Localized boundary layer below the mid‐Pacific velocity anomaly identified from a <i>PcP</i> precursor

Mori, Jim; Helmberger, Donald V.

doi:10.1029/95jb02243

Cited by 142 publications

(93 citation statements)

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“…If this is the case, the pressure-induced acoustic wave velocity changes of the aluminosilicate glasses explored in the present study may offer possible implications for the dynamics of dense magmas that potentially exist at the base of the mantle. The possible existence of dense silicate melts at or near the CMB has been proposed (e.g., Ohtani 1983; Ohtani and Maeda 2001;Williams and Garnero 1996;Labrosse et al 2007) to explain the anomalous reduction of the seismic wave velocities just above the CMB (e.g., Garnero and Helmberger 1995;Mori and Helmberger 1995). Recent high-pressure melting experiments indicated that such dense silicate melts at the CMB can be generated by partial melting of midocean ridge basalts (MORBs) (Andrault et al 2014;Pradhan et al 2015).…”

Section: Resultsmentioning

confidence: 99%

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Ultrahigh-pressure acoustic wave velocities of SiO2-Al2O3 glasses up to 200 GPa

Ohira

Murakami

Kohara

et al. 2016

Prog. in Earth and Planet. Sci.

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Extensive experimental studies on the structure and density of silicate glasses as laboratory analogs of natural silicate melts have attempted to address the nature of dense silicate melts that may be present at the base of the mantle. Previous ultrahigh-pressure experiments, however, have been performed on simple systems such as SiO 2 or MgSiO 3 , and experiments in more complex system have been conducted under relatively low-pressure conditions below 60 GPa. The effect of other metal cations on structural changes that occur in dense silicate glasses under ultrahigh pressures has been poorly understood. Here, we used a Brillouin scattering spectroscopic method up to pressures of 196.9 GPa to conduct in situ high-pressure acoustic wave velocity measurements of SiO 2 -Al 2 O 3 glasses in order to understand the effect of Al 2 O 3 on pressure-induced structural changes in the glasses as analogs of aluminosilicate melts. From 10 to 40 GPa, the transverse acoustic wave velocity (V S ) of Al 2 O 3 -rich glass (SiO 2 + 20. 5 mol% Al 2 O 3 ) was greater than that of Al 2 O 3 -poor glass (SiO 2 + 3.9 mol% Al 2 O 3 ). This result suggests that SiO 2 -Al 2 O 3 glasses with higher proportions of Al ions with large oxygen coordination numbers (5 and 6) become elastically stiffer up to 40 GPa, depending on the Al 2 O 3 content, but then soften above 40 GPa. At pressures from 40 to~100 GPa, the increase in V S with increasing pressure became less steep than below 40 GPa. Abovẽ 100 GPa, there were abrupt increases in the P-V S gradients (dV S /dP) at 130 GPa in Al 2 O 3 -poor glass and at 116 GPa in Al 2 O 3 -rich glass. These changes resemble previous experimental results on SiO 2 glass and MgSiO 3 glass. Given that changes of dV S /dP have commonly been related to changes in the Si-O coordination states in the glasses, our results, therefore, may indicate a drastic structural transformation in SiO 2 -Al 2 O 3 glasses above 116 GPa, possibly associated with an average Si-O coordination number change to higher than 6. Compared to previous acoustic wave velocity data on SiO 2 and MgSiO 3 glasses, Al 2 O 3 appears to promote a lowering of the pressure at which the abrupt increase of dV S /dP is observed. This suggests that the Al 2 O 3 in silicate melts may help to stabilize those melts gravitationally in the lower mantle.

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Section: Resultsmentioning

confidence: 99%

“…There are regions at the base of the mantle where anomalous reductions of seismic wave velocities occur (e.g., Garnero and Helmberger 1995;Mori and Helmberger 1995). These are known as ultralow velocity zones (ULVZs).…”

Section: Introductionmentioning

confidence: 99%

Ultrahigh-pressure acoustic wave velocities of SiO2-Al2O3 glasses up to 200 GPa

Ohira

Murakami

Kohara

et al. 2016

Prog. in Earth and Planet. Sci.

