Thermodynamics of silicate liquids in the deep Earth

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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“…S10). Our experimental isotherms agree with the isotherms from MD simulations along different isotherms (19,21,23) (Fig. 3A).…”

supporting

confidence: 83%

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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“…Planets that are molten at the surface likely possess a magma ocean that extends to hundreds of kilometres depth into the planet. Comparison of the silicate melting curve with the silicate adiabat [17] shows that for T s = 2100 K, similar to that of Kepler-10b assuming d = 4, the adiabat lies above the liquidus (complete melting) to a pressure P = 30 GPa, and lies between liquidus and solidus (partial melting) to a pressure P > 150 GPa. These pressures correspond to depths of 400 km for complete melting and greater than 1500 km for partial melting in the case M p = 5 M .…”

Section: −2β Pmentioning

confidence: 84%

Melting in super-earths

Stixrude

2014

Phil. Trans. R. Soc. A.

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104

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We examine the possible extent of melting in rockiron super-earths, focusing on those in the habitable zone. We consider the energetics of accretion and core formation, the timescale of cooling and its dependence on viscosity and partial melting, thermal regulation via the temperature dependence of viscosity, and the melting curves of rock and iron components at the ultra-high pressures characteristic of superearths. We find that the efficiency of kinetic energy deposition during accretion increases with planetary mass; considering the likely role of giant impacts and core formation, we find that super-earths probably complete their accretionary phase in an entirely molten state. Considerations of thermal regulation lead us to propose model temperature profiles of super-earths that are controlled by silicate melting. We estimate melting curves of iron and rock components up to the extreme pressures characteristic of superearth interiors based on existing experimental and ab initio results and scaling laws. We construct superearth thermal models by solving the equations of mass conservation and hydrostatic equilibrium, together with equations of state of rock and iron components. We set the potential temperature at the core-mantle boundary and at the surface to the local silicate melting temperature. We find that ancient (∼4 Gyr) super-earths may be partially molten at the top and bottom of their mantles, and that mantle convection is sufficiently vigorous to sustain dynamo action over the whole range of super-earth masses.

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“…Magmatic segregations of Ferich peridotitic or komatiitic materials could most easily have formed during early magma ocean crystallization or shortly afterward (36)(37)(38)(39) ) and the corresponding agedepth relationship for sinking slabs (red numbers). We also show the agedepth relationship (black numbers) suggested by van der Meer and colleagues (42) for Mesozoic-Cenozoic times.…”

Section: Long-term Stability Of Deep Mantle Structuresmentioning

confidence: 99%

Deep mantle structure as a reference frame for movements in and on the Earth

Torsvik

Voo

Doubrovine

et al. 2014

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

229

204

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Earth's residual geoid is dominated by a degree-2 mode, with elevated regions above large low shear-wave velocity provinces on the core-mantle boundary beneath Africa and the Pacific. The edges of these deep mantle bodies, when projected radially to the Earth's surface, correlate with the reconstructed positions of large igneous provinces and kimberlites since Pangea formed about 320 million years ago. Using this surface-to-core-mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander, we have developed a model for absolute plate motion back to earliest Paleozoic time (540 Ma). For the Paleozoic, we have identified six phases of slow, oscillatory true polar wander during which the Earth's axis of minimum moment of inertia was similar to that of Mesozoic times. The rates of Paleozoic true polar wander (<1°/My) are compatible with those in the Mesozoic, but absolute plate velocities are, on average, twice as high. Our reconstructions generate geologically plausible scenarios, with large igneous provinces and kimberlites sourced from the margins of the large low shear-wave velocity provinces, as in Mesozoic and Cenozoic times. This absolute kinematic model suggests that a degree-2 convection mode within the Earth's mantle may have operated throughout the entire Phanerozoic.plate reconstructions | thermochemical piles T wo equatorial, antipodal, large low shear-wave velocity provinces ( Fig. 1) in the lowermost mantle (1) beneath Africa (termed Tuzo) (2) and the Pacific Ocean (Jason) are prominent in all shear-wave tomographic models (3-7) and have been argued to be related to a dominant degree-2 pattern of mantle convection that has been stable for long times (3). Most reconstructed large igneous provinces and kimberlites over the past 300 My have erupted directly above the margins of Tuzo and Jason, which we term the plume generation zones (1, 2, 5). This remarkable correlation suggests that the two deep mantle structures have been stable for at least 300 My. Stability before Pangea (before 320 Ma) is difficult to test with plate reconstructions because the paleogeography, the longitudinal positions of continents, and the estimates of true polar wander are uncertain (8). It is similarly challenging to reproduce such long-term stability in numerical models (9). However, if the correlation between the eruption sites of large igneous provinces, kimberlites, and the plume generation zones observed for the past 300 Ma has been maintained over the entire Phanerozoic (0-540 Ma), it can provide a crucial constraint for defining the longitudinal positions of continental blocks during Paleozoic time (250-540 Ma).Here we show that a geologically reasonable kinematic model that reconstructs continents in longitude in such a way that large igneous provinces and kimberlites are positioned above the plume generation zones at the times of their formation ( Fig. 2A and SI Appendix, Fig. S2) can be successfully defined f...

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Thermodynamics of silicate liquids in the deep Earth

Cited by 216 publications

References 58 publications

Fate of MgSiO ₃ melts at core–mantle boundary conditions

Fate of MgSiO ₃ melts at core–mantle boundary conditions

Melting in super-earths

Deep mantle structure as a reference frame for movements in and on the Earth

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Thermodynamics of silicate liquids in the deep Earth

Cited by 216 publications

References 58 publications

Fate of MgSiO 3 melts at core–mantle boundary conditions

Fate of MgSiO 3 melts at core–mantle boundary conditions

Melting in super-earths

Deep mantle structure as a reference frame for movements in and on the Earth

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Fate of MgSiO ₃ melts at core–mantle boundary conditions

Fate of MgSiO ₃ melts at core–mantle boundary conditions