ON TARGET: WHAT CAN ARCHEAN IMPACTS TELL US ABOUT THE ANCIENT CRUST?

Spherule beds represent distal deposits from meteor impacts, and due to their global deposition, may persist even when craters and ejecta have been recycled or buried. A combination of primary mineralogy and platinum group and high field strength element geochemistry provide insight into the composition of the target rock and bolide. Two Archean spherule beds are studied, representative of impacts occurring more than 670 million years apart during a time in which major tectonic changes are occurring and the Earth was undergoing continental crustal growth. Both have preserved Ni-chromites and bulk chemistry indicative of very different bolide and target rock compositions. The mid-Archean S3 impact, which occurred 3.24 billion years ago (Ga) and is preserved in the Barberton greenstone belt of South Africa, retains a geochemical signature consistent with a carbonaceous (CV) chondrite impact into ocean crust and depleted MORB mantle (DMM). The late Archean Paraburdoo spherule layer, which occurred ~2.57 Ga and is preserved in the Hamersley basin of western Australia, retains a geochemical signature of an ordinary (L or LL) type chondrite into a mixed continental and basaltic target with little to no incorporation of mantle material. Ni-chromites are the only primary mineral formed within the spherule that is still preserved in both layers. Electron microprobe analyses of these minerals reveal compositional differences consistent with the differences in source rock. S3 Ni-chromites show greater incorporation of siderophile and chalcophile elements, whereas the Ni-chromites in the PSL are preferentially enriched in the lithophile elements. Analyses of the impact deposits provide windows into randomly sampled and otherwise unpreserved ancient crust, helping to unravel the early Earth’s tectonic history. The vaporization of rock with each impact would have also significantly affected the atmosphere, resulting in sudden climate shifts during the early evolution of life.