The mineralogic distribution of iron in the upper mantle

Abstract

Using the observed distribution of Fe 2+ between garnets and clinopyroxenes and olivines in rocks crystallizing under different geological conditions, for which the temperatures and pressures are assumed, a relative difference in enthalpy (≈3 to ≈6 kcal/mole) is inferred between almandine and pyrope. Using these and thermodynamic data for olivine and ringwoodite, the distribution of Fe 2+ between olivine and ringwoodite, and garnet, is calculated down to 620 km in the mantle, using several different temperature profiles. For an average olivine composition (Mg 0.9Fe 0.1) – SiO 4, the composition of the coexisting garnet (Fe x , Mg 1− x ) 3− Al 2Si 3O 12 has a composition corresponding to x = 0.3 to 0.6 at the top of the mantle. The value of x decrease to a minimum value of between 0.3 and 0.4 at ≈180 km, and then increases again with increasing depth until the olive to spinel transition is encountered. Although the calculations are less certain, the garnet in the spinel zone appears to be relatively less rich in Fe 2+ than in the olivine layer. Estimates of the self-diffusion of Fe 2+ for olivine in the upper mantle suggest that if the grain size and/or one-year convective motions are less than a fraction of a cm, the equilibrium distribution profiles are likely to be a permanent feature of the mantle. A series of theoretical velocity and density profiles for the upper mantle are constructed on the basis of the distribution coefficient calculations and the equation of state of olivines, pyroxenes, garnets and spinels, and compared to some recent seismic observations. It is found that an addition of some 20% garnet (which is necessary richer in iron than the olivine) results in reduction in seismic velocities which is comparable with an increase in iron content of ≈3% and ≈12%, respectively, in the olivine and spinel zone.