Solid solution of CaSiO3 and MgSiO3 perovskites in the lower mantle: The role of ferrous iron

Abstract

The solid solution between CaSiO 3 and MgSiO 3 perovskites is an important control on the properties of the lower mantle but the effect of one of the most important impurity elements (iron) on this solution is largely unknown. Using density functional theory (DFT), ferrous iron's influence on the reciprocal solubility of MgSiO 3 and CaSiO 3 perovskite (forming a single Ca-Mg mixed perovskite phase) was calculated under pressures and temperatures of 25 -125 GPa and 0 -3000 K, respectively. Except at iron-rich conditions, ferrous iron preferentially partitions into the mixed perovskite phase over bridgmanite. This is a small effect (partitioning coefficient K D ~ 0.25 -1), however, when compared to the partitioning of ferrous iron to ferropericlase which rules out perovskite phase mixing as a mechanism for creating iron-rich regions in the mantle. Iron increases the miscibility of Ca and Mg perovskite phases and reduces the temperature at which the two perovskite phases mix but this effect is highly nonlinear. We find that for a pyrolytic mantle (Ca%=12.5 where Ca%=Ca/ (Ca + Mg)) a perovskite ferrous iron concentration of ~13% leads to the lowest mixing temperature and the highest miscibility. With this composition, 1% ferrous iron in a pyrolytic composition would lead to mixing at This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Perovskite-bearing crystal-controlled oxide-silicate mantle xenoliths: Resolution to controversial origins?

“…The latter is readily accommodated in Brg as FeSiO 3 (heroseite), with the interesting property of Fe dissociation (3Fe 3+ = 2Fe 2+ + Fe 0 ) making it a prominent sink for ferric Fe in the lower mantle ( 61 , 62 ). For Ca, there is a limited solid solubility of davemaoite (CaSiO 3 ) in Brg ( 33 ), but increased miscibility to a single phase can be achieved in the presence of high iron (Fe 2+ Fe 3+ ) concentrations ( 63 , 64 ) and extreme P (>90 GPa) as determined experimentally by ( 33 ) and thermodynamically by ( 62 , 64 ). A similar effect is demonstrated for Ti in which Ilm-Cpx intergrowths are stable experimentally at ~55 GPa and Opx-Ilm at 35 GPa ( 33 ), and thermodynamically for the binary Pvr-Brg with the corresponding values of 80 and 45 GPa, respectively ( 34 ).…”

“…As outlined above, P > 80 GPa is required, but as important: Is this in any way related to kimberlite magmatism? Single-phase Ca-Fe 2+ Fe 3+ Ti Brg-Dvm (davemaoite CaSiO 3 ) is predicted at high P-T conditions and is modeled around CMB rising plumes ( 62 ) or in LLSVP seismic anomalies at the CMB ( 63 , 64 ). Although the origin and composition of the large (thousands of kilometer wide × tens of kilometer thick) thermochemical bodies remain uncertain ( 78 ), Dvm is considered a possible candidate ( 79 ), and in combination with Ca, Fe, and Ti, the inferred composition of the Brg progenitor is achieved.…”