Direct shock compression experiments on premolten forsterite and progress toward a consistent high-pressure equation of state for CaO-MgO-Al_2O_3-SiO_2-FeO liquids

We performed shock compression experiments on preheated forsterite liquid (Mg_2SiO_4) at an initial temperature of 2273 K and have revised the equation of state (EOS) that was previously determined by shock melting of initially solid Mg_2SiO_4 (300 K). The linear Hugoniot, U_S = 2.674 ± 0.188 + 1.64 ± 0.06 u_p km/s, constrains the bulk sound speed within a temperature and composition space as yet unexplored by 1 bar ultrasonic experiments. We have also revised the EOS for enstatite liquid (MgSiO_3) to exclude experiments that may have been only partially melted upon shock compression and also the EOS for anorthite (CaAl_2SiO_6) liquid, which now excludes potentially unrelaxed experiments at low pressure. The revised fits and the previously determined EOS of fayalite and diopside (CaMg_2SiO_6) were used to produce isentropes in the multicomponent CaO-MgO-Al_2O_3-SiO_2-FeO system at elevated temperatures and pressures. Our results are similar to those previously presented for peridotite and simplified “chondrite” liquids such that regardless of where crystallization first occurs, the liquidus solid sinks upon formation. This process is not conducive to the formation of a basal magma ocean. We also examined the chemical and physical plausibility of the partial melt hypothesis to explain the occurrence and characteristics of ultra-low velocity zones (ULVZ). We determined that the ambient mantle cannot produce an equilibrium partial melt and residue that is sufficiently dense to be an ultra-low velocity zone mush. The partial melt would need to be segregated from its equilibrium residue and combined with a denser solid component to achieve a sufficiently large aggregate density

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

Shock temperatures of preheated MgO

Shock temperature measurements via optical pyrometry are being conducted on single-crystal MgO preheated before compression to 1905–1924 K. Planar shocks were generated by impacting hot Mo(driver plate)-MgO targets with Mo or Ta flyers launched by the Caltech two-stage light-gas gun up to 6.6 km/s. Quasi-brightness temperature was measured with 2–3% uncertainty by a 6-channel optical pyrometer with 3 ns time resolution, over 500–900 nm spectral range. A high-power, coiled irradiance standard lamp was adopted for spectral radiance calibration accurate to 5%. In our experiments, shock pressure in MgO ranged from 102 to 203 GPa and the corresponding temperature varied from 3.78 to 6.53 kK. For the same particle velocity, preheated MgO Hugoniot has about 3% lower shock velocity than the room temperature Hugoniot. Although model shock temperatures calculated for the solid phase exceeded our measurements by ~5 times the uncertainty, there was no clear evidence of MgO melting, up to the highest compression achieved

Preheated shock experiments in the molten CaAl_2Si_2O_8-CaFeSi_2O_6-CaMgSi_2O_6 ternary: A test for linear mixing of liquid volumes at high pressure and temperature

We performed 17 new shock wave experiments on preheated (1673 K) hedenbergite liquid (CaFeSi_2O_6) and two model basalt liquids (an equimolar binary mix of CaAl_2Si_2O_8 + CaFeSi_2O_6 and an equimolar ternary mix of CaAl_2Si_2O_8 + CaFeSi_2O_6 +CaMgSi_2O_6) in order to determine their equations of state (EOS). Ambient pressure density measurements on these and other Fe-bearing silicate liquids indicate that FeO has a partial molar volume that is highly dependent on composition, which leads to large errors in estimates of the densities of Fe-bearing liquids at ambient pressure based on an ideal mixing of any fixed set of end-member liquids. We formulated a series of mixing tests using the EOS determined in this study to examine whether ideal mixing of volumes might nevertheless suffice to describe the ternary system CaAl_2Si_2O_8-CaFeSi_2O_6-CaMgSi_2O_6 at high temperature and pressure. The ideal mixing null hypothesis is rejected; compositional variations in partial molar volume of FeO appear to extend to high pressure. Only densities of Fe-bearing liquid mixtures with oxide mole fraction of FeO less than 0.06 can be adequately approximated using an ideal solution

Multi‐technique equation of state for Fe 2 SiO 4 melt and the density of Fe‐bearing silicate melts from 0 to 161 GPa

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95366/1/jgrb17308.pd

Multi-technique equation of state for Fe_(2)SiO_4 melt and the density of Fe-bearing silicate melts from 0 to 161 GPa

