Progress Summary The goal of this research project is to measure the phase in the compositions realistic for the mantle at the pressure-temperature conditions of the core-mantle boundary. The target compositional systems include: pyrolitic (average mantle composition), hartzburgitic (depleted component of subducted oceanic lithosphere), basaltic (enriched component of subducted oceanic crust), and chondritic (primitive mantle) compositions. We have conducted synchrotron X-ray diffraction on the phase transitions in pyrolitic, harzburgitic, and basaltic compositions at the Advanced Photon Source, focusing on the depth and thickness of the post-perovskite transition. We found that the post-perovskite transition may exist in the lower mantle with a thickness sufficiently small for seismic detection in harzburgitic and basaltic compositions. However, we found that the post-perovskite transition may not exist in the lower mantle due to higher transition pressure in a pyrolitic composition. Furthermore, we found that the thickness of the post-perovskite transition is very large (400 km) in a pyrolitic composition. This result was published in Proceedings of the National Academy of Sciences (PNAS) in 2012. In a basaltic composition, we found that silica represents a significant fraction of the mineralogy (30%) and undergoes a phase transition to the alpha-PbO2 type structure (seifertite) at a greater depth than the post-perovskite but still within the mantle. Therefore, the existence of basaltic material in the lowermost mantle may result in the observation of double discontinuity. We submit the paper to Journal of Geophysical Research and it is under review.
|Effective start/end date||9/1/12 → 12/31/15|
- National Science Foundation (NSF): $229,066.00