Evidence of denser MgSiO₃glass above 133 gigapascal (GPa) and implications for remnants of ultradense silicate melt from a deep magma ocean

Abstract

Ultralow velocity zones are the largest seismic anomalies in the mantle, with 10-30% seismic velocity reduction observed in thin layers less than 20-40 km thick, just above the Earth's core-mantle boundary (CMB). The presence of silicate melts, possibly a remnant of a deep magma ocean in the early Earth, have been proposed to explain ultralow velocity zones. It is, however, still an open question as to whether such silicate melts are gravitationally stable at the pressure conditions above the CMB. Fe enrichment is usually invoked to explain why melts would remain at the CMB, but this has not been substantiated experimentally. Here we report in situ high-pressure acoustic velocity measurements that suggest a new transformation to a denser structure of MgSiO 3 glass at pressures close to those of the CMB. The result suggests that MgSiO 3 melt is likely to become denser than crystalline MgSiO 3 above the CMB. The presence of negatively buoyant and gravitationally stable silicate melts at the bottom of the mantle, would provide a mechanism for observed ultralow seismic velocities above the CMB without enrichment of Fe in the melt. An ultradense melt phase and its geochemical inventory would be isolated from overlying convective flow over geologic time.dynamics | early Earth evolution | high-pressure experiment | sound velocity measurement | pressure-induced polymorphism T he buoyancy relations between silicate melts and crystals in a deep terrestrial magma ocean are the primary constraints on the possible chemical stratification of Earth's interior (1, 2). The nature of silicate melts under high-pressure conditions is therefore critically important for elucidating the formation and differentiation of the Earth through massive primordial melting of the proto-Earth. Extensive melting of the proto-Earth and the formation of a deep magma ocean, induced by massive planetesimal collisions and possibly encompassing an entire planet, facilitated metal-silicate (core-mantle) segregation, with the subsequent fractional crystallization of silicate melt leading to chemical and gravitational equilibrium (3-6). Because of the high compressibility of melts, a density crossover between crystals and coexisting magmas is expected in the course of fractional crystallization in a deep magma ocean, which would enhance chemical differentiation and could result in a stratified structure of the Earth's interior (7,8). The possible presence of dense, gravitationally stable magmas deep within the Earth at pressures above 100 gigapascals (GPa) has thus been proposed as a consequence of partial melting and a remnant of a deep magma ocean (9, 10), which might explain the observation of anomalously ultralow seismic velocities (ULVZ) above the core-mantle boundary (CMB) (11). However, the behavior of silicate melts under such extreme pressures is poorly understood, and it is still an open question as to whether a density crossover between silicate melt and coexisting mantle phases occurs in the D′′ region above the CMB (approximately 2,900 km dep...

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

“…In contrast, the P-V S profile of SA2 to 40 GPa can be fitted by a low-degree polynomial function such as a concave quadratic function (Fig. 5b), which is quite similar to that of MgSiO 3 glass (Murakami and Bass 2011). The P-V S profile of SA2 below 40 GPa seems to be that of SA1 just shifted to lower pressures (Fig.…”

“…Uncertainties for the wave number of the minimum position of dI∕dv in each spectrum were derived from the errors of the curve fitting. The total uncertainties for pressure were finally estimated from the standard deviation (1σ) of each determined pressure value at each point Figure 5a shows the P-V S profiles of SA1 and SA2 up to 40 GPa as functions of pressure together with those of SiO 2 glass (Murakami and Bass 2010) and MgSiO 3 glass (Murakami and Bass 2011). The values of V S of the SiO 2 -Al 2 O 3 glasses increase rapidly from 10 to 40 GPa, with no discontinuous changes in slope.…”

“…This indicates that acoustic wave velocity can be a good indicator for tracking the pressure evolution of the Si-O coordination number of silicate glasses. Murakami and Bass (2010) also reported an abrupt increase of P-V S gradient (dV S /dP) in SiO 2 glass above~140 GPa, which was also observed in MgSiO 3 glass at~133 GPa (Murakami and Bass 2011) and proposed the possibility of a further structural change such as a gradual increase of the average Si-O coordination number from 6 to 6 + . Such a structural change may increase the densities of silicate melts significantly, and therefore can constrain the dynamics of silicate melts that may exist at the base of the mantle.…”

Fate of MgSiO 3 melts at core–mantle boundary conditions

“…3A). The small density contrast highlights that density cross-overs are probable, and melts can be as dense or even denser than the surrounding solids in the deep mantle, regardless of the presence of a poly-amorphous phase transition near 133 GPa (11), which may lead to a density inversion at even higher pressure as highlighted by shock compression experiments (24) (SI Appendix, Fig. S11).…”

“…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 scenarios for the formation of the Earth's mantle and its geodynamical evolution critically depend on the density and fate of silicate melts at high pressure.…”