Spin crossover and iron-rich silicate melt in the Earth’s deep mantle

Nomura, R.; Ozawa, Haruka; Tateno, Shigehiko; Hirose, Kei; Hernlund, J. W.; Muto, Shunsuke; Ishii, Hirofumi; Hiraoka, Nozomu

doi:10.1038/nature09940

Cited by 224 publications

(252 citation statements)

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“…Although a recent X-ray emission spectroscopic study of iron-bearing silicate glass showed a precipitous change in spin state of iron at B75 GPa (ref. 22), no clear evidence for such a discontinuous feature was observed in the present study, which may require reconsideration of the previously proposed iron partitioning behaviour in silicate melt under pressure 22 . Very recent experimental work on high-pressure Mössbauer spectra of Mg-, Fe-and Al-bearing silicate glass has also shown the absence of such a precipitous change in spin state with pressure 30 .…”

Section: Resultscontrasting

confidence: 94%

“…Knowledge of the pressure dependence of optical absorption bands of iron-bearing dense silicate glasses is therefore essential to understand changes in high-pressure electronic configurations of the glasses and thereby to infer the radiative part of the thermal conductivity. Two types of silicate glasses were used as analogues for dense silicate melts at the CMB: one with (Mg 0.8 Fe 0.2 )SiO 3 composition (E-glass) to isolate the effect of iron 22 , and the other with a multicomponent basaltic composition (M-glass) (Table 1) to simulate a more realistic compositional system at the CMB 6,9,24,25 . In the optical absorption measurements, both samples visibly became optically darker with pressure, as shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

“…Recent experimental research on the possible spin-pairing electronic transition of iron in silicate glasses at midlower mantle pressures 22 has renewed interest in understanding the thermal transport mechanisms in dense silicate melts at the bottom of the mantle, as changes in the optical properties of ironbearing phases associated with spin transitions may significantly influence the radiative component of thermal conductivity. In this work, we conducted visible and near-infrared optical absorption and synchrotron Mössbauer spectroscopic measurements of ironenriched silicate glasses at high pressures up to 85 GPa to clarify the potential influences of pressure-induced electronic structure changes in dense silicate melts on the radiative part of the thermal conductivity.…”

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High-pressure radiative conductivity of dense silicate glasses with potential implications for dark magmas

et al. 2014

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The possible presence of dense magmas at Earth's core-mantle boundary is expected to substantially affect the dynamics and thermal evolution of Earth's interior. However, the thermal transport properties of silicate melts under relevant high-pressure conditions are poorly understood. Here we report in situ high-pressure optical absorption and synchrotron Mössbauer spectroscopic measurements of iron-enriched dense silicate glasses, as laboratory analogues for dense magmas, up to pressures of 85 GPa. Our results reveal a significant increase in absorption coefficients, by almost one order of magnitude with increasing pressure to B50 GPa, most likely owing to gradual changes in electronic structure. This suggests that the radiative thermal conductivity of dense silicate melts may decrease with pressure and so may be significantly smaller than previously expected under coremantle boundary conditions. Such dark magmas heterogeneously distributed in the lower mantle would result in significant lateral heterogeneity of heat flux through the core-mantle boundary.

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confidence: 94%

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High-pressure radiative conductivity of dense silicate glasses with potential implications for dark magmas

et al. 2014

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“…In this scenario, a BMO is separated from the "surficial" MO due to an isochemical density crossover between solids and liquids and/or a crossover in the slopes of the mantle solidus and adiabat (Andrault et al, 2017;Thomas et al, 2012). As any potential crossovers are inferred to occur at very high pressures, if at all (Andrault et al, 2012;de Koker et al, 2013;Ghosh and Karki, 2016;Nomura et al, 2011;Stixrude et al, 2009), they may only be relevant for (near-)complete melting of the early Earth's mantle, e.g. in a giant-impact scenario.…”

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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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“…Although the partitioning of iron (K D ) between solid and melts remains controversial at lower mantle pressures (9,25), it is agreed that iron is an incompatible element. Therefore, iron is enriched in the melt phase with a K D value as low as ∼0.4-0.3 at 25 GPa (26,27) and most likely even lower at higher pressures.…”

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Fate of MgSiO ₃ melts at core–mantle boundary conditions

Petitgirard

Malfait

Sinmyo

et al. 2015

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

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One key for understanding the stratification in the deep mantle lies in the determination of the density and structure of matter at high pressures, as well as the density contrast between solid and liquid silicate phases. Indeed, the density contrast is the main control on the entrainment or settlement of matter and is of fundamental importance for understanding the past and present dynamic behavior of the deepest part of the Earth's mantle. Here, we adapted the X-ray absorption method to the small dimensions of the diamond anvil cell, enabling density measurements of amorphous materials to unprecedented conditions of pressure. Our density data for MgSiO 3 glass up to 127 GPa are considerably higher than those previously derived from Brillouin spectroscopy but validate recent ab initio molecular dynamics simulations. A fourth-order Birch-Murnaghan equation of state reproduces our experimental data over the entire pressure regime of the mantle. At the core-mantle boundary (CMB) pressure, the density of MgSiO 3 glass is 5.48 ± 0.18 g/cm 3 , which is only 1.6% lower than that of MgSiO 3 bridgmanite at 5.57 g/cm 3 , i.e., they are the same within the uncertainty. Taking into account the partitioning of iron into the melt, we conclude that melts are denser than the surrounding solid phases in the lowermost mantle and that melts will be trapped above the CMB.silicate glass density | X-ray absorption | basal magma ocean V ariations in seismic velocities observed in the deep mantle could be attributed to melting phenomena (1), accumulation of dense matter (2), core-mantle interactions (3), or even a deep hidden geochemical reservoir that dates from the early differentiation of the Earth (4). Indeed, melting phenomena play a critical role in the formation and evolution of terrestrial planets. In the early Earth's history, large-scale melting events and magma oceans facilitated the segregation of the iron-rich core and the partitioning of elements between the different layers of the Earth (5). Catastrophic events like the moon-forming giant impact may have melted the entire Earth and modified the thermal and chemical state of the planet's interior (6). It has long been thought that the mantle crystallized from the bottom to the top, with crystals being denser than melts (7). However, the formation of a dense basal magma ocean (BMO) at the bottom of the mantle concomitant to the final accretion of the Earth was proposed as a consequence of partial melting (2). The formation of a BMO requires the accumulation of dense iron-rich silicate melts at the core-mantle boundary (CMB). In this scenario, the mantle starts to crystallize at an intermediate depth, and a raft of crystals continues to grow toward both the bottom and top of the mantle. In the modern mantle, seismic observations highlight ultralow velocity zones (ULVZs) near the CMB (8) that may be caused by melting of deep mantle material (9), potentially feeding sources of hot spots. Alternatively, the ULVZs may be dense remnants from an initial BMO (10, 11). Both scenar...

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Spin crossover and iron-rich silicate melt in the Earth’s deep mantle

Cited by 224 publications

References 40 publications

High-pressure radiative conductivity of dense silicate glasses with potential implications for dark magmas

High-pressure radiative conductivity of dense silicate glasses with potential implications for dark magmas

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

Fate of MgSiO ₃ melts at core–mantle boundary conditions

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Spin crossover and iron-rich silicate melt in the Earth’s deep mantle

Cited by 224 publications

References 40 publications

High-pressure radiative conductivity of dense silicate glasses with potential implications for dark magmas

High-pressure radiative conductivity of dense silicate glasses with potential implications for dark magmas

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

Fate of MgSiO 3 melts at core–mantle boundary conditions

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Fate of MgSiO ₃ melts at core–mantle boundary conditions