Thermodynamic properties of Mg_2SiO_4 liquid at ultra-high pressures from shock measurements to 200 GPa on forsterite and wadsleyite

Polycrystalline samples of Mg_2SiO_4 forsterite and wadsleyite were synthesized and then dynamically loaded to pressures of 39–200 GPa. Differences in initial density and internal energy between these two phases lead to distinct Hugoniots, each characterized by multiple phase regimes. Transformation to the high-pressure phase assemblage MgO + MgSiO_3 perovksite is complete by 100 GPa for forsterite starting material but incomplete for wadsleyite. The datum for wadsleyite shocked to 136 GPa, however, is consistent with the assemblage MgO + MgSiO_3 post-perovksite. Marked increases in density along the Hugoniots of both phases between ∼130 and 150 GPa are inconsistent with any known solid-solid phase transformation in the Mg_2SiO_4 system but can be explained by melting. Density increases upon melting are consistent with a similar density increase observed in the MgSiO_3 system. This implies that melts with compositions over the entire Mg/Si range likely for the mantle would be negatively or neutrally buoyant at conditions close to the core-mantle boundary, supporting the partial melt hypothesis to explain the occurrence of ultra-low velocity zones at the base of the mantle. From the energetic difference between the high-pressure segments of the two Hugoniots, we estimate a Grüneisen parameter (γ) of 2.6 ± 0.35 for Mg_2SiO_4-liquid between 150 and 200 GPa. Comparison to low-pressure data and fitting of the absolute pressures along the melt Hugoniots both require that γ for the melt increases with increasing density. Similar behavior was recently predicted in MgSiO_3 liquid via molecular dynamics simulations. This result changes estimates of the temperature profile, and hence the dynamics, of a deep terrestrial magma ocean

Simultaneous aluminum, silicon, and sodium coordination changes in 6 GPa sodium aluminosilicate glasses

We present the first direct observation of high-coordinated Si and Al occurring together in a series of high-pressure sodium aluminosilicate glasses, quenched from melts at 6 GPa. Using ^(29)Si MAS NMR, we observe that a small amount of Al does not have a significant effect on the amount of ^VSi or ^(VI)Si generated, but that larger Al concentrations lead to a gradual decrease in both these species. ^(27)Al MAS NMR spectra show that samples with small amounts of Al have extremely high mean Al coordination values of up to 5.49, but that larger Al concentrations cause a gradual decrease in both ^VAl and ^(VI)Al. Although mean Al and Si coordination numbers both decrease with increasing Al contents, the weighted combined (Al+Si) coordination number increases. Silicon and Al resonances shift in frequency with increasing pressure or changing Al concentration, indicating additional structural changes, including compression of network bond angles. Increases in the ^(23)Na isotropic chemical shifts indicate decreases in the mean Na-O bond lengths with increasing pressure, which are more dramatic at higher Al contents. Recovered glass densities are about 10 to 15% greater than those of similar ambient pressure samples. However, the density increases due to the combined coordination changes of Al and Si are estimated to total only about 1 to 2%, and are roughly constant with composition despite the large effects of Al content on the individual coordinations of the two cations. Thus, effects of other structural changes must be significant to the overall densification. Apparent equilibrium constants for reactions involving the generation of high-coordinated species show systematic behavior, which suggests an internal consistency to the observed Si and Al coordination number shifts

The MgSiO_3 system at high pressure: Thermodynamic properties of perovskite, postperovskite, and melt from global inversion of shock and static compression data

