Seismic twinning in monazite: Microstructural records of deep crustal earthquakes

Earthquake records preserved in rocks provide key insights into the processes that govern crustal deformation and seismic energy dissipation. This manuscript presents new approaches for identifying mineralogical signatures of paleoearthquakes using advanced microstructural analyses, including electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). These techniques enable observations from the millimetre to nanometre scale of features associated with plastic deformation, including crystal reorientation and deformation twinning. Here, we investigate deformation microstructures in monazite, a key geochronometer, with the aim of assessing the impact of deformation on geochronological interpretations, as deformation-induced crystallographic defects can act as high-diffusivity pathways leading to Pb loss. Understanding the deformational behaviour of monazite is therefore critical for interpreting geochronological data. We examine monazite from an eclogite facies mylonite in the Musgrave Province (central Australia) to elucidate mechanisms of seismic deformation under dry (<0.002 wt% H₂O), lower-crustal conditions. The studied monazite grain is directly cross-cut by a pseudotachylite vein, indicating that the observed microstructures formed during the associated seismic event. EBSD and ECCI analyses reveal crystal-plastic deformation in the form of twinning with three distinct orientations: 180° <100>, 180° <001>, and 95° <201>. The latter is associated with dynamic recrystallization via subgrain boundary rotation. ECCI further reveals nanometre-scale (<15 nm) porosity within both parent grains and twins. These microstructures are consistent with those reported in monazite deformed during impact events. Recent studies of shocked monazite have shown that deformation by twinning can liberate Pb during rupture of rare-earth-element–oxygen (REE–O) bonds, enabling rapid diffusion along crystallographic defects and complete expulsion from the crystal, effectively resetting the geochronometer. The new insight provided by these microstructural focussed observations likely accounts for the disparity of electron probe microanalysis (EPMA)-based geochronology on the same monazite grain, which yielded ages of 1309 to 691 Ma. Seismicity in the Musgrave Province is primarily associated with the Petermann Orogeny (~550 Ma), suggesting that the younger EPMA ages were partially reset as a result of the twinning. Our results demonstrate the potential for monazite to record and date seismicity, opening new avenues for reconstructing paleoearthquake histories from deep crustal rocks.