Li isotopes and trace elements as a petrogenetic tracer in zircon: insights from Archean TTGs and sanukitoids

Abstract

We report d 7 Li, Li abundance ([Li]), and other trace elements measured by ion probe in igneous zircons from TTG (tonalite, trondhjemite, and granodiorite) and sanukitoid plutons from the Superior Province (Canada) in order to characterize Li in zircons from typical Archean continental crust. These data are compared with 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. Most of the TTG and sanukitoid zircon domains preserve typical igneous REE patterns and CL zoning.[Li] ranges from 0.5 to 79 ppm, typical of [Li] in continental zircons. Atomic ratios of (Y ? REE)/(Li ? P) average 1.0 ± 0.7 (2SD) for zircons with magmatic composition preserved, supporting the hypothesis that Li is interstitial and charge compensates substitution of trivalent cations. This substitution results in a relatively slow rate of Li diffusion. The d 7 Li and trace element data constrain the genesis of TTGs and sanukitoids. [Li] in zircons from granitoids is significantly higher than from zircons in primitive magmas in oceanic crust. TTG zircons have d 7 Li (3 ± 8%) and d 18 O in the range of primitive mantlederived magmas. Sanukitoid zircons have average d 7 Li (7 ± 8%) and d 18 O higher than those of TTGs supporting genesis by melting of fluid-metasomatized mantle wedge. The Li systematics in sanukitoid and TTG zircons indicate that high [Li] in pre-3.9-Ga Jack Hills detrital zircons is a primary igneous composition and suggests the growth in proto-continental crust in magmas similar to Archean granitoids.

Multi-mode Li diffusion in natural zircons: Evidence for diffusion in the presence of step-function concentration boundaries

“…Large δ 7 Li variations in zircon were also observed by Bouvier et al (2012), who concluded that it reflected the signatures of the magma source materials.…”

“…Lithium may substitute in a zircon lattice by coupling with trivalent REE and Y ions Hanchar et al, 2001;Ushikubo et al, 2008;Bouvier et al, 2012), and in this case, Li diffuses together with REE and Y, which will slow Li diffusivity by orders of magnitude. However, this mechanism has not been observed by current experimental studies (Cherniak and Watson, 2010;Trail et al, 2016), both of which reported fast Li diffusion in zircon.…”

“…Details for the Tanzanian sample (whole rock data, mineral chemistry data, U-Pb dates, in situ Pb isotopic results and thermobarometry) are given in Bellucci et al (2011) and Mansur et al (2014), and for the remaining samples are given in Bouvier et al (2012). All natural zircons were imaged by ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometry) for multi-element distributions including those of Li and Y. Lithium isotope profiles were then measured on all three TTG zircons and one sanukitoid zircon (DD96-4-5-11).…”

“…All natural zircons were imaged by ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometry) for multi-element distributions including those of Li and Y. Lithium isotope profiles were then measured on all three TTG zircons and one sanukitoid zircon (DD96-4-5-11). The TTG and sanukitoid zircons were previously characterized by CL (cathodoluminescence) imaging and SIMS spot analyses for their Li isotope and trace element compositions (Bouvier et al, 2012).…”

“…Sharp Li concentration gradients frequently seen in zircons suggest that the effective diffusivity of Li is significantly slower than experimentally determined (Cherniak and Watson, 2010;Trail et al, 2016), otherwise the crystallization/metamorphic heating of these zircons would have to be unrealistically fast (years to tens of years). Charge coupling with REE and Y has been suggested as a mechanism that may considerably reduce Li diffusivity in zircon (Ushikubo et al, 2008;Bouvier et al, 2012). We show that Li diffused in the direction of decreasing Li/Y ratio and increasing Li concentration (uphill diffusion) in one of the zircons, demonstrating charge coupling with REE and Y. Quantitative modeling reveals that Li may diffuse in at least two modes in natural zircons: one being slow and possibly coupled with REE+Y, and the other one being fast and not coupled with REE+Y.…”