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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 00 discontinuity. However, recent studies on binary and ternary systems showed that reasonable contents of Fe 2þ 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 00 discontinuity (≤70 km). These results challenge the assignment of the D 00 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 00 discontinuity may be related to the Pv → pPv transition in the differentiated oceanic lithosphere materials transported to the lowermost mantle by subducting slabs.T he lower mantle is a multicomponent (MgO, SiO 2 , FeO, Al 2 O 3 , CaO, Na 2 O, etc.), multiphase (magnesium silicate perovskite (Pv), ferropericlase (Fp), calcium silicate perovskite (Ca-Pv), silica, calcium-ferrite-type aluminous (CF) phase, etc.) system and understanding the effects of chemistry and mineralogy on the depths and thicknesses of mantle phase boundaries is of fundamental interest. Although the discovery of the Pv → pPv transition at lowermost mantle P − T conditions (1-3) has been invoked as an explanation for the D 00 discontinuity, the combined effect of mineralogy and chemistry remain unresolved. Some recent studies (4-14) demonstrated large effects of chemical composition on the depth and thickness of the Pv → pPv boundary. The depth range over which the Pv → pPv transition occurs in mantle-related systems is much wider (4, 5) than the seismically constrained thickness (15,16). It also occurs close to the coremantle boundary (CMB) (135 GPa) even at 2,500 K with a positive Clapeyron slope, making it possible that the transition may not occur at all in the hotter lowermost mantle. Although the effect of Fp on the Pv → pPv transition has been modeled (4), other mineralogical effects are not fully understood due in part to our poor understanding of Al partitioning in these systems. Direct measurements on different mantle-related compositio...

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 (Mg(0.91)Fe(2+)(0.09))SiO(3) and (Mg(0.9)Fe(3+)(0.1))(Al(0.1)Si(0.9))O(3) 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 Fe(2+) 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.