000 km depth) of the products (compositions and phase proportions) over the MgO-FeObinary causes geochemical heterogeneities throughout the lower mantle, but it does not give rise to any sharp discontinuity.In this view, anomalies like the ULVZs, explained with a local and abrupt change of density, do not seem primarily ascribableto the mixing behavior and reactivity of (Mg,Fe)O at subsolidus.
|Numero di pagine||16|
|Rivista||GEOCHIMICA ET COSMOCHIMICA ACTA|
|Stato di pubblicazione||Published - 2017|
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology