An Archean Yellowstone? Evidence from extremely low δ18O in zircons preserved in granulites of the Yilgarn Craton, Western Australia

We report the discovery of Archean (2980-2670 Ma) zircons from the Yilgarn Craton in Western Australia that record unusually low delta O-18 signatures (to -0.5%). These zircons occur in cordierite-orthopyroxene granulites that retain the geochemical signature of intense premetamorphic hydrothermal alteration. We propose a model whereby the low-delta O-18 zircons crystallized within protoliths that record multiple stages of high-temperature interaction and hydrothermal exchange between shallow crustal material and O-18-depleted meteoric fluids, in a setting analogous to that of the Yellowstone caldera. Burial and subsequent granulitefacies metamorphism of this crust led to the crystallization of zircon, which acquired and preserved the extremely O-18-depleted signature of the whole rock. The apparent absence of strongly O-18 depleted Archean zircons has been a puzzling feature of the global zircon record, but we suggest this is an artifact of poor preservation potential. Our findings suggest that long-lived, shallow crustal magmatic-hydrothermal systems similar to those operating in modern caldera complexes were also a feature of Archean Earth.