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

Abstract

Available online xxxx Editor: A. Yin Keywords:Li diffusion zircon charge coupling geospeedometer ToF-SIMS NanoSIMS Micron-to submicron-scale observations of Li distribution and Li isotope composition profiles can be used to infer the mechanisms of Li diffusion in natural zircon. Extreme fractionation (20-30❤) within each single crystal studied here confirms that Li diffusion commonly occurs in zircon. 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. The partitioning of Li between these two modes during its diffusion may depend on the pre-diffusion substitution mechanism of REE and Y in the zircon lattice. Based on our results, sharp Li concentration gradients are not indicative of limited diffusion, and can be preserved at temperatures >700 • C on geologic timescales. Finally, large δ 7 Li variations observed in the Hadean Jack Hills zircons may record kinetic fractionation, rather than a record of ancient intense weathering in the granite source materials.

Episodes of fast crystal growth in pegmatites

“…The possibility that Li diffuses very slowly is at odds with some studies that have inferred short residence times of crystals in magmatic systems from Li concentration profiles 23 . Our findings are similar to the observation of very slow diffusion of Li in zircons due to coupling with slow-diffusing rare-earth elements 24 . The strong coupling of Li to slowly diffusing Al and Ge inferred here suggests that there may have been limited subsolidus diffusive relaxation of Li, Al, and Ge across these boundaries.…”

Trace Elements and Li Isotope Compositions Across the Kamchatka Arc: Constraints on Slab‐Derived Fluid Sources

“…In addition, previous studies have shown that the diffusivity of Li is two or three orders of magnitude faster than that of other elements in natural outcross at contact zones (e.g., Mg, Fe, Ca. Penniston‐Dorland et al, 2010; Richter et al, 2003; Tang et al, 2017). Therefore, the Li concentration gradients and any Li isotope anomalies produced by slab subduction components in the mantle wedge could be equilibrated by diffusion in short time (e.g., 10–10 5 year, Richter et al, 2003; Halama et al, 2009).…”

“…Journal of Geophysical Research: Solid Earth Fe, Ca. Penniston-Dorland et al, 2010;Richter et al, 2003;Tang et al, 2017). Therefore, the Li concentration gradients and any Li isotope anomalies produced by slab subduction components in the mantle wedge could be equilibrated by diffusion in short time (e.g., 10-10 5 year, Richter et al, 2003;Halama et al, 2009).…”

Stability of Zircon and Its Isotopic Ratios in High-Temperature Fluids: Long-Term (4 months) Isotope Exchange Experiment at 850°C and 50 MPa

“…et al, 2017). Slow diffusion could be coupled to rare earth element (REE) and Y abundance or possibly other trivalent cations (e.g., Al), whereas fast diffusion appears to be independent of REE+Y in the zircon lattice (Tang et al, 2017). Our experiments, in particular the hump shaped profile for Li, underscores the presence of multiple diffusion pathways.…”