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

Sm–Nd and Sm–Nd 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/Pb spot ages from 3.95 to 4.19 Ga. Finsch pyropes yield 100×ɛNd=−6±12 ppm, ɛNd=−32.5, and Sm/Nd=0.1150. These results do not confirm previous claims for a 30 ppm Nd excess in South African cratonic mantle. The lack of a Nd 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× ɛNd=8±10 ppm, ɛNd=45±1, and Sm/Nd=0.5891. Back-calculation of this present-day ɛNd yields an unrealistic estimate for the initial ɛNd of −160 ɛ-units, clearly indicating post-crystallization disturbance of the Sm–Nd system. Examination of Sm–Nd 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. ⁎ Corresponding author. Present address. Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, MC 170-25, Pasadena CA 91125, USA. E-mail address: [email protected] (G. Caro). 0009-2541/$ see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2007.10.018 254 G. Caro et al. / Chemical Geology 247 (2008) 253–265 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–Nd systematics in Jack Hills zircons. © 2007 Elsevier B.V. All rights reserved.