Zircon U-Pb isotope,  18O and trace element response to 80 m.y. of high temperature metamorphism in the lower crust: Sluggish diffusion and new records of Archean craton formation

Coordinated cathodoluminescence (CL) imaging and ion microprobe (SHRIMP and CAMECA 1280) analysis document micron-scale U-Pb-O isotope and trace element zoning in zircons from deep crust exposed to 80 m.y. of high temperature and pressure metamorphism. Three, along-strike paragneiss samples across the amphibolite to granulite facies transition in the Kapuskasing Uplift crustal cross-section in the Archean Superior province yield detrital, originally igneous zircon cores overgrown by progressively larger volumes of metamorphic zircon with increasing grade. The cores generally retain primary age (2.85±0.03 to 2.67±0.02 Ga), oxygen isotope (5.1 to 7.0‰) and trace element compositions similar to those reported for magmatic arc sources. Dark CL, metamorphic zircon rims record nearly continuous overgrowth events for ∼80 m.y. from 2.66±0.01 to 2.58±0.01 Ga during uppermost amphibolite to granulite facies regional metamorphism. These rims have significantly higher δ^18^O values (8.4 to 10.4‰) and trace element compositions quite distinct from those of the cores; these differences indicate that their δ^18^O and trace element compositions were not inherited from the igneous cores, consistent with extensive textural evidence for rim formation as metamorphic overgrowths. Multi-spot traverses record steep oxygen isotope discontinuities (4‰ over \<10 μm) at core-rim boundaries, confirming the extremely sluggish rates of volume diffusion of O in non-metamict zircon during extended (∼80 m.y.) granulite-grade metamorphism (peak T=750-800 °C) at substantial f(H~2~O) but water-undersaturated (fluid-absent) conditions. Likewise no evidence of significant diffusive exchange of δ^18^O could be detected along deformation microstructures such as annealed fractures in cores infilled with high δ^18^O zircon. Application of simple diffusion models to detailed δ^18^O profiles in a large number of zircon grains constrain maximum values of the diffusivity of oxygen in zircon (logD^Zrc^~ox~) to the range −27.5 to −26.4 m^2^/s. For the estimated 80 m.y. and 700 to 800 °C time-T window of rim formation, these maximum values are similar to or slower than values reported by Page and others (2007, 2010) and the experimentally-determined "dry" diffusivity of oxygen in zircon (Watson and Cherniak, 1997), but are markedly slower than the experimentally-determined "wet" diffusivity of oxygen in zircon (Watson and Cherniak, 1997). Fast diffusion of oxygen in zircon predicted by hydrothermal experiments may, in nature, require the presence of a hydrous fluid rather than a threshold value of f(H~2~O). Our test demonstrates that unrecrystallized metamorphosed igneous zircons and metamorphic zircons will retain the geochemical (U-Pb age, trace element and δ^18^O) record of their origin and evolution despite prolonged, high-grade metamorphism at significant f(H~2~O) but water under-saturated (fluid-absent) conditions. Such zircons, particularly those that exhibit δ^18^O zoning, are micron-scale records for the T-time-fluid interaction history of deep crustal rocks. Such records will not be preserved in less refractory phases and promise new insights into the processes of continent formation and evolution.

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