[S-IT16] Deep Earth Sciences

[SIT16-06] Transformational faulting in metastable Mn₂GeO₄ olivine: implications for physical mechanisms of deep-focus earthquakes

Feng Shi^2,1, *Yanbin Wang¹, Timothy Officer¹, Tony Yu¹, Lupei Zhu³ (1.Center for Advanced Radiation Sources, The University of Chicago, 2.State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan, Hubei, China, 3.Department of Earth and Atmospheric Sciences, Saint Louis University)

Keywords:intraslab seismicity, deep-focus earthquakes, laboratory earthquake simulation , Acoustic emission, synchrotron , high pressure

The transformational faulting hypothesis postulates that metastable olivine in cold subducting slabs transforms to wadsleyite (a spinelloid) and/or ringwoodite (a spinel) below ~300 km depth, forming localized shear zones leading to deep-focus earthquakes [1]. Detailed physical mechanisms of this hypothesis are still under debate [2, 3]. Several experimental studies have been conducted on an analog olivine Mg₂GeO₄, which transforms directly to the spinel structure, like ringwoodite. In this study, we examine another analog olivine, Mn₂GeO₄, which transforms to beta-Mn₂GeO₄ (a spinelloid isostructural to wadsleyite) at high pressure [4]. Controlled deformation experiments (strain rates from 3x10^-5 to 8x10^-5 s^-1) were conducted on metastable Mn₂GeO₄ olivine within the beta-Mn₂GeO₄ stability field (~4-5 GPa and 673 – 1323 K) with acoustic emission (AE) monitoring. Based on the experimental data, the mechanical behavior of metastable Mn₂GeO₄ olivine is divided into three regimes, depending primarily on temperature. Below ~750 K, olivine is strong but ductile, with no AEs emitted up to ~30% strain. Above ~1150 K, Mn₂GeO₄ olivine is weak and ductile, with no AEs detected. Only between 750 and 1150 K does metastable Mn₂GeO₄ olivine behave in a brittle manner, emitting numerous AEs, associated with large stress drops. Scanning and transmission electron microscopy (SEM and TEM) on recovered samples show that all brittle samples contain macroscopic faults. SEM shows that beta-Mn₂GeO₄ forms primarily sub-parallel long and narrow bands (thickness on the order of 100 nm) cutting through individual olivine grains. Some of the extremely thin shear bands are indicated in Fig. (a) by white arrows. The appearance and topology of these bands are very similar to the nano-shear bands (NSBs) reported in faulted metastable Mg₂GeO₄ olivine after transformational faulting [2, 3]. Electron back-scattered diffraction (EBSD) reveals that these NSBs are primarily located within the boundaries of olivine kink bands [Fig. (b); same area as in Fig. (a)]. Kink bands have been peviously reporrted in olivines of various compositions (silicates, germanates, etc.) when deformed by low-temperature plasticity [5, 6]. Interestingly, in our Mn₂GeO₄ sample deformed at similar temperature condition but within the olivine stability field, virtually no kink bands are present. The observed kink bands thus appear to be related to the metastability of olivine and triggered by existing defects or nuclei of beta-Mn₂GeO₄ within olivine grains. The highly localized spatial distribution of KBBs explains the narrowness of the NSBs. Aided by local temperature increase due to the exothermic nature of the phase transformation, these NSBs propagate and self-organize with increasing deformation, leading to macroscopic failure. Such a micro-mechanism is likely to occur in metastable olivine in subducting slabs below ~300 km depth.
References cited
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