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“…According to results of the seismological studies conducted in the D" layer (a 10 to 40 km-thick layer directly neighbouring the core), the velocity of longitudinal waves, Vp is decreased by 5-10 to 30 % and thus significantly differs from that of the overlying mantle [Garnero, 2004;Mori, Helmberger, 1995;Williams, Garnero, 1996]. With account of the available experimental data [Khitarov et al, 1983], we can state that the degree of melting of silicate rocks in this area of ultra-low velocities is high, and it is close to complete meltdown on some sites.…”

Section: The Asthenospheric D" Layer and Its Role For Pluming In Thementioning

confidence: 99%

Seismic Evidence for Partial Melt at the Base of Earth's Mantle

Williams

Garnero

1996

Science

540

406

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Abstract:The content of hydrogen in the outer core of the Earth is roughly quantified from the dependence of the density of iron (viewed as the main component of the core) on the amount of hydrogen dissolved in the core, with account of the most likely presence of iron hydrogen in the outer core, and the matter's density jumps at the boundaries between the outer liquid core and the internal solid core (that is devoid of hydrogen) and the mantle. Estimations for the outer liquid core show that the hydrogen content varies from 0.67 wt. % at the boundary with the solid inner core to 3.04 wt. % at the boundary with the mantle.Iron occlusion is viewed as the most likely mechanism for the iron-nickel core to capture such a significant amount of hydrogen. Iron occlusion took place at the stage of the young sun when the metallic core emerged in the cooling protoplanetary cloud containing hydrogen in high amounts, and non-volatile hydrogen was accumulated. Absorption (occlusion) of molecular hydrogen was preceded by dissociation of molecules into atoms and ionization of the atoms, as proved by results of studies focused on Fe-H2 system, and hydrogen dissipation was thus prevented. The core matter was subject to gravitational compression at high pressures that contributed to the forced rapprochement of protons and electrons which interaction resulted by the formation of hydrogen atoms. Highly active hydrogen atoms reacted with metals and produced hydrides of iron and nickel, FeH and NiH. While the metallic core and then the silicate mantle were growing and consolidating, the stability of FeH and NiH was maintained due to pressures that were steadily increasing. Later on, due to the impacts of external forces on the Earth, marginal layers at the mantle-core boundary were detached and displaced, pressures decreased in the system, and iron and nickel hydrides were decomposed to produce molecular hydrogen. Consequences of the hydrides transformation into molecular hydrogen are important in terms of petrology, mineralogy and geodynamics. At high temperatures, molecular hydrogen can be involved in redox reactions with iron silicates and carbonaceous gases (CO and CO2), and the synthesis of water becomes possible throughout the entire mantle. It is known that water can significantly reduce the temperature of rock melting, which leads to partial melting of the rocks and pluming in the asthenosphere (in the D" layer) at the bottom of the mantle, and causes the hydrolysis of magnesium silicates, which results in the chemically bound state (hydroxyl ions). Highly ductile hydroxyl-containing magnesium silicates can alter rheological properties of the rocks. A combination of rheologically weak areas in the mantle rocks and the external cosmic effects can cause significant impacts on the tectonic activity and facilitate its manifestation throughout the entire mantle. GEODYNAMICS & TECTONOPHYSICS P U B L I S H E D B Y T H E I N S T I T U T E O F T H E E A R T H ' S C R U S T S I B E R I A N B R A N C H O F R U S S I A N A C A D E M Y O...

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“…Seismic data indicate that it has a thickness of 5 -40 km and P and S wave velocity reductions of the order of 5 -30%, suggesting a possible partial melting [Garnero and Helmberger, 1995;Mori and Helmberger, 1995;Williams and Garnero, 1996;Revenaugh and Meyer, 1997;Vidale and Hedlin, 1998;Wen and Helmberger, 1998]. …”

Section: Ultralow-velocity Zonementioning

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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Localized boundary layer below the mid‐Pacific velocity anomaly identified from a PcP precursor

Cited by 142 publications

References 23 publications

Ultrahigh-pressure acoustic wave velocities of SiO2-Al2O3 glasses up to 200 GPa

Ultrahigh-pressure acoustic wave velocities of SiO2-Al2O3 glasses up to 200 GPa

Seismic Evidence for Partial Melt at the Base of Earth's Mantle

Small‐scale convection in the D″ layer

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