DECOUPLING OF ZIRCON U-PB AND TRACE-ELEMENT SYSTEMATICS DRIVEN BY U DIFFUSION IN ECLOGITE-FACIES ZIRCON (MONVISO META-OPHIOLITE, W. ALPS)

Zircon is widely used to date metamorphic processes, particularly due to slow cation diffusion under crustal conditions. Further, in cases where U-Pb isotopic resetting is observed, it has often been demonstrated to be due to the migration of Pb. Here, we present laser-ablation depth profiling data that demonstrates rapid U diffusion in partially altered, high-pressure zircon. The zircons are hosted in metagabbros that underwent eclogite-facies (~550°C, ~2.6 GPa) recrystallization during subduction of the Monviso meta-ophiolite. One metagabbro contains only newly-grown zircons (50.2±1.1 Ma); two coarser-grained samples exhibit thin metamorphic rims on igneous cores. Most profiles in the coarse-grained samples record discrete Pb_C-rich and Pb*-, U-, Th-, and trace-element poor rims in the outermost ≤5 µm of each grain, but U shows apparent diffusion profiles that extend ~10–15 µm into zircon crystals and correlate with U-Pb date resetting. The data define three populations (cores, diffusively reset rims, and newly precipitated rims) that form two two-component mixtures, indicating that recrystallization was everywhere coupled with U addition. Data from fully equilibrated rims form a single age population (51.1±0.4 Ma) within error of the newly grown zircon and compatible with ~1 My fluid-rock interaction timescales. We interpret the U profiles as evidence of inward U diffusion associated with fluid-induced resorption, and systematically exclude other mechanisms for their formation. However, calculated diffusivity estimates are >20 orders of magnitude faster than predicted by experiments. The absence of zircon lattice damage, and the propagation of diffusion inward of a reaction front, indicates a link between fluid-saturated zircon alteration and fast U diffusion in zircon. Our results emphasize that – even at low temperature – zircon U-Pb systematics may be affected by parent and/or daughter diffusion over length scales large enough to affect laser-ablation or ion microprobe spot analyses.