Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p32a..02g&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P32A-02
Physics
[3924] Mineral Physics / High-Pressure Behavior, [6296] Planetary Sciences: Solar System Objects / Extra-Solar Planets, [8147] Tectonophysics / Planetary Interiors
Scientific paper
Newly discovered super-Earth type extrasolar planets have brought up questions about their mantle structure and the stability of magnesium silicates at the extreme high pressures in these objects. Computer simulations by Umemoto et al. (2006) indicate a breakdown of MgSiO3 into MgO + SiO2 with a large negative Clayepron slope at a pressure achieved in the interiors of 10 Earth-mass super-Earths (~1000 GPa). The large and negative Clapeyron slope of the decomposition in super-Earths would result in a basal oxide layer with quasi-periodic upwellings or even an impermeable boundary (van der Berg et al., 2010). Our current static compression techniques are not capable of reaching the predicted breakdown pressure. However, we can investigate analog materials which undergo the same sequence of phase transitions but at much lower pressures. Umemoto et al. (2006) predicted that silicate perovskite analog mineral neighborite NaMgF3 (MgSiO3 analog) dissociates into NaF (MgO analog) and MgF2 (SiO2 analog) at 40 GPa. We have conducted in situ X-ray diffraction measurements on neighborite and MgF2 in the laser-heated diamond-anvil cell at the GSECARS sector of Advanced Photon Source. We do not observe the breakdown of NaMgF3 up to 70 GPa and 2500 K, much higher than the predicted breakdown pressure. This is in part due to the assumption that SiO2 and MgF2 transitions from the pyrite-type to the cotunnite-type phase, with an increase in the Si--O coordination number from six to nine. We find a new phase of MgF2 whose stability field exists between these two structures. This new phase appears to be one (or more) of the several possible distorted pyrite types with seven-fold coordination. SiO2 likely has a similar high pressure phase, expanding the stability field of MgSiO3 post-perovskite to pressures in excess of those found in the rocky mantle fraction of super-Earths. Our experimental results suggest that mantle silicates are stable to the highest pressures found in super-Earths and super-Earth mantles are unlikely to have a deep oxide layer. Therefore, major impenetratable phase boundaries for mantle flow may not exist in super-Earths.
Grocholski B.
Prakapenka Vitali
Shim Sangwoo
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