K. C. Catalli

Thickness and Clapeyron slope of the post-perovskite boundary

The thicknesses and Clapeyron slopes of mantle phase boundaries strongly influence the seismic detectability of the boundaries and convection in the mantle. The unusually large positive Clapeyron slope found for the boundary between perovskite (Pv) and post-perovskite (pPv) (the ‘pPv boundary’) would destabilize high-temperature anomalies in the lowermost mantle, in disagreement with the seismic observations. Here we report the thickness of the pPv boundary in (Mg0.91Fe2+0.09)SiO3 and (Mg0.9Fe3+0.1)(Al0.1Si0.9)O3 as determined in a laser-heated diamond-anvil cell under in situ high-pressure (up to 145 GPa), high-temperature (up to 3,000 K) conditions. The measured Clapeyron slope is consistent with the D′′ discontinuity. In both systems, however, the pPv boundary thickness increases to 400–600 ± 100 km, which is substantially greater than the thickness of the D′′ discontinuity (<30 km). Although the Fe2+ buffering effect of ferropericlase could decrease the pPv boundary thickness, the boundary may remain thick in a pyrolitic composition because of the effects of Al and the rapid temperature increase in the D′′ layer. The pPv boundary would be particularly thick in regions with an elevated Al content and/or a low Mg/Si ratio, reducing the effects of the large positive Clapeyron slope on the buoyancy of thermal anomalies and stabilizing compositional heterogeneities in the lowermost mantle. If the pPv transition is the source of the D′′ discontinuity, regions with sharp discontinuities may require distinct compositions, such as a higher Mg/Si ratio or a lower Al content.

Mineralogical effects on the detectability of the postperovskite boundary

The discovery of a phase transition in Mg-silicate perovskite (Pv) to postperovskite (pPv) at lowermost mantle pressure-temperature (P - T) conditions may provide an explanation for the discontinuous increase in shear wave velocity found in some regions at a depth range of 200 to 400 km above the core-mantle boundary, hereafter the D′′ discontinuity. However, recent studies on binary and ternary systems showed that reasonable contents of Fe2+ and Al for pyrolite increase the thickness (width of the mixed phase region) of the Pv - pPv boundary (400–600 km) to much larger than the D′′ discontinuity (≤ 70 km). These results challenge the assignment of the D′′ discontinuity to the Pv - pPv boundary in pyrolite (homogenized mantle composition). Furthermore, the mineralogy and composition of rocks that can host a detectable Pv → pPv boundary are still unknown. Here we report in situ measurements of the depths and thicknesses of the Pv → pPv transition in multiphase systems (San Carlos olivine, pyrolitic, and midocean ridge basaltic compositions) at the P - T conditions of the lowermost mantle, searching for candidate rocks with a sharp Pv - pPv discontinuity. Whereas the pyrolitic mantle may not have a seismologically detectable Pv → pPv transition due to the effect of Al, harzburgitic compositions have detectable transitions due to low Al content. In contrast, Al-rich basaltic compositions may have a detectable Pv - pPv boundary due to their distinct mineralogy. Therefore, the observation of the D′′ discontinuity may be related to the Pv → pPv transition in the differentiated oceanic lithosphere materials transported to the lowermost mantle by subducting slabs.

Crystal structure and thermoelastic properties of (Mg0.91Fe0.09)SiO3 postperovskite up to 135 GPa and 2,700 K

Intriguing seismic observations have been made for the bottom 400 km of Earths mantle (the D″ region) over the past few decades, yet the origin of these seismic structures has not been well understood. Recent theoretical calculations have predicted many unusual changes in physical properties across the postperovskite transition, perovskite (Pv) → postperovskite (PPv), that may provide explanations for the seismic observations. Here, we report measurements of the crystal structure of (Mg0.91Fe0.09)SiO3-PPv under quasi-hydrostatic conditions up to the pressure (P)–temperature (T) conditions expected for the core-mantle boundary (CMB). The measured crystal structure is in excellent agreement with the first-principles calculations. We found that bulk sound speed (VΦ) decreases by 2.4 ± 1.4% across the PPv transition. Combined with the predicted shear-wave velocity (VS) increase, our measurements indicate that lateral variations in mineralogy between Pv and PPv may result in the anticorrelation between the VΦ and VS anomalies at the D″ region. Also, density increases by 1.6 ± 0.4% and Grüneisen parameter decreases by 21 ± 15% across the PPv transition, which will dynamically stabilize the PPv lenses observed in recent seismic studies.

X-ray diffraction and Mössbauer spectroscopy of Fe3+-bearing Mg-silicate post-perovskite at 128-138 GPa

Abstract The effect of ferric iron on the properties of Mg-silicate post-perovskite (PPv) were studied up to 138 GPa using synchrotron X-ray diffraction and Mössbauer spectroscopy. Our diffraction measurements revealed that the incorporation of Fe3+ has virtually no effect on the volume of PPv, in contrast to Fe2+, which increases the volume. Therefore, incorporation of Fe3+ increases the density of PPv much more effectively than Fe2+. Mössbauer spectroscopy suggests that Fe3+ enters PPv through chargecoupled substitution and is high spin in the bipolar prismatic site and low spin in the octahedral site (i.e., mixed spin state). Our results may have important implications for the gravitational stability of lower-mantle heterogeneities.