Geochemistry of greater than 3900 Ma detrital zircons from Jack Hills, Western Australia.

摘要

Rare detrital zircons as old as 4400 million years (Ma) from the Jack Hills, Western Australia, are currently the oldest identified pieces of Earth. The original host rocks for the Jack Hills zircons have not been identified. The zircons are up to 400 Ma older than the oldest known terrestrial rocks, the 4030 Ma Acasta gneisses in northwest Canada.; The chemistry of the Jack Hills zircons may record clues about what rocks were on Earth prior to ∼3900 Ma. By analyzing the U-Th-Pb and oxygen isotope ratios, trace elements, cathodoluminescence zoning, grain morphology, and mineral inclusions, the Jack Hills zircons can be compared with zircons from known environments.; Most of the >3900 Ma Jack Hills zircons preserve growth zoning in CL, have magmatic Th/U ratios, delta18O values from 5.3 to 7.3‰, and have trace element compositions that are typical for magmatic zircons. Values of delta18O above about ∼6.0‰ are elevated relative to zircons that form in equilibrium with uncontaminated mantle melts. Elevated delta18O values are thus evidence for the operation of crustal recycling of altered rocks into magmas, and also evidence for the presence of fluid alteration of the protoliths at low temperatures.; Crustal alteration and recycling are common geological processes today, and occur not only at plate margins, but also in areas of thickened or buried crust. The Jack Hills zircons are similar to zircons found in evolved felsic rocks (e.g. granitoids) that are ubiquitous in the continental crust.; To evaluate if the Jack Hills zircons could have crystallized in mafic melts, zircons in gabbros and serpentinites drilled from the Mid-Atlantic Ridge (MAR) were studied. The MAR zircons preserve growth zoning, and are of similar size to the Jack Hills zircons. However, the oceanic zircons preserve a narrower range of delta18O values from 4.8 to 5.6‰, values that record high temperature equilibrium with uncontaminated melts. The delta18O values from the MAR zircons thus provide strong evidence that the Jack Hills zircons required the input of altered crust into their melt sources, and could not have crystallized in a setting analogous to a modern mid-ocean ridge environment.