Li zoning in zircon as a potential geospeedometer and peak temperature indicator

Abstract

920 to 650 °C, calculated diffusivities are in agreement with a previously established Arrhenius relationship calibrated on trace-element-poor Mud Tank zircon. Our revised Arrhenius relationship that includes both datasets is:We also observed that synthetic sector-zoned zircon exhibits near-step-function Li concentration profiles across sectors that correlate with changes in the rare earth element (REE) and P concentrations. This allowed us to examine how Li diffusion might couple with REE diffusion in a manner different than that described above. In particular, re-heating these grains revealed significant Li migration, but no detectable migration of the rare earth elements. Thus, unlike most elements in zircon which are not mobile at the micrometer scale under most timetemperature paths in the crust, Li zoning, relaxation of zoning, or lack of zoning altogether could be used to reveal time-temperature information. Discrete ~10 μm concentration zones of Li within zircon may be partially preserved at 700 °C for tens to hundreds of years, and at 450 °C for millions of years. In this regard, Li zoning in zircon holds significant potential as a geospeedometer, and in some instances as a qualitative indicator of the maximum temperature experienced by the zircon.

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

“…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. Cherniak and Watson (2010) conducted "in" diffusion experiments using REE-depleted Mud Tank zircons, and thus their experiments are unlikely to have evaluated charge coupling effects.…”

“…Cherniak and Watson (2010) conducted "in" diffusion experiments using REE-depleted Mud Tank zircons, and thus their experiments are unlikely to have evaluated charge coupling effects. Using Lu-and P-doped synthetic zircons, Trail et al (2016) carried out "in" diffusion and heating experiments, and their "in" diffusion experiments showed no discernable difference in Li diffusivity when compared to Cherniak and Watson's results. These Hoskin et al (2000) experimental studies reveal that another substitution mechanism for Li in zircon that does not involve coupling with REE and Y creates a fast diffusion mode.…”

“…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).…”

“…The experimental studies of Cherniak and Watson (2010) and Trail et al (2016) showed that Li diffusion is fast in zircon. However, if Li diffuses at the rate determined by Cherniak and Watson (2010) and Trail et al (2016), the sharp Li concentration gradients would require extremely fast zircon growth rates and metamorphic events.…”

“…On the other hand, the rapid diffusivity of Li and the associated large kinetic isotopic fractionations make Li a promising geospeedometer for determining rates of rapid geologic processes that can be difficult to determine using other perimental studies (Cherniak and Watson, 2010), which showed that Li can diffuse rapidly in zircon under crustal P-T-X conditions. Rather than using Li in zircon to track magmatic sources, the work by Cherniak and Watson (2010) suggested the possibility of using Li in zircon as a magmatic and metamorphic geospeedometer, with the first attempt made by Trail et al (2016). Lithium diffusion in zircon was also reported by Gao et al (2015), who studied Li isotopes in several zircon standards.…”