233-5 Snapshots of an Evolving Atmosphere and Crust Recorded in Precambrian Spherule Beds

Session: Impact Cratering and the Evolution of Life

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Archean and Paleoproterozoic impact spherule beds offer a unique window into crustal evolution during early Earth history. At least 13 distinct large meteorite impacts between 3.47 and 2.49 Ga have been identified, represented by spherule beds in the Kaapvaal (South Africa) and Pilbara (Western Australia) cratons. This talk will summarize some of the changes in the atmosphere and crust recorded by these beds over nearly 1 billion years of Earth’s early history; samples of these unique beds will be presented for inspection by the audience.

Geochronologic, geochemical, and petrologic analyses of these spherule beds illuminate evolving crustal conditions through time. Consistency in the Fe³⁺/Fe_T ratios of Ni-rich spinels in layers up to the 2.57 Paraburdoo Spherule layer indicate persistent low oxygen atmosphere conditions, in contrast to the more oxidized spinels observed in Phanerozoic impacts. However, differences in trace element content in the spinels, as well as in the bulk chemistry, are consistent with the progressive emergence and stabilization of continental crust. Increasing variance in the size distributions of spherules in coeval layers between the Kaapvaal and Pilbara cratons point to increasing tectonic separation during the late Archean and early Paleoproterozoic. The presence and changing chemistry of TiO₂-II, Cr isotopes, and platinum group elements (PGE) across these beds also track the variation in impactor population.

Beyond their impact origin, these beds record subtle but widespread sedimentological and geochemical signals to constrain crust-hydrosphere-atmosphere interactions during early Earth history. The study of impact spherule beds offers not only a record of extraterrestrial bombardment, but also a framework for reconstructing Precambrian crustal growth, tectonic reorganization, and continental evolution.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8962

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