The 176Lu-176Hf systematics of ALM-A: A sample of the recent Almahata Sitta meteorite fall

Abstract

The application of Lu-Hf chronometry to meteorites has been compromised by arbitrary results such as dates up to 300 Myr older than the Pb-Pb age of the Solar System, unsubstantiated isochron scatter among different meteorite fractions, and varying initial Hf isotope ratios (176Hf/177Hfi). To determine the cause of the discrepancies and presence of unsupported radiogenic 176Hf, we collected Lu-Hf data for the ureilitic trachyandesite ALM-A, a fragment of the recent Almahata Sitta meteorite fall. The purest feldspar and pyroxene fractions and all 2 M HNO3 washes (i.e. selectively dissolved phosphate minerals) yield a 13-point isochron with a reasonable age of 4569 ± 24 Ma and 176Hf/177Hfi of 0.279796 ± 0.000011. Most impure mineral fractions, in contrast, scatter above this regression. Terrestrial contamination causes the 176Hf excesses, but is effectively removed by handpicking the purest mineral grains. Our study demonstrates 1) the successful application of the Lu-Hf chronometer to ALM-A, and 2) an internal consistency among the Pb-Pb age of the Solar System, the 176Lu decay constant, the Lu-Hf CHUR parameters, and robust estimates of the 176Hf/177Hfi of the Solar System from meteorites.