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From the Lab to the Slab: Transformational Faulting at High Pressure and Temperature in Fe-Rich Olivine and Implications for Deep-Focus Earthquakes

Description: 

A major unresolved question in earth science is how earthquakes can occur under the high-pressure (P), high-temperature (T) conditions found hundreds of km below the surface. One theory is that deep-focus earthquakes (350-700 km depth) occur in association with metastable olivine transforming to its high-pressure polymorphs wadsleyite and/or ringwoodite. While this mechanism has been shown to operate in the olivine/ringwoodite analogue Mg₂GeO₄ at <4 GPa, there is a scarcity of experimental evidence confirming it conclusively in silicate olivine at higher pressures. To test this hypothesis, deformation experiments were performed on Fe-rich olivine, (Mg_0.25,Fe_0.75)₂SiO₄, under various P,T conditions in both the olivine and ringwoodite stability fields. Acoustic activity was monitored in situ, as well as stress and strain, which were measured via synchrotron X-ray diffraction and radiography respectively. Experiments run under conditions that promoted transformation to ringwoodite (P = 6-9 GPa; T = 600-900 ^oC) generated bursts of acoustic activity which, in contrast to ordinary brittle failure, was enhanced with increasing pressure. Focal mechanisms of acoustic events indicate exclusively shear failure with no detectible volumetric change or tensile crack components. Tomographic imaging of the samples recovered from >7 GPa reveals the existence of macroscopic faults. In every seismogenic experiment, the presence of ringwoodite was confirmed through SEM imaging. The ringwoodite phase develops in the form of long thin bands of nano-grained material which, due to the reduced grain size, cannot support shear stress leading to strain localization at seismogenic strain rates. In contrast, experiments run under P,T conditions that prohibited transformation were acoustically quiet and the samples remained intact. The results of this study demonstrate that seismogenic fracture is associated with the olivine to ringwoodite transformation in Fe-rich silicate olivine lending experimental support to the hypothesis that transformational faulting is responsible for deep-focus earthquakes in the mantle transition zone.

Session: Structure and Properties of Subducting Slabs and Deep Earthquakes

Type: Oral

Date: 4/19/2023

Presentation Time: 02:00 PM (local time)

Presenting Author: Timothy Officer

Student Presenter: No

Invited Presentation: Yes

Authors