Due to the higher temperature obtained in the quartz samples, the slope of the stishovite-CaCl2 phase boundary is constrained to be ~180 K/GPa. These observations raise the possibility of the existence of a significant amount of partial melt in the lower mantle, e.g. the ultra-low speed zone. The temperature drop is assumed to be due to complete melting of the samples subjected to pressures greater than 115 GPa.
A projectile with a flyer plate of standard material embedded in it is fired at a target of the material under investigation (Figure 2.1a). The arrival of the shock destroys the mirrored surface and the reflection of light that is detected is extinguished. Time calibration of the streak velocity allows the travel time of the shock and therefore the velocity through the sample of known thickness to be calculated.
To prepare the sample for shock temperature measurement, a silver layer was sputter coated to the driver side of the sample to block radiation emitted from shock compressed air trapped at the sample–driver interface [ Lyzenga , 1980 ]. Radiation produced by shock heating of the central 5 millimeters of the sample is reflected by a. Solving for PH, an equation is developed to calculate the Hugoniot for the initial sample shocked to H.P.P., as a function of volume.
603.7 nm) Pyro 4 (661.5 nm)
The jump in density observed upon reaching final shock states in the melt is discussed in detail by Lyzenga et al. The room temperature compression curves of stishovite and the CaCl2 structure, observed in the diamond anvil study by Andrault et al. The EOS for stishovite used in this study is further supported by the reasonable agreement between the calculated stishovite Hugoniot and the data from Luo et al.
The interpretation of the quartz Hugoniot presented in this study, based largely on the EOS of stishovite, is in stark contrast to the conclusion of Panero et al. With increasing pressure, the speed of sound decreases in anticipation of the transition to the CaCl2 structure which occurs at ~65 GPa. The longitudinal wave velocity in the CaCl2 structure (metastable shocked in the melting region above 70 GPa) then increases again until at a Hugoniot pressure of 115 GPa, melting occurs, as inferred from the sudden drop in bulk sound speed.
The only data for porous coesite at 85 GPa are assumed to correspond to a point in the stishovite melting regime which reaches a. In the case of the porous cristobalite data, the two peaks in the 45 to 70 GPa region are interpreted as being in the stishovite regime. A possible explanation for the discrepancy with the calculated Hugoniot in this pressure regime will be discussed together with the observed anomalous behavior of the fused quartz Hugoniot data in the section on shock temperature calculations, below.
Agreement between the calculated Hugoniot and the data indicates that shock temperatures of molten quartz below 55 GPa are in the stability region of stishovite, data between 55 and 65 GPa appear to correspond to superheated stishovite, and above 75 GPa lie along hugionite melting segment. It now appears that in the case of crystalline quartz, the Hugoniot between 70 and 115 GPa is in the CaCl2 structure and not in the superheated stishovite phase as suggested by Lyzenga et al. It has been proposed that the molten quartz and porous cristobalite data are sampling a P-V-T region in the melt where some of the silicon is still in 4-fold coordination, unlike the H.P.P. quartz.
Pressure density data (Figure 3.3) suggest that melting of superheated stishovite occurs at ∼ 85 GPa and above ∼ 90 GPa, the Hugoniot is assumed to be at full H.P.P. Other experimental and calculated constraints on the slope of the phase boundary are shown in Figure 3.6b. Given the ∼2% uncertainty in Hugoniot density measurements inferred from shock travel times through samples during shock temperature experiments [ Lyzenga et al ., 1983 ], the 2.7% density decrease inferred across the melting boundary from Lyzenga et al.
At higher pressures the Hugoniots of crystal quartz and coesite are in the melting regime.
M91 Melt
P.P. melt
Calculated Hugoniots in melting regime: dotted curves are for CV = constant and solid curves for CV(T) from equation 3.1. QTZ, This studyFQ, This study.
The lower density of the sample used in shot 315 (Table 4.2) was due to air bubbles in the bulk sample. It appears that the ~10 ns rise time of the shock experiments is sufficient time for the majority of the glass to transform to the perovskite structure.in the 50 to 100 GPa peak shock pressure. These findings are consistent with the interpretation of the pressure density Hugoniot data presented in Figure 4.1.
