A Micro-Mechanism for the Nucleation of High-Pressure Phases During Transformational Faulting in Olivine
Description:
In the transformational faulting hypothesis for deep-focus earthquakes, metastable olivine in the subducted lithosphere transforms to wadsleyite and/or ringwoodite below ~300 km depth, triggering mechanical instability. Detailed physical mechanisms of this hypothesis are still under debate. We conducted controlled deformation experiments with acoustic emission (AE) monitoring to investigate this process. We examined three olivines: Mg2GeO4, Mn2GeO4, and (Mg1-xFex)2SiO4 (with x=0.25 and 0.5). Under our experimental conditions, Mg2GeO4 olivine transforms directly to spinel (ringwoodite) structure, Mn2GeO4 to wadsleyite, whereas (Mg1-xFex)2SiO4 into a mixture of olivine and ringwoodite.
Mechanical behavior of all three metastable olivines can be divided into three regimes, depending primarily on temperature. At low temperatures, metastable olivines are strong but ductile, with no AEs produced up to ~30% strain. At high temperatures, they are weak and ductile, with no AEs observed. Only within a narrow range of intermediate temperatures do these olivines behave in a brittle manner, emitting numerous AEs. Recovered brittle samples contain macroscopic faults. High-pressure products are primarily in the form of sub-parallel, long and narrow bands (widths on the order of 100 nm) cutting through individual olivine grains, sometimes shearing the olivine grains into several parts. These nano-shear bands (NSBs) are oriented about 30 – 60 degrees from the maximum compressive principal stress. Detailed microstructure analyses show that at intermediate temperature, olivines deformed by kink band formation and NSBs are primarily located within kink bands boundaries (KBBs). Within KBBs, olivine crystal lattice is severely distorted due to high density of dislocations, thereby promoting nucleation of high-pressure phases. The self-organization of NSB network produces macroscopic faulting, as evidenced by subsets of NSBs, which are either in the fault zone or very close to the faults.
Session: Structure and Properties of Subducting Slabs and Deep Earthquakes [Poster]
Type: Poster
Date: 4/19/2023
Presentation Time: 08:00 AM (local time)
Presenting Author: Yanbin Wang
Student Presenter: No
Invited Presentation: No
Authors
Yanbin Wang Presenting Author Corresponding Author wang@cars.uchicago.edu University of Chicago |
Feng Shi shifeng@cug.edu.cn China University of Geosciences |
Timothy Officer officer@cars.uchicago.edu University of Chicago |
Tony Yu tyu@cars.uchicago.edu University of Chicago |
Man Xu mxu@cars.uchicago.edu University of Chicago |
Lupei Zhu lupei.zhu@slu.edu St. Louis University |
Zhigang Peng zpeng@gatech.edu Georgia Institute of Technology |
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A Micro-Mechanism for the Nucleation of High-Pressure Phases During Transformational Faulting in Olivine
Category
Structure and Properties of Subducting Slabs and Deep Earthquakes