U-Pb isotopic behaviour of zircon during upper-amphibolite facies fluid infiltration in the Napier Complex, east Antarctica

摘要

Understanding the factors that contribute to U-Pb discordance in zircon is essential for interpreting isotopic data and for assessing the validity of concordia intercept ages. Modification caused by interaction with metamorphic fluids is often cited as a primary means by which significant or even complete isotopic resetting of U-Pb systematics in zircon might be achieved under subsolidus conditions. We present a field example from the Napier Complex, east Antarctica, in which a Palaeoproterozoic (2450-70 Ma) zircon population interacted locally with an Early Palaeozoic (498 ± 1.7 Ma) aqueous fluid at upper-amphibolite facies conditions. Conventional ion microprobe analysis of sectioned and polished grain surfaces indicates that fluid interaction resulted in minor disturbance of U and Pb in zircons (both normal and reverse discordance) with limited displacement along a chord with a lower intercept that coincides with the timing of fluid infiltration. In contrast, ion probe 'drilling' or depth profiling into unpolished natural zircon crystal surfaces revealed extensive disturbance of U-Pb systematics, to depths of ～0.2 μm, with near-surface ages consistent with the timing of fluid influx at ～498 Ma. Although zircon underwent some radiogenic Pb redistribution during fluid interaction, infiltrating fluids resulted in minimal grain-scale isotopic modification of zircon. Based on ion probe depth profiling results, we propose that limited normal discordance observed in the conventional ion microprobe zircon analyses, in this case, is controlled by an analytical mixture of reset and/or recrystallised zircon along penetrative micro-fracture networks with that of adjacent unaffected zircon. We also suggest that the observed reverse discordance is genuine, resulting from localised intra-grain net accumulations of radiogenic Pb. We conclude that the isotopic response of zircon, in this case, is controlled by the interaction of an aqueous metamorphic fluid, of low to moderate salinity, resulting in sub-micrometre depth scale isotopic modification at natural crystal faces and along penetrative micro-fracture networks, and that grain-scale isotopic modification was negligible. Therefore, we urge caution when considering regional chronological interpretations that appeal to significant zircon isotopic resetting caused exclusively by metamorphic fluid interaction at upper-amphibolite facies conditions.