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3.2 g. Fe3+ content of (Mg,Fe)SiO3 perovskite and (Mg,Fe)O in laser-heated diamond anvil experiments (C.A. McCammon, in collaboration with A. Duba/Lawrence Livermore and P. Peyronneau, J.-P. Poirier and F. Visocekas/Paris)

Recent work in Paris has shown that temperature gradients in laser-heated diamond anvil cell experiments can have large effects on oxygen fugacity conditions, and therefore potentially large effects on physical properties such as electrical conductivity. To quantify the amount of Fe3+ in quenched phases synthesised by laser heating, we used the Mössbauer milliprobe (Annual Report, 1993), which enables different areas of the sample to be studied (Fig. 3.2-6).

Fig. 3.2-6: 57Fe0.15Mg0.85SiO3 orthopyroxene after laser heating in the diamond anvil cell. The horizontal stripe shows the area that was heated. Mössbauer spectra were taken from three areas: (1) transparent area in the middle (perovskite phase); (2) dark area at the edges (heated, not transformed); and (3) white area (unheated). The diameter of the spots studied with Mössbauer spectroscopy (indicated by circles) is 100 µm.

We found that Fe3+ only occurs in the perovskite phase; no evidence for Fe3+ was found in either the unheated or heated areas of orthopyroxene. In experiments with (Mg,Fe)O, we found that samples heated in the diamond anvil cell become transparent and contain significantly less Fe3+ than the starting material, consistent with results from multianvil press experiments (Annual Report, 1995). Experiments were also performed to compare results from orthopyroxene loaded in air and under argon. Mössbauer spectra recorded at the centres of heated spots showed a significant difference: samples loaded in air gave Fe3+/ Fe values of 5-7% in 57Fe0.15Mg0.85SiO3 perovskite; whereas samples loaded in argon gave values of 2-3% in the same phase. In addition, there was a magnetic phase present in the samples loaded in argon whose parameters corresponded to metallic iron. No magnetic phase was detected in the samples loaded in air.

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