Thermal History of the Ibitira Noncumulate Eucrite: Evidence for Reheating and Fast Cooling

Ibitira is an unbrecciated noncumulate eucrite and shows fine-grained, holocrystalline, and vesicular texture. It belongs to an annealing grade of type 5, in which pyroxene is thermally equilibrated. It is generally considered that noncumulate eucrites were crystallized from basaltic melts on the surface of the HED parent body. Although Ibitira shows old Pb-Pb ages, an Ar-39-Ar-40 age of 4.495 Ga for Ibitira ranks it among the oldest such ages for eucrites, and suggests that Ibitira escaped the widespread impact resetting of K-Ar ages about 4.13.5 Ga ago indicated by other HED meteorites. 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 the thermal history of Ibitira and on the evolution of eucritic crust of the HED parent body.