Melting of subducted basalt at the core-mantle boundary

Andrault, Denis; Pesce, G.; Bouhifd, Mohamed Ali; Bolfan-Casanova, Nathalie; Hénot, Jean-Marc; Mézouar, M.

doi:10.1126/science.1250466

Cited by 129 publications

(140 citation statements)

References 32 publications

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“…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). The densification in SA1 and SA2, which probably reflects an average Si-O coordination number in excess of 6, occurs at lower pressures than in pure MgSiO 3 glass (Fig.…”

Section: Resultsmentioning

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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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%

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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“…This is not a surprise that mantle melting would be predicted from calculations of the evolution of the core since current presence of partial melt at the bottom of the mantle has been invoked to explain the occurrence of ultra low velocity zones (ULVZ) (eg Williams and Garnero, 1996;Rost et al, 2005). The present CMB temperature is already very close to the solidus, or even higher for basalt compositions (Andrault et al, 2014), and even milder core evolution calculations would predict significant melting of the lower mantle in the past. This is the reason behind the basal magma ocean model which argues that ULVZs are the remnant of a once much thicker magma ocean.…”

Section: Thermal Evolution Modelsmentioning

confidence: 97%

Thermal evolution of the core with a high thermal conductivity

Labrosse

2015

Physics of the Earth and Planetary Interiors

190

271

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The rate at which heat is extracted across the core mantle boundary (CMB) is constrained by the requirement of dynamo action in the core. This constraint can be computed explicitly using the entropy balance of the core and depends on the thermal conductivity, whose value has been revised upwardly. A high order model (fourth degree polynomial of the radial position) for the core structure is derived and the implications for the core cooling rate and thermal evolution obtained, using the recent values of the thermal conductivity. For a thermal conductivity increasing with depth as proposed by some of these recent studies, a CMB heat flow equal to the isentropic value (13.25TW at present) leads to a 700 km thick layer at the top of the core where a downward convective heat flow is necessary to maintain an isentropic and well mixed average state. Considering a CMB heat flow larger than the well mixed isentropic value leads to an inner core less than 700 Myr old and the thermal evolution of the core is largely constrained by the conditions for dynamo action without an inner core. Analytical calculations for that period show that a CMB temperature larger than 7000 K must have prevailed 4.5 Gyr ago if the geodynamo has been driven by thermal convection for that whole time. This raises questions regarding the onset of the geodynamo and its continuous operation for the last 3.5 Gyr. Implications regarding the evolution of a basal magma ocean are also considered.

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“…Melting in the hot thermal boundary layer near the CMB is expected in the hot Hadean eon (e.g., Korenaga, 2006) for any material with significant Fe-enrichment. Note that even today, moderately-enriched rocks such as basalts are thought to almost reach melting temperatures at the CMB (Andrault et al, 2014;Kato et al, 2016). A secondary BMO made up of (hybridized) Fe-rich cumulates would be gravitationally stable due to its intrinsic density anomaly, independent of the postulated density crossover between solids and liquids (Andrault et al, 2017).…”

Section: Discussionmentioning

confidence: 99%

Reconciling magma‐ocean crystallization models with the present‐day structure of the Earth's mantle

Ballmer

Lourenço

Hirose

et al. 2017

Geochem Geophys Geosyst

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Terrestrial planets are thought to experience episode(s) of large‐scale melting early in their history. Fractionation during magma‐ocean freezing leads to unstable stratification within the related cumulate layers due to progressive iron enrichment upward, but the effects of incremental cumulate overturns during MO crystallization remain to be explored. Here, we use geodynamic models with a moving‐boundary approach to study convection and mixing within the growing cumulate layer, and thereafter within the fully crystallized mantle. For fractional crystallization, cumulates are efficiently stirred due to subsequent incremental overturns, except for strongly iron‐enriched late‐stage cumulates, which persist as a stably stratified layer at the base of the mantle for billions of years. Less extreme crystallization scenarios can lead to somewhat more subtle stratification. In any case, the long‐term preservation of at least a thin layer of extremely enriched cumulates with Fe# > 0.4, as predicted by all our models, is inconsistent with seismic constraints. Based on scaling relationships, however, we infer that final‐stage Fe‐rich magma‐ocean cumulates originally formed near the surface should have overturned as small diapirs, and hence undergone melting and reaction with the host rock during sinking. The resulting moderately iron‐enriched metasomatized/hybrid rock assemblages should have accumulated at the base of the mantle, potentially fed an intermittent basal magma ocean, and be preserved through the present‐day. Such moderately iron‐enriched rock assemblages can reconcile the physical properties of the large low shear‐wave velocity provinces in the present‐day lower mantle. Thus, we reveal Hadean melting and rock‐reaction processes by integrating magma‐ocean crystallization models with the seismic‐tomography snapshot.

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Melting of subducted basalt at the core-mantle boundary

Cited by 129 publications

References 32 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

Thermal evolution of the core with a high thermal conductivity

Reconciling magma‐ocean crystallization models with the present‐day structure of the Earth's mantle

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