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Mantle earthquakes frozen in mylonitized ultramafic pseudotachylytes of spinel-lherzolite facies

¹ Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan
² Dipartimento di Geoscienze, Padova University, Via Giotto 1, 35317 Padova, Italy, and Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy
³ Department of Earth Sciences, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba City 263-8522, Japan
⁴ Department of Earth and Planetary Sciences, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Correspondence: *E-mail: t_ueda{at}kueps.kyoto-u.ac.jp

We report a new type of ultramafic pseudotachylyte that forms a fault- and injection-vein network hosted in the mantle-derived Balmuccia peridotite (Italy). In the fault vein the pseudotachylyte is now deformed and recrystallized into a spinel-lherzolite facies ultramylonite, made of a fine (<2 µm) aggregate of olivine, orthopyroxene, clinopyroxene, and spinel, with small amounts of amphibole and dolomite. Electron backscattered diffraction study of the ultramylonite shows a clear crystallographic preferred orientation (CPO) of olivine. The fault vein pseudotachylyte overprints a spinel-lherzolite facies amphibole-bearing mylonite, indicating that shear localization accompanying chemical reaction had taken place in the peridotite before seismic slip produced frictional melting. The occurrence of amphibole in the host mylonite and that of dolomite as well as amphibole in the matrices of ultramylonite and pseudotachylyte may indicate that fluid was present and had evolved in its composition from H₂O-rich to CO₂-rich during ductile deformation with metamorphic reactions, which may account for the observed rheological transition from ductile to brittle behavior. The spinel-lherzolite facies assemblage in mylonites, P-T estimations from pyroxene geothermometry and carbonate reactions, and the type of olivine CPO in deformed pseudotachylyte indicate that both the preseismic and the postseismic ductile deformations occurred at ~800 °C and 0.7–1.1 GPa.

Key Words: pseudotachylyte • mantle • peridotite • mylonite • shear localization • earthquake • fluid • dolomite • Balmuccia • Ivrea zone