Evidence for a Ubiquitous Seismic Discontinuity at the Base of the Mantle

Sidorin, Igor; Gurnis, Michael; Helmberger, Donald V.

doi:10.1126/science.286.5443.1326

Cited by 167 publications

(121 citation statements)

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“…The vast reservoirs of iron and silicates at the core-mantle boundary provide favorable chemical-physical conditions for the formation of high-iron ppv silicate, which holds the key to understanding the geophysical and geochemical properties of the DЉ layer (6,(22)(23)(24). Contrary to the previous thinking that the mantle composition was essentially unchanged by contact with the core, i.e., the composition of the mantle silicate remains within its iron-poor solubility limit of x Ͻ 0.15, this scenario calls for a reaction layer of denser silicates with much higher iron content, resulting in the observed low-velocity zones and ultralow-velocity zones (25).…”

Section: Methodsmentioning

confidence: 99%

Iron-rich silicates in the Earth's D″ layer

Mao

Meng

Shen

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

106

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High-pressure experiments and theoretical calculations demonstrate that an iron-rich ferromagnesian silicate phase can be synthesized at the pressure-temperature conditions near the coremantle boundary. The iron-rich phase is up to 20% denser than any known silicate at the core-mantle boundary. The high mean atomic number of the silicate greatly reduces the seismic velocity and provides an explanation to the low-velocity and ultra-low-velocity zones. Formation of this previously undescribed phase from reaction between the silicate mantle and the iron core may be responsible for the unusual geophysical and geochemical signatures observed at the base of the lower mantle.core-mantle boundary ͉ high pressure ͉ mineral physics ͉ post-perovskite M odern deep-Earth mineralogical research began with highpressure experiments on iron silicate, a major component in the solid Earth. The discovery of the fayalite (Fe 2 SiO 4 ) olivine-spinel transition in 1959 (1) marked the first known transition beyond the upper mantle. The disproportionation of fayalite spinel into mixed oxides using the newly invented laser-heated and resistive-heated diamond-anvil cell in the early 1970s (2, 3) marked the first phase transition under lower mantle conditions. In the Earth's crust, upper mantle, and transition zone, iron silicates form extensive solid solutions with the magnesium endmembers in major rock-forming minerals, e.g., fayalite in ␣-(Fe,Mg) 2 SiO 4 (olivine), ferrosilite in (Fe,Mg)SiO 3 (pyroxene), almandine in (Fe,Mg) 3 Al 2 Si 3 O 12 (garnet), and fayalite spinel in ␥-(Fe,Mg) 2 SiO 4 (ringwoodite). No iron-rich silicate, however, was known to exist under the high pressuretemperature (P-T) conditions beyond the 670-km discontinuity that accounts for approximately three-quarters of the Earth's total silicates and oxides. Following Birch's 1952 postulation (4), iron-rich silicates break down to mixed oxides in the lower mantle.In the lower-mantle silicate, (Fe x Mg 1Ϫx )SiO 3 perovskite, iron can only participate as a minor component with x Ͻ 0.15 (5), even at the core-mantle boundary with an unlimited supply of iron from the core. Without a stable iron-rich silicate phase, previous explanations of the complex geochemical and geophysical signatures of the DЉ layer have been limited to heterogeneous, solid͞melt mixtures of iron-poor silicates and iron-rich metals and oxides (6, 7). Recently, MgSiO 3 has been found to transform from perovskite to CaIrO 3 structure under the P-T conditions of the DЉ layer (8)(9)(10)(11)(12). This postperovskite (ppv) phase also was observed to coexist with silicate perovskite and magnesiowüstite in experiments with orthopyroxene and olivine starting materials with x up to 0.4, but the iron content in this phase is undefined because of the unknown Fe͞Mg distributions among multiple coexisting ferromagnesian phases (13). Here, we report experimental and theoretical investigations across the ferrosilite (FeSiO 3 )-enstatite (MgSiO 3 ) join. We found that iron-rich (Fe x Mg 1Ϫx )SiO 3 with x as high as 0...

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Section: Methodsmentioning

confidence: 99%

Iron-rich silicates in the Earth's D″ layer

Mao

Meng

Shen

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

106

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“…[42] An alternative explanation for the D 00 discontinuity is a phase transformation [Nataf and Huard, 1993;Sidorin et al, 1999]. However, no evidence of phase transformations in the lower mantle has been found yet [Kesson et al, 1998;Serghiou et al, 1998].…”

