Apatite Lu–Hf dating of late Archean banded iron formations

Abstract Precambrian banded iron formations (BIFs) preserve unique records of the oxygenation of Earth’s oceans but have proven challenging to date robustly. U–Pb dating of authigenic phosphate minerals has been attempted but, given the generally low U and high common Pb contents of these minerals and the propensity for Pb mobility during post-depositional hydrothermal and/or metamorphic events, the U–Pb isotope system is not robust in BIF phosphates and largely produces (partial) reset dates. We employ the newly developed in situ apatite laser ablation–inductively coupled plasma–reaction cell–mass spectrometry (LA-ICP-MS/MS) Lu–Hf dating technique to constrain the depositional age of late Archean BIFs in the Eastern Block of the North China Craton that have experienced greenschist-amphibolite facies metamorphism. Petrographic observations combined with major element and trace rare earth element (REE) fingerprinting demonstrate that the targeted apatite grains are of marine authigenic origin. The apatite Lu–Hf inverse isochron age of 2501 ± 28 Ma (MSWD = 2.1; n = 62) is within uncertainty of previously reported zircon U–Pb dates from interbedded volcanic rocks, which we interpret as the primary apatite crystallization age. In contrast, the U–Pb system in apatite records reset dates of ca. 2.1 Ga with large uncertainties. Our study demonstrates, for the first time, that precise authigenic apatite 176Lu/176Hf ages can be obtained for Precambrian metamorphic BIFs, providing a new tool to robustly and directly date the timing of BIF deposition, regardless of amphibolite-grade metamorphic overprints.