Superplasticity of minerals 3: Grain growth effect

2022 Fiscal Year Final Research Report

• Project/Area Number: 18H03734
• Research Category: Grant-in-Aid for Scientific Research (A)
• Allocation Type: Single-year Grants
• Section: 一般
• Review Section: Medium-sized Section 17:Earth and planetary science and related fields
• Research Institution: The University of Tokyo
• Principal Investigator: Hiraga Takehiko 東京大学, 地震研究所, 教授 (10444077)
• Project Period (FY): 2018-04-01 – 2023-03-31
• Keywords: 超塑性 / 粒成長 / 粘性率 / 下部マントル / 拡散クリープ

Outline of Final Research Achievements

In forsterite (Mg2SiO4) + 10 vol% periclase (MgO) polycrystal, the slowest diffusing species, Si, is only present in the primary phase, forsterite. Uniaxial compression creep and grain growth experiments were carried out on this polycrystalline material. The results show that the diffusivities estimated from the creep and grain growth rates are the same, i.e. creep (superplastic deformation) and grain growth are governed by a common diffusion mechanism. In many rocks, the slowest diffusing species are in the primary phase. The viscosity of polycrystals during diffusion creep is determined by grain size and the common diffusivity. We finally proposed a grain size-time viscometer, which gives the viscosity from the time taken to reach that grain size.

Free Research Field

地球内部ダイナミックス

Academic Significance and Societal Importance of the Research Achievements

地球内部のマントル対流を司るマントル粘性率に対して、岩石を構成する鉱物の粒径およびそれに至るまでの時間から粘性率を求める手法を確立した。実際に観察される粒径と推定された地質時間を用いて、従来困難であった下部マントル、特に核ーマントル境界という下部マントル最深部の粘性率推定を行った。最深部においては、大きな粒径変化（粒成長）が予想され、最大粘性率の形で提案された。