Multiple Amorphous–Amorphous Transitions

Loerting, Thomas; Бражкин, В. В.; Morishita, Tetsuya

doi:10.1002/9780470508602.ch2

Cited by 38 publications

(36 citation statements)

References 284 publications

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“…2). Our results for SiO 2 glass along with those of previous studies (17)(18)(19)(20)(21)(22)(23)(24) indicate that the velocity-pressure trends in SiO 2 glass can be interpreted as a gradual transition from four-to sixfold coordination of Si below 40 GPa, predominantly sixfold coordination from 40 to 140 GPa, and a transition from sixfold to a higher coordination state above 140 GPa (16) (Fig. 2).…”

Section: Resultssupporting

confidence: 66%

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

Murakami

Bass

2011

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

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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...

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Section: Resultssupporting

confidence: 66%

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

Murakami

Bass

2011

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

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“…Therefore, the inflection points under ultrahigh pressure may primarily reflect a change of the average Si-O coordination number from 6 to 6 + . Density increases caused by structural transitions accompanied by Si-O coordination number growth occur continuously in glasses and melts and occur discontinuously in crystalline materials (e.g., Loerting et al 2009). Although it is not obvious whether the analogy between silicate glasses and silicate melts works well as a model of their structural or density changes with pressure, experimental results on the evolution of average Si-O coordination number of SiO 2 glass (Meade et al 1992;Lin et al 2007; Funamori 2008, 2010;Benmore et al 2010;Zeidler et al 2014) shows good agreement with that of SiO 2 melt calculated by MD simulations (Karki et al 2007).…”

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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“…In these studies, and in the IR studies of pure CO2 ice (see Kataeva et al (2015) and references cited therein) the ice is much thicker than that of actual ice mantles. It is known that crystallization is characterized by long range order in the solid; in thin films, it depends on the thickness, with the thinner films often being amorphous or partially amorphous with nanocrystals (Loerting et al 2009). It is more realistic to study the segregation and crystallization in the thickness range comparable with the thickness of ice mantles (less than a few tens of a monolayer (ML)) in the ISM.…”

Section: Introductionmentioning

confidence: 99%

Characterization of thin film CO2 ice through the infrared ν1 + ν3 combination mode

He¹,

Vidali²

2017

Monthly Notices of the Royal Astronomical Society

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Carbon dioxide is abundant in ice mantles of dust grains; some is found in the pure crystalline form as inferred from the double peak splitting of the bending profile at about 650 cm −1 . To study how CO 2 segregates into the pure form from water-rich mixtures of ice mantles and how it then crystallizes, we used Reflection Absorption InfraRed Spectroscopy (RAIRS) to study the structural change of pure CO 2 ice as a function of both ice thickness and temperature. We found that the ν 1 + ν 3 combination mode absorption profile at 3708 cm −1 provides an excellent probe to quantify the degree of crystallinity in CO 2 ice. We also found that between 20 and 30 K, there is an ordering transition that we attribute to reorientation of CO 2 molecules, while the diffusion of CO 2 becomes significant at much higher temperatures. In the formation of pure crystalline CO 2 in ISM ices, the rate limiting process is the diffusion/segregation of CO 2 molecules in the ice instead of the phase transition from amorphous to crystalline after clusters/islands of CO 2 are formed.

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Multiple Amorphous–Amorphous Transitions

Cited by 38 publications

References 284 publications

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

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

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

Characterization of thin film CO2 ice through the infrared ν1 + ν3 combination mode

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Multiple Amorphous–Amorphous Transitions

Cited by 38 publications

References 284 publications

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

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

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

Characterization of thin film CO2 ice through the infrared ν1 + ν3 combination mode

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

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