  Zircons are found in extraterrestrial rocks from the Moon, Mars, and some
differentiated meteorite parent-bodies. These zircons are rare, often of small
size, and have been affected by neutron capture induced by cosmic ray exposure.
The application of the 176Lu-176Hf decay system to zircons from planetary
bodies such as the Moon can help establish the chronology of large-scale
differentiation processes, like the crystallization of the lunar magma ocean.
Here, we present methods to measure the isotopic composition of Hf of
extraterrestrial zircons dated using ID-TIMS U-Pb after chemical abrasion. We
introduce a 2-stage elution scheme to separate Hf from Zr while preserving the
unused Zr fraction for future isotopic analysis. The effect of neutron capture
is also re-examined using the latest thermal neutron capture cross sections and
epithermal resonance integrals. Our tests show that the precision of Hf
isotopic analyses is close to what is theoretically attainable. We have tested
this method to a limited set of zircon grains from lunar rocks returned by the
Apollo missions (lunar soil 14163, fragmental polymict breccia 72275, and
clast-rich breccia 14321). The model ages align with previously reported
values, but further work is needed to assess the chronology of lunar magma
ocean crystallization as only a handful of small zircons (5 zircons from 3
samples) were analyzed, and the precision of the analyses can be improved by
measuring more and larger lunar zircon grains.