We present new equation-of-state (EoS) data acquired by shock loading to pressures up to 245 GPa on both low-density samples (MgSiO_3 glass) and high-density, polycrystalline aggregates (MgSiO_3 perovskite + majorite). The latter samples were synthesized using a large-volume press. Modeling indicates that these materials transform to perovskite, postperovskite, and/or melt with increasing pressure on their Hugoniots. We fit our results together with existing P-V-T data from dynamic and static compression experiments to constrain the thermal EoS for the three phases, all of which are of fundamental importance to the dynamics of the lower mantle. The EoS for perovskite and postperovskite are well described with third-order Birch-Murnaghan isentropes, offset with a Mie-Grüneisen-Debye formulation for thermal pressure. The addition of shock data helps to distinguish among discrepant static studies of perovskite, and for postperovskite, constrain a value of K' significantly larger than 4. For the melt, we define for the first time a single EoS that fits experimental data from ambient pressure to 230 GPa; the best fit requires a fourth-order isentrope. We also provide a new EoS for Mg_2SiO_4 liquid, calculated in a similar manner. The Grüneisen parameters of the solid phases decrease with pressure, whereas those of the melts increase, consistent with previous shock wave experiments as well as molecular dynamics simulations. We discuss implications of our modeling for thermal expansion in the lower mantle, stabilization of ultra-low-velocity zones associated with melting at the core-mantle boundary, and crystallization of a terrestrial magma ocean

Configurational entropy of basaltic melts in Earth’s mantle

Although geophysical observations of mantle regions that suggest the presence of partial melt have often been interpreted in light of the properties of basaltic liquids erupted at the surface, the seismic and rheological consequences of partial melting in the upper mantle depend instead on the properties of interstitial basaltic melt at elevated pressure. In particular, basaltic melts and glasses display anomalous mechanical softening upon compression up to several GPa, suggesting that the relevant properties of melt are strongly pressure-dependent. A full understanding of such a softening requires study, under compression, of the atomic structure of primitive small-degree basaltic melts at their formation depth, which has proven to be difficult. Here we report multiNMR spectra for a simplified basaltic glass quenched at pressures up to 5 GPa (corresponding to depths down to ∼150 km). These data allow quantification of short-range structural parameters such as the populations of coordination numbers of Al and Si cations and the cation pairs bonded to oxygen atoms. In the model basaltic glass, the fraction of ^([5,6])Al is ∼40% at 5 GPa and decreases to ∼3% at 1 atm. The estimated fraction of nonbridging oxygens at 5 GPa is ∼84% of that at ambient pressure. Together with data on variable glass compositions at 1 atm, these results allow us to quantify how such structural changes increase the configurational entropy of melts with increasing density. We explore how configurational entropy can be used to explain the anomalous mechanical softening of basaltic melts and glasses

Configurational entropy of basaltic melts in Earth’s mantle

Although geophysical observations of mantle regions that suggest the presence of partial melt have often been interpreted in light of the properties of basaltic liquids erupted at the surface, the seismic and rheological consequences of partial melting in the upper mantle depend instead on the properties of interstitial basaltic melt at elevated pressure. In particular, basaltic melts and glasses display anomalous mechanical softening upon compression up to several GPa, suggesting that the relevant properties of melt are strongly pressure-dependent. A full understanding of such a softening requires study, under compression, of the atomic structure of primitive small-degree basaltic melts at their formation depth, which has proven to be difficult. Here we report multiNMR spectra for a simplified basaltic glass quenched at pressures up to 5 GPa (corresponding to depths down to ∼150 km). These data allow quantification of short-range structural parameters such as the populations of coordination numbers of Al and Si cations and the cation pairs bonded to oxygen atoms. In the model basaltic glass, the fraction of ^([5,6])Al is ∼40% at 5 GPa and decreases to ∼3% at 1 atm. The estimated fraction of nonbridging oxygens at 5 GPa is ∼84% of that at ambient pressure. Together with data on variable glass compositions at 1 atm, these results allow us to quantify how such structural changes increase the configurational entropy of melts with increasing density. We explore how configurational entropy can be used to explain the anomalous mechanical softening of basaltic melts and glasses

