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

Abstract

146Sm– 142Nd and 147Sm– 143Nd 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 207Pb/ 206Pb spot ages from 3.95 to 4.19 Ga. Finsch pyropes yield 100 × ɛ 142Nd = − 6 ± 12 ppm, ɛ 143Nd = − 32.5, and 147Sm/ 144Nd = 0.1150. These results do not confirm previous claims for a 30 ppm 142Nd excess in South African cratonic mantle. The lack of a 142Nd 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 × ɛ 142Nd = 8 ± 10 ppm, ɛ 143Nd = 45 ± 1, and 147Sm/ 144Nd = 0.5891. Back-calculation of this present-day ɛ 143Nd yields an unrealistic estimate for the initial ɛ 143Nd of − 160 ɛ-units, clearly indicating post-crystallization disturbance of the 147Sm– 143Nd system. Examination of 146,147Sm– 142,143Nd 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 146Sm– 142Nd systematics in Jack Hills zircons.