Stability of magnesiowüstite in Earth's lower mantle

Abstract

[(Mg,Fe)SiO 3 ] (1, 2), which together are likely the most important mineral assemblage of Earth's interior. The stability of the magnesiowüstite and silicate perovskite plays a crucial role in understanding the geophysical and geochemical properties of Earth. At ambient conditions, the end members of the MgOFeO (periclase-wüstite) system form a solid solution and have the same rock-salt (B1) structure. Periclase remains in the B1 structure to at least 227 GPa (3, 4). Wüstite transforms to a rhombohedral structure at pressures above 18 GPa at 300 K (5) and then to the NiAs or anti-NiAs structure (6-8). The topological difference between the pressure-temperature (P-T) phase diagrams of periclase and wüstite indicates that regions of two-phase equilibria should exist. A thermodynamically calculated P-T-composition phase diagram for the system suggests that an increase in pressure in the system would result in a gradual exsolution of an almost pure FeO and an Fe-depleted (Mg,Fe)O (9). Recent studies of three magnesiowüstites [(Mg 0.5 ,Fe 0.5 )O, (Mg 0.6 ,Fe 0.4 )O, and (Mg 0.8 ,Fe 0.2 )O] in an externally heated diamond anvil cell (DAC) up to 86 GPa and 1,000 K suggested that magnesiowüstite decomposes into Mg-rich and Fe-rich magnesiowüstites (10, 11). The decomposition of magnesiowüstite was proposed to occur at the P-T conditions of the lower mantle (10, 11) and to contribute significantly to the seismic-wave heterogeneity of the lower mantle (12, 13). On the other hand, no evidence for a phase transformation in (Mg 0.6 ,Fe 0.4 )O was found in shock-wave experiments to 201 GPa (14). Here we report the in situ study of structure and stability of magnesiowüstites at P-T conditions of the lower mantle. A rhenium or stainless steel gasket was preindented to a thickness of 30 m and then a hole of 220-m diameter was drilled in it. An amorphous boron and epoxy mixture (4:1 by weight) was filled and compressed in the hole. Subsequently, another hole of 100 m was drilled and used as the sample chamber. A sandwich configuration, consisting of dried NaCl as the thermal insulator and pressure medium on both sides of the sample, was used (16-18). The amorphous boron provided higher strength to create a deeper sample chamber, giving stronger x-ray diffraction from a thicker sample and better laser-heating spots attributable to thicker thermal insulating layers. ʈ Moreover, use of amorphous boron as an inner gasket also avoided unwanted x-ray diffraction peaks from Re or the stainless steel gasket. Experimental MethodsWe have used a double-sided Nd:YLF (neodymium: yttrium lithium fluoride) laser heating system, operating in multimode (TEM 00 ϩTEM 01 ), to heat the sample from both sides of a DAC at the 13-IDD GeoSoilEnviro-Consortium for Advanced Radiation Sources (GSECARS) sector of the Advanced Photon Source, Argonne National Laboratory (18). The laser beam diameter was Ϸ25 m. Graybody temperatures were determined by fitting the thermal radiation spectrum between 670 nm and 830 nm to the Planck radiation function. The tempe...

Pressure-induced spin-state transition of iron in magnesiowüstite (Fe,Mg)O

“…High-pressure x-ray emission and Mössbauer spectroscopy experiments show that the Fe 2+ ion of FeO and (Fe,Mg)O undergoes a high-spin (HS) to low-spin (LS) transition at pressures relevant for the Earth's lower mantle [2,5,11,12,[19][20][21][22][23][24][25][26][27][28][29][30][31]. It has been confirmed that FeO makes a Mott-insulatorto-metal transition at about 70 GPa, retaining the B1-type lattice structure at high temperature [11][12][13].…”

“…It has a rocksalt B1 crystal structure with Mg 2+ and high-spin (S = 2) Fe 2+ ions having octahedral environments. Furthermore, (Fe,Mg)O is likely to keep the B1-type lattice structure throughout the Earth's lower-mantle conditions as suggested by recent x-ray diffraction measurements [2]. (Fe,Mg)O comprises two end-member oxides with remarkably different electronic properties: MgO and FeO.…”

Texture development and elastic stresses in magnesiowűstite at high pressure

“…McCammon, 1993), and periclase (MgO) have a NaCl (B1) structure at ambient conditions. Intermediate and iron-rich compositions of magnesiowűstite show a phase transition to a rhombohedral phase at high pressure (Yagi et al, 1985, Richet et al, 1989, Kondo et al, 2004, Lin et al, 2003. There is also a pressure-induced electronic spin transitions of iron in (Mg,Fe)O at high pressure (Sherman 1988, Sherman 1991, Sherman and Jansen 1995, Cohen et al 1997, Badro et al 2003, Lin et al 2005, Speziale et al 2005, and a possible dissociation of intermediate compositions into magnesium-rich and iron-rich components (Dubrovinsky et al, 2000;2001;.…”

Shear waves in the diamond-anvil cell reveal pressure-induced instability in (Mg,Fe)O

“…25 Fe 0.75 )O display the rhombohedral distortion above 20 and 60 GPa, respectively (12,13). At 80-90 GPa and 1,000 K, (Mg 0.50 Fe 0.50 )O and (Mg 0.60 Fe 0.40 )O were reported to separate into ferropericlase plus FeO in the externally heated diamond-anvil cell (DAC) (14).…”