Aluminium distribution in an Earth's non–primitive lower mantle.

The aluminium incorporation mechanism of perovskite was explored by means of quantum mechanics in combination with equilibrium/off-equilibrium thermodynamics under the pressure-temperature conditions of the Earth's lower mantle (from 24 to 80 GPa). Earth's lower mantle was modelled as a geochemically non-primitive object because of an enrichment by 3 wt% of recycled crustal material (MORB component). The compositional modelling takes into account both chondrite and pyrolite reference models. The capacity of perovskite to host Al was modelled through an Al 2 O 3 exchange process in an unconstrained Mg-perovskite + Mg-Al-perovskite + free-Al 2 O 3 (corundum) system. Aluminium is globally incorporated principally via an increase in the amount of Al bearing perovskite [ Mg-Al-pv (80 GPa)/ Mg-Al-pv (24 GPa) ≈ 1.17], rather than by an increase in the Al 2 O 3 content of the average chemical composition which changes little (0.11–0.13, mole fraction of Al 2 O 3) and tends to decrease in Al. The Al 2 O 3 distribution in the lower mantle was described through the probability of the occurrence of given compositions of Al bearing perovskite. The probability of finding Mg-Al-perovskite is comparable to Mg-perovskites. Perovskite with Al 2 O 3 mole fraction up to 0.15 has an occurrence probability of ∼28% at 24 GPa, increasing up to ∼43% at 80 GPa; on the contrary, perovskite compositions in the range 0.19–0.30 Al 2 O 3 mole fraction drop their occurrence probability from 9.8 to 2.0%, over the same P -range. In light of this, the distribution of Al in the lower mantle shows that, among the possible Al bearing perovskite phases, the (Mg 0.89 Al 0.11)(Si 0.89 Al 0.11)O 3 composition is the likeliest, providing from 5 to 8% of the bulk perovskite in the pressure range from 24 to 80 GPa. The occurrence of the most Al rich composition, i.e. (Mg 0.71 Al 0.29)(Si 0.71 Al 0.29)O 3 , is a rare event (probability of occurrence < 1.7%). This study predicts that perovskite may globally host Al 2 O 3 in terms of 4.3 and 4.8 wt% (with respect to the non-primitive lower mantle mass), thus accounting for ∼90% and 100% of the bulk Al 2 O 3 estimated in the framework of pyrolite and chondrite reference models, respectively. A calcium-ferrite type phase (on the MgAl 2 O 4 -NaAlSiO 4 join) seems to be the only candidate that can compensate for the 10% gap of the perovskite Al incorporation capacity, in the case of a pyrolite non-primitive lower mantle model. [ABSTRACT FROM AUTHOR]