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Absorption spectrum of shock-compressed Fe^(2+)-bearing MgO and the radiative conductivity of the lower mantle

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New shock wave measurements of the optical absorption spectrum from 410 to 580 nm of 0.14 and 0.26 mole percent Fe^(2+)-bearing MgO (synthetic periclase) have been carried out to pressures of 42 GPa, and demonstrate considerably lower opacities at short wavelengths than inferred from static high pressure measurements on more Fe^(2+)-rich samples. Although the spectra obtained under dynamic pressure conditions demonstrate a marked increase in the absorption coefficient at 410 nm (e.g., from 0.3 to 0.65 cm−1 for 0.26 mole percent Fe^(2+)), in qualitative agreement with the effect of pressure on more Fe^(2+)-rich samples, the absolute values of the absorptivity coefficients observed are ∼10^2 lower than those inferred from static high pressure spectral data for iron-rich materials probably containing relatively more ferric iron. The spectra for (Mg_(0.9), Fe_(0.1))O inferred from Mao and Bell's (1977) static results to 31 GPa imply that radiative thermal conductivity for the lower mantle (∼3000 K) is effectively blocked by significant opacity in the optical range. When the present shock-wave results, for which the Fe^(3+) content is believed to be less than 5% of the total iron, are extrapolated to a typical mantle Fe^(2+) composition, temperatures of ∼3000 K and pressure of 31 GPa, a significant radiative thermal conductivity, 4 J (s K m)^(−1) is calculated, implying that significant radiative thermal conductivity of the lower mantle may take place in Fe^(2+)-depleted minerals under highly reducing conditions.

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