Advances in Shock Compression of Mantle Materials and Implications

Hugoniots of lower mantle mineral compositions are sensitive to the conditions where they cross phase boundaries including both polymorphic phase transitions and partial to complete melting. For SiO_2, the Hugoniot of fused silica passes from stishovite to partial melt (73 GPa, 4600 K) whereas the Hugoniot of crystal quartz passes from CaCi_2 structure to partial melt (116 GPa, 4900 K). For Mg_2SiO_4, the forsterite Hugoniot passes from the periclase +MgSiO_3 (perovskite) assemblage to melt before 152 GPa and 4300 K, whereas the wadsleyite Hugoniot transforms first to periclase +MgSiO_3 (post-perovskite) and then melts at 151 GPa and 4160 K. Shock states achieved from crystal enstatite are molten above 160 GPa. High-pressure Grüneisen parameters for molten states of MgSiO_3 and Mg_2SiO_4 increase markedly with compression, going from 0.5 to 1.6 over the 0 to 135 GPa range. This gives rise to a very large (>2000 K) isentropic rise in temperature with depth in thermal models of a primordial deep magma ocean within the Earth. These magma ocean isentropes lead to models that have crystallization initiating at mid-lower mantle depths. Such models are consistent with the suggestion that the present ultra-low velocity zones, at the base of the lowermost mantle, represent a dynamically stable, partially molten remnant of the primordial magma ocean. The new shock melting data for silicates support a model of the primordial magma ocean that is concordant with the Berkeley-Caltech iron core model [1] for the temperature at the center of the Earth

Hydrogen Incorporation in Natural Mantle Olivines

Constraints on water storage capacity and actual content in the mantle must be derived not only from experimental studies, but also from investigation of natural samples. Olivine is one of the best-studied, OH-bearing "nominally anhydrous" minerals, yet there remain multiple hypotheses for the incorporation mechanism of hydrogen in this phase. Moreover, there is still debate as to whether the mechanism is the same in natural samples vs. experimental studies, where concentrations can reach very high values (up to ~0.6 wt% H_2O) at high pressures and temperatures. We present new observations and review IR and TEM data from the literature that bear on this question. Hydrogen incorporation in natural olivine clearly occurs by multiple mechanisms, but in contrast to some previous assertions we find that there are strong similarities between the IR signatures of experimentally annealed olivines and most natural samples. At low pressures (lower than ~2 GPa) in both experiments and natural olivines, hydrogen incorporation might be dominated by a humite-type defect, but the nature of the defect may vary even within a single sample; possibilities include point defects, planar defects and optically detectable inclusions. IR bands between 3300 and 3400 cm^(-1), ascribed previously to the influence of silica activity, are apparently related instead to increased oxygen fugacity. At higher pressures in experiments, the IR band structure changes and hydrogen is probably associated with disordered point defects. Similar IR spectra are seen in olivines from xenoliths derived from deeper parts of the mantle (below South Africa and the Colorado Plateau) as well as in olivines from the ultra-high pressure metamorphic province of the Western Gneiss Region in Norway

Zonation of H_(2)O and F Concentrations around Melt Inclusions in Olivines

Studies of both naturally quenched and experimentally reheated melt inclusions have established that they can lose or gain H_(2)O after entrapment in their host mineral, before or during eruption. Here we report nanoSIMS analyses of H2O, Cl and F in olivine around melt inclusions from two natural basaltic samples: one from the Sommata cinder cone on Vulcano Island in the Aeolian arc and the other from the Jorullo cinder cone in the Trans-Mexican Volcanic Belt. Our results constrain olivine/basaltic melt partition coefficients and allow assessment of mechanisms of volatile loss from melt inclusions in natural samples. Cl contents in olivine from both samples are mostly below detection limits (≤0·03 ± 0·01 ppm), with no detectable variation close to the melt inclusions. Assuming a maximum Cl content of 0·03 ppm for all olivines, maximum estimates for Cl partition coefficients between olivine and glass are 0·00002 ± 0·00002. Olivines from the two localities display contrasting H_(2)O and F compositions: Sommata olivines contain 27 ± 11 ppm H_(2)O and 0·28 ± 0·07 ppm F, whereas Jorullo olivines have lower and proportionately more variable H_(2)O and F (11 ± 12 ppm and 0·12 ± 0·09 ppm, respectively; uncertainties are two standard deviations for the entire population). The variations of H_(2)O and F contents in the olivines exhibit clear zonation patterns, increasing with proximity to melt inclusions. This pattern was most probably generated during transfer of volatiles out of the inclusions through the host olivine. H_(2)O concentration gradients surrounding melt inclusions are roughly concentric, but significantly elongated parallel to the crystallographic a-axis of olivine. Because of this preferential crystallographic orientation, this pattern is consistent with H_(2)O loss that is rate-limited by the ‘proton–polaron’ mechanism of H diffusion in olivine. Partition coefficients based on olivine compositions immediately adjacent to melt inclusions are 0·0007 ± 0·0003 for H_(2)O and 0·0005 ± 0·0003 for F. The H_(2)O and F diffusion profiles most probably formed in response to a decrease in the respective fugacities in the external melt, owing to either degassing or mixing with volatile-poor melt. Volatile transport out of inclusions might also have been driven in part by increases in the fugacity within the inclusion owing to post-entrapment crystallization. In the case of F, because of the lack of data on F diffusion in olivine, any interpretation of the measured F gradients is speculative. In the case of H_(2)O, we model the concentration gradients using a numerical model of three-dimensional anisotropic diffusion of H, where initial conditions include both H2O decrease in the external melt and post-entrapment enrichment of H_(2)O in the inclusions. The model confirms that external degassing is the dominant driving force, showing that the orientation of the anisotropy in H diffusion is consistent with the proton–polaron diffusion mechanism in olivine. The model also yields an estimate of the initial H_(2)O content of the Sommata melt inclusions before diffusive loss of 6 wt % H_(2)O. The findings provide new insights on rapid H_(2)O loss during magma ascent and improve our ability to assess the fidelity of the H_(2)O record from melt inclusions

