Geocheminar fall-2017

Owing to both its ability to record U-Pb ages and its mechanical and chemical resilience, zircon is the premier crustal geochronometer in most geologic settings. Its ability to survive multiple sedimentary cycles makes it a key tracer for continental evolution and indeed, zircon is to date the only material known to survive from Earth's first 500 million years. However, zircon's ability to serve as a "time capsule" preserving information about this period is only commensurate with its ability to resist later alteration, and radiation damage can render it susceptible to recrystallization or hydrothermal alteration. By examining zircon from known geologic settings with well-constrained tectonothermal histories, we are able to establish patterns of alteration affecting many of the lines of evidence useful for constraining continental evolution: zircon trace element chemistry and the cargo of mineral inclusions which most zircons contain. Using the Jack Hills detrital zircons, we have developed textural criteria for recognizing altered inclusion assemblages, and using zircon from a suite of recent granites, we can further constrain the frequency of inclusion alteration and telltale signs. We have also used the Jack Hills zircons to develop an index for chemical alteration of zircon. Application to Phanerozoic granite zircon reveals its general applicability. Alteration to zircon trace chemistry and mineral inclusions appears to usually be identifiable either texturally or chemically, respectively. Secondary features in zircon can in some circumstances yield their own useful information -- for example, secondary inclusions which are amenable to U-Pb dating, when examined on a 5-10 micron scale, provide a chronicle of the metamorphic events undergone by their host zircons throughout their residence in the crust.