Photon Factory Activity Report 2005 #23 Part B (2006)
High Pressure Science

13A, 18C/ 2005G143
3+

Fe and Al partitioning between silicate perovskites
in the lower mantle condition
Daisuke NISHIO-HAMANE, Yusuke SETO, Takaya NAGAI*, Kiyoshi FUJINO
Hokkaido Univ., N10W8, Sapporo 060-0810, Japan
Introduction
ABO3 perovskites with MgSiO3 (Mg-Pv) and CaSiO3(Ca-Pv) are though to be the most major silicate minerals
in the Earth’s lower mantle. Usually, an A site in silicate
perovskites is occupied by a divalent ion. However,recent experimental studies indicated that Mg-Pv
3+
incorporates significant amount of Fe in the lower
mantle conditions [1, 2]. Furthermore, it was suggested
that the presence of Al in Mg-Pv favors the incorporation
3+
3+of Fe as Fe AlO3 component [3]. Therefore, taking into
3+
consideration of the MgSiO3-CaSiO3-Fe AlO3 system isfundamental to understand the behaviour of trivalent ionsin major portions of the Earth's lower mantle. Especially,3+
Fe and Al partitioning between Mg-Pv and Ca-Pv is an3+
important indicator to understand the distribution of Feand Al in the lower mantle. Here, we report the3+partitioning of Fe and Al between Mg-Pv and Ca-Pv atcorresponding to the Earth’s lower mantle condition byusing the combination methods of a laser heating
diamond anvil cell (LHDAC), high pressure in-situ
synchrotron X-ray experiments, and an analyticaltransmission electron microscopy (ATEM).
Experiments
3+
A gel with a composition of CaMgSi2O6 : Fe AlO3 =85 : 15 (85CM) was prepared by a conventional sol-gel
method and was used as the starting materials. Highpressure and temperature experiments at 27-106 GPa and1600-2500 K were conducted by using a LHDAC. Aftereach high pressure and high temperature experiment,
temperature was quenched to room temperature andpressure was kept loading in the DAC.
In order to identify mineral assemblies at in-situ highpressure and those of recovered samples, synchrotron Xray diffraction measurements were carried out at highpressure and room temperature in the beam line 13A and18C, Photon Factory. X-ray diffraction spectra wererecorded with an imaging plate. Two dimensional X-ray
diffraction images were integrated as a function of twotheta in order to give a conventional one-dimensional
diffraction profile.
In addition, since it is also important to know chemicalcompositions of coexisting minerals quantitatively, after
synchrotron X-ray diffraction measurements, ATEM
observation was performed for recovered samples fromhigh pressure and temperature.
Results and discussion
In-situ X-ray experiments at high pressure above 30
GPa revealed the two-phase assemblage of Mg-Pv andCa-Pv is stable. On the other hand, small amount of190

Users' Reportstishovite coexists with Mg-Pv and Ca-Pv below 30 GPa.
Ca-Pv cannot be observed in X-ray diffraction patternobtained at ambient conditions recovered from 27-100
GPa, because it is well known that Ca-Pv is unstable at3+
ambient conditions. The cell volumes of (Fe , Al)bearing Mg-Pv at ambient condition are almost constantin spite of their synthesized conditions and larger thanthat of pure Mg-Pv.
ATEM observation also showed that the productphases in 85CM are Mg-Pv, Ca-Pv and small amount ofstishovite below 30 GPa, and two phases of Mg-Pv and3+
Ca-Pv above 30 GPa. Fe /Al ratio in Mg-Pv was almost
1.0 in all conditions and this evidence suggests that most3+3+
Fe is incorporated into Mg-Pv with Al as Fe AlO3
component. Since (Mg+Ca)/Si ratio in Mg-Pv is also
constant at 1.0 in all conditions, the coupled substitution
3+(Fe Al → MgSi) in Mg-Pv seems to be the dominant
substitution mechanism. However, the presence ofstishovite in the run products obtained below 30 GPa
suggests that oxygen vacancy forming substitution
3+(Fe Al → 2Si) partly occurs at such pressure-temperature
conditions. These evidences indicate that the oxygen
vacancy forming substitution is possibly importantmechanism at pressures corresponding t the uppermost
part of the lower mantle. However, the coupled
substitution mechanism seems to be increasingly favoredin the most part of the lower mantle.
ATEM observation also showed that Ca-Pv is almost
3+DFe -free and Al-free. The partitioning coefficient (K ) of3+
Fe and Al between Mg-Pv and Ca-Pv can be defined asDCa-Pv
Mg-Pv
Cathe concentration ratio as K = X FA/X FA, where XMg-Pv
3+Pv
and X FA mean Fe + Al concentration in Ca-Pv andFAMg-Pv, respectively. The partitioning coefficients vary inlow values (0.1-0.2) in all conditions and show no
significant dependence on pressure and temperature
3+conditions. This means that Fe and Al are likely to be3+
preferentially incorporated into Mg-Pv as Fe AlO3
component, when Mg-Pv coexists with Ca-Pv at lower
mantle conditions.
References[1] D.J. Frost et al, nature. 428, 409-412 (2004).
[2] C.A. McCammon, science, 308, 807-808 (2005).
[3] C.B. Vanpeteghem et al, Phys. Earth Planet. Inter, 155,
96-103 (2006)* nagai@ep.sci.hokudai.ac.jp