R5: Extraterrestrial materials

[R5-12] Low-temperature thermal history of cumulate eucrites as inferred from tridymite polymorphs

*Haruka Ono¹, Atsushi Takenouchi², Takashi Mikouchi³, Akira Yamaguchi² (1. Tokyo Uni. Sci., 2. NIPR, 3. Tokyo Uni. Mus.)

Keywords:Eucrite, Silica minerals, Thermal history, Tridymite, Low-temperature

Eucrites are considered to have formed in Vesta’s crust and have 2 subgroups, cumulate and non-cumulate. Cooling rates of cumulate eucrites have been estimated by pyroxene exsolution, but these rates are only at high temperature (>600 C). We studied tridymite polymorphs in 3 cumulate eucrites to understand their thermal histories below 400 C. We found orthorhombic tridymite lamellae in the monoclinic host in Moama, suggesting its faster cooling at low temperature compared to other 2 samples.