The breakdown of the perovskite structure to its component oxides was observed by Meade et al. His conclusion that the majorite crystallized from the melt was based on small elemental differences in the composition of the majorite and unshocked orthopyroxene of the Coorara meteorite. The SiO2 sound velocity data indicate the mass sound velocity of the melt at high pressure and temperature is similar to that of the solid at high pressure and room temperature (Figure 3.2).
Based on these observations, the present analysis shows that the MgSiO3 melt is 2-3% denser than the solid at pressures and temperatures corresponding to the lowermost 1000 km of the mantle. A further implication of the melt being denser than the solid is a negative slope for the melting boundary based on the Clapeyron equation. A negative Clapeyron slope is inconsistent with a smooth extrapolation of the Sweeney and Heinz [1998] melting curve to pressures greater than 100 GPa.
Using the aforementioned bulk sound speed constraint and the transition of the melting curve of Sweeney and Heinz (1998) to a negative slope, an H.P.P. With this slope, the calculated density of the solid (perovskite) along the melt boundary at 170 GPa is 5.5. 1983] along the melt boundary with positive slope of 11 K/GPa in the high-pressure regime of the SiO2 system.
Such behavior may be due to the solid being in a superheated metastable state. It is unlikely that the temperature in the melt would be higher than the temperature of the solid, so the calculation of the crystal tip to perovskite temperature at 170 GPa constrains the melting temperature to be ~5500 K. This is in contrast to the observed increase in melting density at 170 GPa along the Hugoniot of the crystal.
This is attributed to impurities in the pyroxenite samples and weathering of the Bamble bronzite.
298K DAC 2900K LHDAC
The data in the range 70 – 105 GPa are less dense than predicted for the perovskite structure. Inverted Hugoniot calculated for a MgO/fused silica mixture shocked onto a perovskite due to high thermal pressure. Calculated Hugoniot temperature of ultraporous MgO-SiO2 mixture shocked to perovskite and melting shown.
Hugoniots of enstatite indicate that the perovskite structure is achieved in the range of 110 - 170 GPa, followed by melting. The presence of peak shock density for initial enstatite (001) on the perovskite and melt segments indicates that the increase in density at ~170 GPa is not due to a denser Hugoniot achieved due to orientation, as observed with synthetic Mg2SiO4. 3 Enstatite to Periclase + SiO2 HPP melt 5 Enstatite to H.P.P. Figure 4.5 a) Temperature versus pressure shows a single measurement on MgSiO3 synthetic glass at 107 ± 4 GPa and K. Calculations of glass and enstatite shocked to the final shock states of perovskite and H.P.P. melt1 are shown.
Accounting for overheating, the melting transition of the crystal is fixed at ∼170 GPa, as observed in the P-ρ data, and a melting Hugoniot temperature consistent with a -10 K/GPa deviation, at 90 GPa, from the extrapolation of Sweeney and Heinz [1998].
CMBG
Simon (SH98) corresponds to the extrapolation of the melting curve of Sweeney and Heinz [1998] to 170 GPa; G-C corresponds to the low-temperature, negatively sloped melting curve of Figure 4.5a; and G'-C' corresponds to the high temperature negative Clapeyron curve with greater slope of Figure 4.5b. The bifurcation in the (Mg,Fe)SiO3 data at 60 GPa shows the Akimotoite and Perovskite phases reached at similar pressure ranges as interpreted for the MgSiO3 data in Figure 4.2 with a leg of higher density assumed to be due to compressibility more shorter. The low compressibility is most likely due to partial or complete melting occurring at lower pressures than expected due to impurities (pyroxenites) or alteration products (Bamle bronze), not a phase transition to a phase of denser solid.
Mg,Fe)SiO
P.P. melt Calculated
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Ahrens, Pyroxenes and olivines: Structural implications of shock wave data for high-pressure phases, in High-Pressure Research: Applications to Geophysics, edited by M.H. Wentzcovitch, High-pressure elastic properties of bulk materials in the Earth's mantle from first principles, Rev. Lyzenga, G.A., Shock temperatures of materials: Experiments and application to the high-pressure equation of state, Ph.D.
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