Application of precise Nd-142/Nd-144 analysis of small samples to inclusions in diamonds (Finsch, South Africa) and Hadean Zircons (Jack Hills; Western Australia)

摘要

Sm-146-Nd-142 and Sm-147-Nd-143 systematics were investigated in garnet inclusions in diamonds from Finsch (S. Africa) and Hadean zircons from Jack Hills (W. Australia) to assess the potential of these systems as recorders of early Earth evolution. The study of Finsch inclusions was conducted on a composite sample of 50 peridotitic pyropes with a Nd model age of 3.3 Ga. Analysis of the Jack Hills zircons was performed on 790 grains with ion microprobe Pb-207/Pb-206 spot ages from 3.95 to 4.19 Ga. Finsch pyropes yield 100 x epsilon Nd-142=-6 +/- 12 ppm, epsilon Nd-143=-32.5, and Sm-147/Nd-144=0.1150. These results do not confirm previous claims for a 30 ppm Nd-142 excess in South African cratonic mantle. The lack of a Nd-142 anomaly in these inclusions suggests that isotopic heterogeneities created by early mantle differentiation were remixed at a very fine scale prior to isolation of the South African lithosphere. Alternatively, this result may indicate that only a fraction of the mantle experienced depletion during the first 400 Myr of its history. Analysis of the Jack Hills zircon composite yielded 100 X epsilon Nd-142=8 +/- 10 ppm, epsilon Nd-143=45 +/- 1, and Sm-147/Nd-144=0.5891. Back-calculation of this present-day epsilon Nd-143 yields an unrealistic estimate for the initial epsilon Nd-143 of -160 epsilon-units, clearly indicating post-crystallization disturbance of the Sm-147-Nd-143 system. Examination of (146,147)sm-Nd-142,Nd-143 data reveals that the Nd budget of the Jack Hills sample is dominated by non-radiogenic Nd, possibly contained in recrystallized zircon rims or secondary subsurface minerals. This secondary material is characterized by highly discordant U-Pb ages. Although the mass fraction of altered zircon is unlikely to exceed 5-10% of total sample, its high LREE content precludes a reliable evaluation of Sm-146-Nd-142 systematics in Jack Hills zircons.