Section: Discontinuitymentioning

confidence: 99%

Small‐scale convection in the D″ layer

Solomatov

Moresi

2002

J. Geophys. Res.

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[1] Small-scale convection has been suggested as a possible explanation for seismic heterogeneities in the D 00 layer of the Earth's mantle. Recently developed scaling laws for convection with realistic viscosities allow quantitative assessment of this hypothesis. Large temperature and viscosity contrasts across the thermal boundary layer at the core-mantle boundary suggest that small-scale convection starts at the bottom of the thermal boundary layer and propagates upward before the thermal boundary layer as a whole becomes unstable. The convection boundary is likely to become a chemical boundary as a result of mixing within the convective layer. This implies that the D 00 discontinuity can represent both convective and chemical boundary and that the presence or absence of small-scale convection can be responsible for the observed intermittent nature of the D 00 discontinuity. Most lateral heterogeneities in the D 00 layer are concentrated near the convection boundary, consistent with seismic data. The length scale of lateral temperature variations, the topography of the core-mantle boundary, and the viscosity of the D 00 layer are in agreement with observational constraints. The thickness of the secondary thermal boundary layer formed at the bottom of the convective layer is similar to the thickness of the ultralow-velocity zone. The magnitude of lateral variations in temperature can only marginally be responsible for lateral variations in seismic velocities. Variations in the topography of the convection boundary and chemical heterogeneities are likely to be more important factors. A large seismic velocity drop in the ultralow-velocity zone cannot be explained by thermal effects alone and must be caused by other factors such as partial melting.

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“…Seismic studies show DЉ to be complex and laterally heterogeneous, including evidence for a DЉ seismic discontinuity, anomalous seismic anisotropy, and ultra-low velocity zones (1)(2)(3). The ferromagnesian silicate perovskite (pv) phase, (Mg,Fe)SiO 3 , has been regarded as the dominant lower mantle mineral but its properties are not consistent with those of the DЉ region.…”

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confidence: 99%

Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO ₃ orthopyroxene

Shieh

Duffy

Kubo

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

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Using the laser-heated diamond anvil cell, we investigate the stability and equation of state of the postperovskite (ppv, CaIrO3-type) phase synthesized from a natural pyroxene composition with 9 mol.% FeSiO3. Our measured pressure-volume data from 12-106 GPa for the ppv phase yield a bulk modulus of 219(5) GPa and a zero-pressure volume of 164.9(6) Å 3 when K0 ‫؍‬ 4. The bulk modulus of ppv is 575(15) GPa at a pressure of 100 GPa. The transition pressure is lowered by the presence of Fe. Our x-ray diffraction data indicate the ppv phase can be formed at P > 109(4) GPa and 2,400(400) K, corresponding to Ϸ400 -550 km above the core-mantle boundary. Direct comparison of volumes of coexisting perovskite and CaIrO3-type phases at 80 -106 GPa demonstrates that the ppv phase has a smaller volume than perovskite by 1.1(2)%. Using measured volumes together with the bulk modulus calculated from equation of state fits, we find that the bulk sound velocity decreases by 2.3(2.1)% across this transition at 120 GPa. Upon decompression without further heating, it was found that the ppv phase could still be observed at pressures as low at 12 GPa, and evidence for at least partial persistence to ambient conditions is also reported.phase transition ͉ x-ray diffraction ͉ high pressure ͉ diamond anvil cell ͉ lower mantle T he DЉ region of the Earth's mantle lying just above the core-mantle boundary has long been among the leastunderstood regions of the planet. Seismic studies show DЉ to be complex and laterally heterogeneous, including evidence for a DЉ seismic discontinuity, anomalous seismic anisotropy, and ultra-low velocity zones (1-3). The ferromagnesian silicate perovskite (pv) phase, (Mg,Fe)SiO 3 , has been regarded as the dominant lower mantle mineral but its properties are not consistent with those of the DЉ region. Thus, the discovery of a postperovskite (ppv) phase at 125 GPa and 2,500 K (4) has attracted considerable attention because of its potential relevance to DЉ. First-principles quantum mechanical calculations (5-9) support the thermodynamic stability of the new phase and place constraints on some physical properties including the elastic tensor and the Clapeyron slope of the transition. However, theoretical studies involve varying degrees of approximation and thus comparison with direct experiment is essential. Experimental studies have placed constraints on the transition pressure and unit cell parameters of the ppv phase (4, 5, 10-16). Besides the crystal structure, the equation of state (EOS) is the most fundamental parameter obtained from high-pressure experiments. As yet, there has been no experimental study that directly compares the EOSs of the pv and ppv phases over a broad range of pressure conditions for a mantle-relevant chemical composition. Results and DiscussionPv and ppv phases were synthesized from an orthopyroxene sample containing 9 mol.% Fe by using the laser-heated diamond anvil cell (see Experimental Methods). By using synchrotron x-ray diffraction, we obtained pressure-volume data at 300 K b...

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Evidence for a Ubiquitous Seismic Discontinuity at the Base of the Mantle

Cited by 167 publications

References 20 publications

Iron-rich silicates in the Earth's D″ layer

Iron-rich silicates in the Earth's D″ layer

Small‐scale convection in the D″ layer

Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO ₃ orthopyroxene

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Evidence for a Ubiquitous Seismic Discontinuity at the Base of the Mantle

Cited by 167 publications

References 20 publications

Iron-rich silicates in the Earth's D″ layer

Iron-rich silicates in the Earth's D″ layer

Small‐scale convection in the D″ layer

Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO 3 orthopyroxene

Contact Info

Product

Resources

About

Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO ₃ orthopyroxene