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Our current understanding of the electronic state of iron in lower-mantle minerals leads to a considerable disagreement in bulk sound speed with seismic measurements if the lower mantle has the same composition as the upper mantle (pyrolite). In the modeling studies, the content and oxidation state of Fe in the minerals have been assumed to be constant throughout the lower mantle. Here, we report high-pressure experimental results in which Fe becomes dominantly Fe2+ in bridgmanite synthesized at 40-70 GPa and 2,000 K, while it is in mixed oxidation state (Fe3+/∑Fe = 60%) in the samples synthesized below and above the pressure range. Little Fe3+ in bridgmanite combined with the strong partitioning of Fe2+ into ferropericlase will alter the Fe content for these minerals at 1,100- to 1,700-km depths. Our calculations show that the change in iron content harmonizes the bulk sound speed of pyrolite with the seismic values in this region. Our experiments support no significant changes in bulk composition for most of the mantle, but possible changes in physical properties and processes (such as viscosity and mantle flow patterns) in the midmantle.
RESUMO
The discovery of a phase transition in Mg-silicate perovskite (Pv) to postperovskite (pPv) at lowermost mantle pressure-temperature (P - T) conditions may provide an explanation for the discontinuous increase in shear wave velocity found in some regions at a depth range of 200 to 400 km above the core-mantle boundary, hereafter the D('') discontinuity. However, recent studies on binary and ternary systems showed that reasonable contents of Fe(2+) and Al for pyrolite increase the thickness (width of the mixed phase region) of the Pv - pPv boundary (400-600 km) to much larger than the D('') discontinuity (≤ 70 km). These results challenge the assignment of the D('') discontinuity to the Pv - pPv boundary in pyrolite (homogenized mantle composition). Furthermore, the mineralogy and composition of rocks that can host a detectable Pv â pPv boundary are still unknown. Here we report in situ measurements of the depths and thicknesses of the Pv â pPv transition in multiphase systems (San Carlos olivine, pyrolitic, and midocean ridge basaltic compositions) at the P - T conditions of the lowermost mantle, searching for candidate rocks with a sharp Pv - pPv discontinuity. Whereas the pyrolitic mantle may not have a seismologically detectable Pv â pPv transition due to the effect of Al, harzburgitic compositions have detectable transitions due to low Al content. In contrast, Al-rich basaltic compositions may have a detectable Pv - pPv boundary due to their distinct mineralogy. Therefore, the observation of the D('') discontinuity may be related to the Pv â pPv transition in the differentiated oceanic lithosphere materials transported to the lowermost mantle by subducting slabs.
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Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.
RESUMO
Highlights from the Science family of journals.