Hf isotopic compositions of the Napier Complex, Antarctica

The geochemical community is now assessing the evidence for the origin
of the earliest crust and depletion of the Archean mantle through a
variety of methods, including documentation of whether a 142Nd
positive anomaly resulting from decay of the now extinct 146Sm nuclide
exists in the mantle, and also measurement of initial 176Hf/177Hf
values in zircons and other phases. It has been in the spirit of such
investigations that we have made Hf isotopic measurements on zircon,
garnet, orthopyroxene, osumilite, rutile and sapphirine - all from the
Napier Complex of East Antarctica. Individual zircon grains have
yielded a remarkably uniform range of initial 176Hf/177Hf values
between 0.280399 +/- 5 and 0.280469 +/- 7. Because the samples were
collected from different localities and in different rock types, it
had been assumed that they would have a wide range in isotopic
compositions, indicating a complex history of mantle melting and
crustal recycling. Instead, these results now indicate that the source
of the crustal materials that formed the Napier Complex at 3.8 Ga were
depleted relative to the chondritic uniform reservoir, or CHUR, which
is based on analyses of chondritic meteorites. Moreover, the results
demonstrate that even the silicic rocks in the Napier Complex are not
likely to have formed from remobilized older crustal materials, but
were instead juvenile products of mantle melting.

Measurement of Lu-Hf isotopic compositions for garnet, opx,
sapphirine, osmulite, rutile and whole rock samples from several
localities in the Napier Complex have yielded isochrons with ages
between 2459 - 23 Ma and 2173 - 37 Ma with
rather uniform initial ratios between 0.280876 and 0.280884. Although
the calculated errors are larger than those often obtained from zircon
U-Pb dating, it should be noted that they are only slightly large than
one percent. Attempts are being made to add data points to the
isochrones with the anticipation that this will reduce the errors
further. Besides producing isochrons, one major contribution of this
work has been demonstration of what happens to the Lu-Hf system at the
mineral and whole-rock scale during ultra high temperature (UHT)
metamorphism. The main rock-forming minerals record an initial Hf
ratio acquired during metamorphism while zircons ?see through? this
event and record the initial ratio of the rocks acquired at the time
of crystallization.