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Journal Articles Geophysical Monograph series Year : 2005

Thermochemical State of the Lower Mantle: New Insights From Mineral Physics

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Abstract

We repo recent findings in the field ofhigh-pressure mineral physics with impor- tant implications for Eafth's lower mantle. W€ show that the two main constituents ofthe lower mantle, namely, (Mg,Fe)Sio1-magnesium silicate perovskite and (Mg,Fe)O-ferropedclase undergo electronic tftnsitions at lower mantle pres- sures (70 and 120 GPa), in which iron transforms from tle high-spin state to the low-spin state. The transformations modify the thermochemical state of Earth's lower mantle. Minerals beadng high-spin iron have characteristic absorption lines in the near-infrared, hindedng radiative conductivity at lower-man e temperatures. These absorption lines shill to the visible (green to violet) €nge in the low-spin state, and their int nsic intensities decrcase; the minerals thus become incrcasingly transparcnt in the near-infrared and their radiative and total thermal conductivi- ties rise. Thus, the heat conductivity ofthe lowermost mantle could be higher than previously thought. Moreover, the spin-driven partitioning of ircn between the two mineral phases can explain large-scale chemjcal heterogeneities in the mantle that are diven by regional temperatuie vadations. It is noteworthY that the translhon pressures corrcspond to the bottom third ofthe lower mantle (70 GPa, 1700-km depth), and to tle last 300 km above the core-mantle boundary (120 GPa, 2600-km depth); these rcgions have very special geophysical signatures, as chemical hetero- geneities have been reported by seismology in the first case and the bottom 300 km ofEarth's mantle constitutes the D" layer. Our observations provid€ a mineral physics basis for these features in Earth's lower mantle.
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Dates and versions

hal-00133634 , version 1 (27-02-2007)

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  • HAL Id : hal-00133634 , version 1

Cite

James Badro, Guillaume Fiquet, François Guyot. Thermochemical State of the Lower Mantle: New Insights From Mineral Physics. Geophysical Monograph series, 2005, 160, pp.241-260. ⟨hal-00133634⟩
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