[S-MP24] Deformed rocks, Metamorphic rocks and Tectonics

[SMP24-08] Dehydration melting of lawsonite eclogite xenolith in Colorado Plateau: Microtextural analyses for the fluid migration history

*Ryo Fukushima¹, Tatsuki Tsujimori¹, Nobuyoshi Miyajima² (1.Tohoku University, 2.Bayerisches Geoinstitut, University of Bayreuth)

Keywords:dehydration melting, Colorado Plateau, lawsonite eclogite, focused ion beam scanning electron microscopy (FIB SEM), transmission electron microscopy (TEM)

Ultrahigh-pressure lawsonite eclogite xenolith from central Colorado Plateau is likely to be a fragment of the Farallon plate, which was once subducted with a low angle under the west coast of North America. Previous petromineralogical–geochemical studies suggested that the diatreme formation bringing such xenoliths to the surface was caused by a series of the following events: (1) a high-angle subduction of the slab; (2) dehydration of serpentinite and hydration of slab constituents ('Great hydration'); and (3) heating due to plume invasion. However, it remains unresolved as to the causal relationship among these events and durations of the hydration/thermal pulse. In this study, we attempt to further constrain the history of the slab eclogitization and the diatreme formation, by analyzing microtextures of omphacite and lawsonite pseudomorph. Based on the presence of rhyolitic glass along omphacite's micro crack, and the presence of aggregates of spherulitic zoisites + plagioclase microlites in the lawsonite pseudomorph, we propose that this eclogite was heated up at least to the wet solidus (~870°C) at 3 GPa; the local presence of fluid along grain boundaries would have caused flux melting, whereas lawsonite would have undergone dehydration melting. These findings would facilitate discussion on the melting process of hydrated slab constituents at high-pressure conditions.