Mineralogical effects on the detectability of the postperovskite boundary

Abstract

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 Fe 2+ 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.