TY - JOUR
T1 - Mineralogical effects on the detectability of the postperovskite boundary
AU - Grocholski, Brent
AU - Catalli, Krystle
AU - Shim, Sang Heon
AU - Prakapenka, Vitali
PY - 2012/2/14
Y1 - 2012/2/14
N2 - 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.
AB - 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.
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U2 - 10.1073/pnas.1109204109
DO - 10.1073/pnas.1109204109
M3 - Article
C2 - 22308329
AN - SCOPUS:84857127147
SN - 0027-8424
VL - 109
SP - 2275
EP - 2279
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 7
ER -