U-Pb systematics of the unique achondrite Ibitira: Precise age determination and petrogenetic implications

Abstract

Ibitira is an unbrecciated, equilibrated vesicular basaltic achondrite that is considered to have originated on a parent body distinct from all other known meteorites. We present the first combined high-precision U and Pb isotopic data for this unique meteorite. The 238U/235U value of 137.777±0.013 determined for the whole rock is comparable to values determined for bulk chondrites and other basaltic achondrites. This value results in corrections of -1.1Ma for Pb-Pb dates calculated using the previously assumed invariant 238U/235U value of 137.88. Using the determined 238U/235U value, the 7 most radiogenic Pb isotopic analyses for acid-leached pyroxene-rich and whole rock fractions yield an isochron Pb-Pb age of 4556.75±0.57Ma, in excellent agreement with the results of Mn-Cr chronology which give the ages of 4557.4±2.5Ma and 4555.9±3.2Ma using the U-corrected Pb-Pb age of D'Orbigny as a time anchor. Along with the previously proposed thermal history of Ibitira and our closure temperature estimates for Pb diffusion, the Pb-Pb age is interpreted as the timing of the last chemical equilibration and coarse pyroxene exsolution that occurred during high temperature metamorphism. The metamorphism may have been caused by burial of Ibitira lava under successive lava flows and, if so, the Pb-Pb age should post-date the crystallization by a short time interval. The Pb isotopic data for acid leachates suggest partial re-equilibration of Pb between plagioclase and phosphate, perhaps during an impact event at 4.49Ga, as recorded by K-Ar systematics. The whole rock 238U/204Pb indicates that compared to CI chondrites, Ibitira is less depleted in Pb than in some alkali elements despite a lower condensation temperature of Pb than the alkali elements. The restricted Pb depletion may reflect preferential concentration of metals with high fluid/melt partition coefficients including Pb and Zn as a result of fluid exsolution and migration within the parent magma. We discuss the implications of the U-Pb systematics for the origin and differentiation of the parent body.