MAGNETITE-MAGNESIOFERRITE PHASE RELATIONS AND APPLICATION TO ALH84001. Andrea

Introduction: Oxygen fugacity (fO2) is an important factor in planetary formation and evolution. Certain minerals or assemblages of minerals are stable only under certain fO2 conditions (at a constant T and P) and can be recorders of ambient fO2 during geologic processes. The best estimate of oxygen fugacity from mineral assemblages has implications, from large-scale models of planetary evolution to interpretation of single meteorites. For example, redox conditions are part of the discussion of the origin and history of shergottites, basaltic rocks in meteorites from Mars [1-4]. In addition oxygen fugacity may be an important factor to consider in the interpretation of the carbonate and magnetite assemblages seen in ALH84001 [5]. Background: Oxygen fugacity. For many planetary materials, the presence/coexistence of iron oxide minerals such as hematite (hem), magnetite (mt) and wü (wü) are used to estimate fO2 at the time of formation. The P, T and fO2 dependence for the equilibria below are well known and calibrated. 3 Fe2O3 (hem) = 2 Fe3O4 (mt) + 1/2 O2 (1) 2 Fe3O4 (mt) = 6 FeO (wü) + O2 (2) 2 FeO (wü) = 2 Fe (iron) + O2 (3) O2 can be delivered to the minerals via a fluid or a mineral phase. On magnesioferrite. Full consideration of fO2 should consider magnesioferrite (mfer) (MgFe2O4; Mg endmember of a solid solution of Mg and Fe with mt) and magnesiowüstite(Mg-wü) (Fe, Mg)O solid solutions. Addition of Mg will affect the activities of mt and wü in reactions (1) and (2) above. At a constant T and P, calculated fO2 will be different than that of pure iron phases. Some workers have investigated this effect, either through experiments [6] or thermodynamic calculations [7-9]. Analysis: The petrologic and experimental literature has differing reports of the stability of MgFe2O4 (mfer) versus FeFe2O4 (mt) plus MgO (periclase). The occurrence of mt + periclase versus mfer and how this depends on fO2 is best explained by figure 1 below. This phase diagram is redrawn after a figure in [9] and based on data from [10]. At one bar and 1000°C, figure 1 indicates how the phase assemblage varies with Fe-Mg composition and oxygen partial pressure (essentially fO2). The familiar iron oxide buffer reactions (reactions1, 2, 3) are arrayed along the y-axis, at XMg = 0. Phase stability as a function of Fe-Mg composition. As Mg is added to the system, it becomes part of a solid solution in mt (spinel) or wü. (As the mt phase varies in composition, it is referred to as spinel in the diagram and in future discussions.) Mg cannot enter hem. Taking reaction (1) as an example, the fO2