Shock-induced temperatures of CaMgSi_2O_6

Abstract

Optical radiation from CaMgSi_2O_6 crystal (diopside) shock-compressed to 145–170 GPa yields shock-induced temperatures of 3500–4800 K, while that from CaMgSi_2O_6 glass, with a density 86% that of CaMgSi_2O_6 crystal, shock-compressed to 96–98 GPa, yields shock-induced temperatures of 3700–3900 K. The observed radiation histories of of the targets containing CaMgSi_2O_6 crystal and glass imply that the shock-compressed states of both are highly absorptive, with effective absorption coefficients of ≥ 500–1000 m^(−1). Calculated shock-compressed states for both CaMgSi_2O_6 crystal and glass, when compared to experimental results, imply the presence of a high-pressure phase (HPP) along both Hugoniots over the respective pressure ranges. The CaMgSi_2O_6 crystal experimental results are consistent with a standard temperature and pressure (STP) HPP mass density of 4100±100 kg/m^3, a STP HPP bulk modulus of 250±50 GPa, and a difference in specific internal energy (SIE) between (metastable) HPP and the CaMgSi_2O_6 crystal states at STP ("energy of transition") of 2.2±0.5 MJ/kg. The CaMgSi_2O_6 glass results are "best-fit" by the same (median) values of all three parameters; except for the STP SIE difference between the CaMgSi_2O_6 glass and HPP states, however, they are less sensitive to parameter variations than the crystal results because they are at lower pressure. All these model constraints are insensitive to the range of values (1–2) assumed for the STP HPP Gruneisen's parameter. The relatively high value of the STP SIE difference between HPP and CaMgSi_2O_6 crystal or glass most likely implies that CaMgSi_2O_6 glass and crystal experience both solid-solid and solid-liquid phase transformations along their respective Hugoniots below 96 and 144 GPa, respectively. The HPP CaMgSi_2O_6 Hugoniot constrained by the crystal experimental results lies between 2500–3000 K in the pressure range (110–135 GPa) of the lowermost mantle (D′′)] our results imply that CaMgSi_2O_6 is at least partly molten at these pressures and temperatures. Seismically constrained compositional models for this region of Earth's lower mantle suggest that it could contain a significant amount of Ca (25–30 wt % CaO). If so, our results imply that the temperature of the D′′ region must be below ≈ 3000 K, since the finite S-wave velocity of the D′′ region implies that it must be (at least at seismic frequencies) predominantly solid.

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