Mineralogical effects on the detectability of the postperovskite boundary

Journal Article · Tue May 02 04:00:00 EDT 2017 · Proc. Natl. Acad. Sci. USA

DOI:https://doi.org/10.1073/pnas.1109204109· OSTI ID:1036338

Grocholski, Brent; Catalli, Krystle; Shim, Sang-Heon; Prakapenka, Vitali ^[1]

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1. UC

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{double_prime} discontinuity. However, recent studies on binary and ternary systems showed that reasonable contents of Fe{sup 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{double_prime} discontinuity ({le} 70 km). These results challenge the assignment of the D{double_prime} discontinuity to the Pv - pPv boundary in pyrolite (homogenized mantle composition). Furthermore, the mineralogy and composition of rocks that can host a detectable Pv {yields} pPv boundary are still unknown. Here we report in situ measurements of the depths and thicknesses of the Pv {yields} 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 {yields} 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{prime} discontinuity may be related to the Pv {yields} pPv transition in the differentiated oceanic lithosphere materials transported to the lowermost mantle by subducting slabs.

• Research Organization: Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, IL (US)
• Sponsoring Organization: DOE - OTHERNSF
• OSTI ID: 1036338
• Journal Information: Proc. Natl. Acad. Sci. USA, Journal Name: Proc. Natl. Acad. Sci. USA Journal Issue: (7) ; 02, 2012 Vol. 109; ISSN PNASA6; ISSN 0027-8424
• Country of Publication: United States
• Language: ENGLISH