(Mg,Fe)SiO 3 -Perovskite Stability and Lower Mantle Conditions

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Abstract
The stability of (Mg,Fe)SiO3-perovskite at lower mantle conditions has been the subject of three recent studies ([1][1], [2][2], [3][3]). While our work ([2][2]) and the study of Meade et al . ([3][3]) show that perovskite may not be stable in the lower mantle and dissociates into its component oxides, G. Serghiou et al . ([1][1]) come to the opposite conclusion. In that report ([1][1]), perovskite was synthesized from a mixture of oxides, and attempts were made to decompose a glass of MgSiO3 composition between pressures of 73 to 100 GPa and at temperatures to 3000 K. Their experiments ([1][1]) were performed in CO2-laser heated diamond anvil cells (DACs) in Ar or CsCl pressure mediums, and processes in the samples were characterised by Raman spectroscopy or by study of fluorescence Cr-doped samples. Serghiou et al . ([1][1]) state that the results of conflicting studies ( 2 or 3 ) are incorrect and result from significant pressure gradients in experiments done without pressure medium and under temperature gradients introduced by heating with an Nd–yttrium-aluminum-garnet (Nd-YAG) laser. The difference between experiments ([1][1]) and ([2][2]) appears to be mostly the result of different ways of characterizing the conditions of experiments and specimens; perovskite does indeed dissociate into its component oxides. The temperature gradients introduced in the sample by laser heating are very large, in the range of 10 to 50 K/μm ([4][4], [5][5]), but temperature distributions within the hot spot are quite similar (10 to 15 K/μm) for both CO2 lasers and Nd-YAG lasers running in multimode ([5][5]). Moreover, the thermal stresses produced by a temperature gradient on the order of 10 K/μm (107 K/m) could increase to 10 GPa at a peak temperature of 3000 K at an average pressure of 100 GPa, which would affect results of the experiments ([5][5], [6][6]). Thus, the thermal gradient by itself is not likely to be responsible for dissociation of perovskite in experiments with Nd-YAG laser, in contrast with experiments with CO2 laser. The pressure gradients also could be quite different in experiments in ([1][1]) as contrasted with those in ([2][2], [3][3]) because of the difference in sample preparation and pressure medium. Meade et al . ([3][3]) conduct experiments in NaCl and detected decomposition of (Mg,Fe)SiO3- perovskite. But even compression of pure perovskite or perovskite in CsCl pressure medium (for example, at 100 GPa) would not make any significant difference; estimated bulk moduli of MgSiO3-perovskite at such pressure is ∼700 GPa ([7][7]) and for CsCl it is ∼600 GPa, which means that perovskite alone is not much harder than CsCl ([8][8]). The methods of pressure measurement in ([1][1]) and ([2][2],[3][3]) were different. While the pressure in ([2][2], [3][3]) was based mostly on in situ x-ray measurements of pressure on the heated spots (during heating or in quench samples still under pressure), in ([1][1]) pressure was determined from a secondary ruby scale, with the pieces of ruby placed outside of the heated spots ([9][9]). Serghiou et al . ([1][1]) state that “the pressure difference between ruby chips in the heated area did not exceed 1 GPa.” First, in a heated area, ruby could react with any or all of the compounds (MgO, SiO2, and MgSiO3), which would affect pressure determination and the results of experiments. There is theoretical and experimental evidence of phase transition in corundum at a pressure of 78 to 100 GPa, especially at high temperature (≳ 1000 K), which might make the ruby pressure scale at >78 GPa problematic ([10][10]). It was already discussed by Tshuchida and Yagi ([11][11]) that in the laser heated spots pressure changes greatly. For example, they found ([11][11]) that pressure dropped by 10 to 20 GPa after heating of stishovite over 1000°C at 70 to 100 GPa. For platinum with in situ recorded diffraction patterns, the splitting of reflections corresponding to 20 to 25 GPa on laser heating at 60 GPa ([11][11]). It was also shown ([11][11]) that the pressure measured before and after heating do not actually reflect pressure (or stress) variations during the laser heating. We observed the change of 5 to 7 GPa in ruby heated with CO2 laser at initial pressure 50 to 60 GPa in CsCl pressure medium. In transition of silica polymorph, with four-fold coordinated silicon to the polymorph with six-fold coordinated Si, or when enstatite (or MgSiO3glass) transforms to perovskite, the reduction of the volumes were almost 50%, which caused the pressure to drop as a result of the rheological properties of the sample discussed above. In other words, experiments ([1][1]) described as conducted in the pressure range of 70 to 80 GPa probably were conducted in a range between 50 to 60 GPa, which is below the pressure expected (above 75 GPa at ∼2250 K) for dissociation of perovskite ([12][12]). Another significant difference between data in ([1][1]) and ([2][2], [3][3]) arises from the methods of the phase analysis; as mentioned above, in ([2][2], [3][3]), x-ray was used, while Serghiou et al . ([1][1]) applied Raman spectroscopy to detect the products of heating of MgO + SiO2 mixture at high pressure. Periclase (or magnesiowustite) is Raman-inactive, and only perovskite or possible silica phases could be detected by Raman spectroscopy. Raman spectroscopy is widely used in high-pressure studies, and problems associated with broadening and decreasing intensities of the peaks, loss of the signal for crystallites of small size, and so forth, are well known ([13][13]). For example, Zerr et al . ([14][14]), in studying the solidus of pyrolite-like composition after heating samples at pressure above 24 GPa, “were unable to detect any of major Raman lines of Mg-Si-perovskite” ([15][15]). Silica (amorphous and stishovite particularly) is known to produce weak Raman scattering at high pressure (above 20 GPa) or in samples...
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