Rheological weakening due to phase mixing

2019 Fiscal Year Final Research Report

• Project/Area Number: 18K13634
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 17040:Solid earth sciences-related
• Research Institution: Shizuoka University (2019); Shimane University (2018)
• Principal Investigator: Tasaka Miki 静岡大学, 理学部, 准教授 (80772243)
• Project Period (FY): 2018-04-01 – 2020-03-31
• Keywords: かんらん岩 / かんらん石 / 輝石 / ２相系 / 鉱物混合層 / 歪み弱化 / 剪断帯

Outline of Final Research Achievements

To understand the effects of secondary minerals on changes in the mechanical properties of upper mantle rocks due to phase mixing, we conducted high-strain torsion experiments on aggregates of olivine + orthopyroxene (opx) with orthopyroxene volume fractions of fopx = 0.15, 0.26, and 0.35. For samples with larger amounts of orthopyroxene, fopx = 0.26 and 0.35, the deformation mechanism changes as strain increases. In contrast, for samples with fopx = 0.15, no change in deformation mechanism occurs.
The microstructures of samples with larger amounts of orthopyroxene provide insight into the change in deformation mechanism derived from the mechanical data. These mechanical and associated microstructural properties imply that rheological weakening due to phase mixing only occurs in the samples with larger fopx, which is an important constraint for understanding strain localization in the upper mantle of Earth.

Free Research Field

岩石鉱物学

Academic Significance and Societal Importance of the Research Achievements

本研究はかんらん岩ねじり実験を行い鉱物混合層形成と歪み弱化における化学効果を明らかにした。マントルの代表的な鉱物であるかんらん石＋輝石系で鉱物量比の異なる多結晶体を用いて歪み速度、歪み量の違う実験を行った。輝石の多い試料(fpx = 0.26)では歪み増加に伴い粒径依存型クリープで変形、歪み弱化が起きることが分かった。一方輝石の少ない試料(fpx = 0.15,0)では歪み弱化は確認できなかった。実験により示唆された「変形に起因する物質拡散により鉱物混合層が形成される」ならば、細粒化と歪み弱化の程度は応力と温度、岩石の化学組成に依存する。この結果は地球のプレート強度を考える上で重要である。