Analysis of hydrogen and fluorine in pyroxenes: II. Clinopyroxene

Studies of coexisting, nominally anhydrous minerals in mantle samples show that clinopyroxene is an especially important host for hydrogen. Recent experimental studies have also shown that clinopyroxene may contain significant amounts of fluorine, which has implications for the F budget of the mantle. More accurate quantification of H and F is therefore a desirable goal. We measured H in 13 natural clinopyroxenes using Fourier transform infrared (FTIR) spectroscopy. ^(16)O^1H/^(30)Si and ^(19)F/^(30)Si were also measured in the samples using secondary ion mass spectrometry (SIMS). H data were compared between the two techniques and F was calculated with reference to F-bearing silicate glass standards. Four of the clinopyroxenes are used as standards for SIMS calibration in multiple laboratories, and three have been measured previously using hydrogen manometry and/or elastic recoil detection analysis. Compared to clinopyroxenes in previous surveys comparing FTIR and SIMS, the 13 samples cover a broader range in chemistry and band positions in the O-H vibrational spectrum. They also all lack detectable amphibole lamellae, which are otherwise commonly present in this mineral group. In contrast to orthopyroxene, the SIMS and FTIR data for clinopyroxene show significantly better correlations (r^2 = 0.96–0.98) when the frequency-dependent IR calibration of Libowitzky and Rossman (1997) is applied, as opposed to the Bell et al. (1995) calibration (r^2 = 0.92–93). We derive a frequency-dependent molar absorption coefficient with parameters different from those of Libowitzky and Rossman’s calibration, which was established using data on stoichiometric hydrous phases and gives poor agreement with the manometrically determined value for PMR-53. Comparison of data for PMR-53 to our SIMS calibrations for orthopyroxene and olivine suggests that the matrix effect among these phases is less than 20% relative. Fluorine concentrations vary depending on geological context, with the highest concentrations (up to 214 ppm) found in diopsides from crustal metamorphic environments. Mantle samples follow similar geographic trends as olivines and orthopyroxenes, with higher F in xenocrysts from Kilbourne Hole (46 ppm) and South African kimberlites (up to 29 ppm) compared to the Colorado Plateau (8 ppm). On the basis of chemical correlations, we propose two different incorporation mechanisms for F: (1) coupled subsititution with Al^(3+) and/or Fe^(3+) in tetrahedral sites; and (2) coupled substitution with monovalent cations (Na and K) in the M2 site. The second substitution is more relevant to mantle augites than crustal diopsides. Our measured F concentrations are much lower than those in some clinopyroxenes synthesized in recent high P-T studies. Nevertheless, our data support suggestions that the F budget of the mantle can be entirely accommodated by incorporation in nominally anhydrous/fluorine-free minerals