Li isotopes in zircon: a petrogenetic tool

Li content and δ7Li in zircons have been recently been suggested to be useful for characterizing a zircon’s parent rock. However, interpreting the significance of [Li] and δ7Li in zircons requires better understanding of the possible effects of alteration on Li and δ7Li distribution in zircon, Li substitution mechanisms, and Li diffusion rates in zircon. For that purpose, well studied zircons with known parent rocks have been studied for δ7Li, Li abundance and other trace elements.
Combining trace element and Li content measurements allows determining the Li substitution in unaltered magmatic zircon. The acquired data supports the hypothesis that Li is interstitial and charge compensates substitution of trivalent cations. This substitution results in a relatively slow rate of Li diffusion. Li content in TTG and Sanukitoid zircons are similar to the Li content of zircons from continental crust. Their δ7Li have been compared to δ7Li of bulk rocks, no existing δ7Li being published before. The δ7Li and trace element data constrain the genesis of TTGs and sanukitoids.
These data are compared to detrital zircons from the Jack Hills (Western Australia) with U-Pb ages greater than 3.9 Ga for which parent rock-type is not known. The Li systematics in sanukitoid and TTG zircons indicates that high [Li] in pre-3.9 Ga Jack Hills detrital zircons is a primary igneous composition and suggests growth in protocontinental crust in magmas similar to Archean granitoids.

Figure 1: Histograms of Li contents and Li isotopes in TTG, Sanukitoid and Jack Hills zircons. For the Li content histogram, the samples are compared to previous data obtained in oceanic and continental crust. For the δ7Li histogram, no data on zircon have been published before this study. Obtained values are thus compared to bulk rocks.

 

For more information:

Bouvier A.-S., Ushikubo T., Kita N., Cavosie A., Kozdon R., Valley J. (2012): Li isotopes and trace elements as a petrogenetic tracer in Archean zircons, Contributions to Mineralogy and Petrology 163, 745–768. [DOI]

 

Collaborations
Aaron Cavosie
Noriko Kita
Reinhard Kozdon
Takayuki Ushikubo
John Valley

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