We have conducted new equation of state measurements on liquid Fe_(2)SiO_4 in a collaborative, multi-technique study. The liquid density (ρ), the bulk modulus (K), and its pressure derivative (K′) were measured from 1 atm to 161 GPa using 1-atm double-bob Archimedean, multi-anvil sink/float, and shock wave techniques. Shock compression results on initially molten Fe_(2)SiO_4 (1573 K) fitted with previous work and the ultrasonically measured bulk sound speed (C_o) in shock velocity (U_S)-particle velocity (u_p) space yields the Hugoniot: U_S = 1.58(0.03) u_p + 2.438(0.005) km/s. Sink/float results are in agreement with shock wave and ultrasonic data, consistent with an isothermal K_T = 19.4 GPa and K′ = 5.33 at 1500°C. Shock melting of initially solid Fe_(2)SiO_4 (300 K) confirms that the Grüneisen parameter (γ) of this liquid increases upon compression where γ = γ_o(ρ_(o)/ρ)^q yields a q value of –1.45. Constraints on the liquid fayalite EOS permit the calculation of isentropes for silicate liquids of general composition in the multicomponent system CaO-MgO-Al_(2)O_3-SiO_2-FeO at elevated temperatures and pressures. In our model a whole mantle magma ocean would first crystallize in the mid-lower mantle or at the base of the mantle were it composed of either peridotite or simplified “chondrite” liquid, respectively. In regards to the partial melt hypothesis to explain the occurrence and characteristics of ultra-low velocity zones, neither of these candidate liquids would be dense enough to remain at the core mantle boundary on geologic timescales, but our model defines a compositional range of liquids that would be gravitationally stable

High-pressure melt curve of shock-compressed tin measured using pyrometry and reflectance techniques

We have developed a new technique to measure the melt curve of a shocked metal sample and have used it to measure the high-pressure solid-liquid phase boundary of tin from 10 to 30 GPa and 1000 to 1800 K. Tin was shock compressed by plate impact using a single-stage powder gun, and we made accurate, time-resolved radiance, reflectance, and velocimetry measurements at the interface of the tin sample and a lithium fluoride window. From these measurements, we determined temperature and pressure at the interface vs time. We then converted these data to temperature vs pressure curves and plotted them on the tin phase diagram. The tin sample was initially shocked into the high-pressure solid γ phase, and a subsequent release wave originating from the back of the impactor lowered the pressure at the interface along a constant entropy path (release isentrope). When the release isentrope reaches the solid-liquid phase boundary, melt begins and the isentrope follows the phase boundary to low pressure. The onset of melt is identified by a significant change in the slope of the temperature-pressure release isentrope. Following the onset of melt, we obtain a continuous and highly accurate melt curve measurement. The technique allows a measurement along the melt curve with a single radiance and reflectance experiment. The measured temperature data are compared to the published equation of state calculations. Our data agree well with some but not all of the published melt curve calculations, demonstrating that this technique has sufficient accuracy to assess the validity of a given equation of state model

Molybdenum sound velocity and shear modulus softening under shock compression

We measured the longitudinal sound velocity in Mo shock compressed up to 4.4 Mbars on the Hugoniot. Its sound speed increases linearly with pressure up to 2.6 Mbars; the slope then decreases up to the melting pressure of ∼3.8 Mbars. This suggests a decrease of shear modulus before the melt. A linear extrapolation of our data to 1 bar agrees with the ambient sound speed. The results suggest that Mo remains in the bcc phase on the Hugoniot up to the melting pressure. There is no statistically significant evidence for a previously reported bcc→hcp phase transition on the Hugoniot

Tantalum sound velocity under shock compression

We used several variations of the shock compression method to measure the longitudinal sound velocity of shocked tantalum over the pressure range 37–363 GPa with a typical uncertainty of 1.0%%. These data are consistent with Ta remaining in the bcc phase along the principal Hugoniot from ambient pressure to ≈300 GPa, at which pressure melting occurs. These data also do not support the putative melting phenomena reported below 100 GPa in some static compression experiments

Tantalum sound velocity under shock compression

We used several variations of the shock compression method to measure the longitudinal sound velocity of shocked tantalum over the pressure range 37–363 GPa with a typical uncertainty of 1.0%%. These data are consistent with Ta remaining in the bcc phase along the principal Hugoniot from ambient pressure to ≈300 GPa, at which pressure melting occurs. These data also do not support the putative melting phenomena reported below 100 GPa in some static compression experiments