Thermal history of the Ibitira noncumulate eucrite as inferred from pyroxene exsolution lamella: Evidence for reheating and rapid cooling

Abstract— Ibitira is a strongly recrystallized and unbrecciated noncumulate eucrite. We measured Ca compositional profiles of Ibitira pyroxene by electron microprobe and computed the cooling rate and burial depth from pyroxene exsolution profiles to gain information on early thermal history of Ibitira. Pyroxene begins to exsolve at 1082 °C and cools down to 550 °C at a rate of 0.02 °C/year, forming an augite lamella about 7.0 μm in width. A notable characteristic of the Ca profile of augite lamellae in Ibitira pyroxene is a gradient near the interface between augite and low‐Ca pyroxene (pigeonite). This profile suggests that after thermal metamorphism Ibitira pyroxene experienced a sudden temperature rise to above solidus temperature of pyroxene (～1082 °C), and subsequent rapid cooling. The ³⁹Ar‐⁴⁰Ar age of 4.485 Ga for Ibitira, which is the oldest ³⁹Ar‐⁴⁰Ar age for noncumulate eucrites, may date this reheating event.