The effect of Ti on Ca-pv and Mg-pv phase stability

Abstract

Magnesium silicate perovskite in the form of bridgmanite (bdg) and Calcium silicate perovskite (Ca-pv) have similar chemical structures and may mix into a single perovskite phase in the lower mantle which would have profound effects on many seismic properties. While we have previously found that this is unlikely to occur in pure bdg and ca-pv in this paper we examine whether phase mixing can be induced by Titanium.We predict that even small amounts of Ti can cause significant increases in mixing of the two phases. Miscibility of the phases has a strong dependence upon how Ti is partitioned between the two phases before mixing and thus the source and history of introduced Ti is important in determining miscibility.We predict basalts, even those with heavy Ti enrichment (10%), will not form a single phase in subducted slabs as their mixing temperatures remain above 2500 K for most compositions throughout lower mantle pressures. In pyrolytic mantle it is predicted that at shallow depths large amounts of Ti are needed to induce phase mixing (~40% Ti at 25 GPa and geotherm temperatures) but less Ti is needed to induce mixing with depth (~1% Ti at 125 GPa and geotherm temperatures). Thus we predict that enriched Ti regions will see perovskite mixing near the bottom of the lower mantle. These mixed perovskite regions partition Ti out of unmixed regions and thus provide a mechanism for Ti enriched regions to form in the deep lower mantle. Both ferrous iron and the Ca:Mg ratio are predicted to have a larger control on the mixing temperature of pyrolytic systems than Ti, however. For Ti and Ca rich pyroxene megacrysts we find that they should become a single phase along a lower mantle geotherm at around 40–115 GPa depending heavily upon Ti and Ca concentration.