Evidence from kimberlitic zircon for a decreasing mantle Th/U since the Archean

Abstract

A decoupling in MORB of measured Th/U ( κ = 2.5) from that calculated by Pb isotopes ( κ = 3.8) for the depleted asthenosphere is well established, and has been referred to as the second Pb paradox ( Kramers, J.D., and Tolstikhin, I.N., 1997. Two terrestrial lead isotope paradoxes, forward transport modelling, core formation and the history of the continental crust. Chem. Geol., 139, 75–110.) or the kappa conundrum ( Elliott, T., Zindler, A., and Bourdon, B., 1999. Exploring the kappa conundrum: the role of recycling in the lead isotope evolution of the mantle. Earth Planet. Sci. Lett., 169, 129–145.). More controversial has been the cause and timing of this phenomenon, although a higher return flux of U 6+ relative to Th 4+ and(or) the recycling of crustal Pb into the mantle have become the preferred explanations of most workers. Such a combined mechanism effectively operating over the past 2.5 Ga was modelled in plumbotectonics ( Zartman, R.E., and Haines, S., 1988. The plumbotectonics model for Pb isotopic systematics among major terrestrial reservoirs—a case for bi–directional transport. Geochim. Cosmochim. Acta, 52, 1327–1339. 709.33.), and found to be quantitatively feasible. A large TIMS, SIMS and LA-ICPMS database of Th and U concentrations for kimberlite-hosted zircon, particularly from Cr-poor megacrystic suites, now exists ( Kinny, P.D., Compston, W., Bristow, J.W., and Williams, I.S., 1989. Archaean mantle xenocrysts in a Permian kimberlite: two generations of kimberlitic zircon in Jwaneng DK2, southern Botswana. In: Ross, J., et al. (Eds.), Kimberlites and Related Rocks. Geol. Soc. Aust. Spec. Publ., vol. 146. Blackwell, Melbourne, pp. 833–842.; Berryman, A.K., Stiefenhofer, J., Shee, S.R., Wyatt, B.A. and Belousova, E.A., 1999. The discovery and geology of the Timber Creek kimberlite, Northern Territory, Australia. In: J. J. Gurney et al. (Editors), Proceedings of the VIIth International Kimberlite Conference, Cape Town, pp. 30–39.; Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., van Achterbergh, E., O–Reilly, S.Y., and Shee, S.R., 2000. The Hf isotope composition of cratonic mantle: LAM–MC–ICPMS. analysis of zircon megacrysts in kimberlites. Geochim. Cosmochim. Acta, 64, 133–147.; Peltonen, P., and Manttari, I., 2001. An ion microprobe U–Th–Pb study of zircon xenocrysts from the Lahtojoki kimberlite pipe, eastern Finland. Bull. Geol. Soc. Finl., 73, Parts 1–2, 47–58.; Spetsius, Z.V., Belousova, E.A., Griffin, W.L., O'Reilly, S.Y., and Pearson, N.J., 2002. Archean sulfide inclusions in Paleozoic zircon megacrysts from the Mir kimberlite, Yakutia: implications for the dating of diamonds. Earth Planet. Sci. Lett., 175, 1–16.; Appendices A and B, this work). Six suites comprising 10 or more zircon grains with ages between 90 and 2550 Ma reveal consistent patterns when plotted on Th/U vs. U diagrams. We interpret these patterns as resulting from fractional crystallization of a melt with kimberlite affinity presumably derived from the asthenosphere, permitting the extrapolation to an initial Th/U at the time zircon crystallization began. A two-fold decrease is seen in this ratio over the past 2.5 Ga, suggesting that during this time a similar change has occurred in the parent silicate melt. Estimates of Th and U distribution coefficients between zircon and coexisting melt permit calculation of Th/U in the melt, which, for these highly incompatible elements, presumably is the same as for its mantle source rock. Kimberlitic zircon may thus indeed give evidence of a reduction in κ, tentatively calculated as from ∼ 4 to ∼ 2, since the Archean for the depleted asthenosphere.