ZIRCON U-Pb ISOTOPE, δ~(8O) 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

摘要

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 crosssection 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~(8O) values (8.4 to 10.4‰) and trace element compositions quite distinct from those of the cores; these differences indicate that their δ~(8O) 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_2O) but water-undersaturated (fluid-absent) conditions. Likewise no evidence of significant diffusive exchange of δ~(8O) could be detected along deformation microstructures such as annealed fractures in cores infilled with high δ~(8O) zircon. Application of simple diffusion models to detailed δ~(8O) 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 experimentallydetermined “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_2O). Our test demonstrates that unrecrystallized metamorphosed igneous zircons and metamorphic zircons will retain the geochemical (U-Pb age, trace element and δ~(8O)) record of their origin and evolution despite prolonged, high-grade metamorphism at significant f(H_2O) but water under-saturated (fluid-absent) conditions. Such zircons, particularly those that exhibit δ~(8O) 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.