Pb isotopes reveal intracrustal exhalative hydrothermalism during deposition of the ∼3.43 Ga Strelley Pool Formation

Abstract

Hydrothermal systems are widely considered to have supported development of Earth's early microbial ecosystems by providing energy and nutrients to surface and subsurface environments. We studied a remarkably extensive and well-preserved hydrothermal dike complex active during deposition of the ∼3.43 Ga Strelley Pool Formation (Warrawoona Group, Pilbara Region, Western Australia). We report field observations, mineralogy, Pb isotope ratios, and selected trace metal concentrations for black, silica-rich dikes at the study site. The majority of the samples fall into two distinct arrays in the common Pb isotope diagram. One group of samples, with many very unradiogenic isotope ratios, defines a slope whose apparent age is roughly coeval with deposition of the Strelley Pool Formation. The other group, containing samples with elevated 206Pb/204Pb ratios and generally low 208Pb/204Pb ratios, has a moderately well-defined slope of apparent Neoarchean age. The original Pb isotope ratios of this latter group were likely disturbed by a combination of Pb isotope memory resetting, Pb-loss, and U-addition. Using the trace metal concentrations of samples associated with Paleoarchean Pb isotope memory, we identify a crustal rather than a mantle source for metals carried in the hydrothermal system. An intracrustal (as opposed to an oceanic ridge) setting is also consistent with local and regional geology. Combined 208/207/206/204Pb systematics of the dike complex reveal a striking similarity to unconformable Pb found in older Pilbara Craton sediment-hosted metal deposits of the Doolena Gap Formation, Apex Basalt, Coppin Gap, and the Dresser Formation. The unusual Pb isotope systematics, plotting far above conformable Pb isotope evolution curves, is currently poorly understood. We discuss the use of Pb isotopes to constrain the timing of geological events including the age of initial deposition, and the loss and resetting of original geochemical signatures. We propose that improving our understanding of early Earth environments influenced by hydrothermalism requires combining geochemical fingerprints with proxies, such as Pb isotopes, which are very sensitive to potential contribution from crustal sources.