Determination of the three phase region of the post-spinel transition in (Mg,Fe)2SiO4: explanation of the extreme sharpness of the 660-km discontinuity and implication for chemical structure and dynamics of the deep mantle

• Applicant: Professor Dr. Tomoo Katsura
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2014 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 257122795

Seismic observations indicate that the 660-km discontinuity is less than 2 km thick, corresponding to <0.1 GPa in pressure. Such sharpness is in contrast with that of the 410-km discontinuity, whose thickness is estimated to be 7 km. The 660-km discontinuity is usually explained by the postspinel transition in (Mg,Fe)2SiO4, in which (Mg,Fe)2SiO4 ringwoodite dissociates into (Mg,Fe)SiO3 perovskite plus periclase (Mg,Fe)O. Because these three minerals have Mg-Fe solid solutions, the postspinel transition should have a finite interval, and therefore we need a special explanation to account for the extreme sharpness of the 660-km discontinuity. For this reason, the transition interval of the postspinel transition must be determined. If the pressure interval of the transition is >0.1 GPa, we will have to reconsider the structure and dynamics of the deep mantle. Firstly, it is possible that the chemical compositions are different between the upper and lower mantles, and that mantle convection is at least partially layered. An alternative explanation is that the sluggish kinetics of nucleation prevents initiation of the postspinel transition, and once nucleation occurs, the transition proceeds very rapidly. If it is proved that the interval of the postspinel transition is extremely thin, the presence of vertical mantle flow could be assessed by global variation of the thickness of the 660-km discontinuity. Previous experimental studies have not successfully determined the pressure interval of the postspinel transition because they have lacked sufficient precision in pressure determination and suffered from the effects of sluggish kinetics. The precision of pressure determination in previous studies was no better than 0.3 GPa, which was too large to determine the transition interval, which could be less than 0.1 GPa. In contrast, the applicant has already established experimental techniques to determine sample pressure with a precision of 0.04 GPa, which should be sufficient for the present project. He has also established an experimental technique to obtain the equilibrium compositions of minerals by using a flux. Combining these two techniques, the whole three phase region of ringwoodite+perovskite+periclase will be determined in pressure-composition space at a constant temperature of 2000 K. The pressure interval of the postspinel transition in the real mantle will be estimated based on the geometry of the three phase region by considering the expected Mg-Fe exchange with majorite.

• DFG Programme: Research Grants
• International Connection: Japan
• Participating Persons: Yuji Higo, Ph.D.; Dr. Norimasa Nishiyama