INVESTIGATING THE POTENTIAL INFLUENCES ON CHANGES IN IMPACT SPHERULE DIAMETERS BETWEEN CORRELATED LAYERS THROUGHOUT THE ARCHEAN

Impact spherule layers form from large impact events, which vaporize target rock and meteorite material which subsequently condenses into spherical ejecta particles termed spherules. Spherule layers can be deposited on global scales, making them exceptional stratigraphic markers, and are especially relevant to Archean rocks that were deposited during a time when Earth experienced a higher frequency of bombardment. This study uses petrographic imaging methods and considers a sequence of four impact events deposited from 3.47-2.49 Ga, which have been correlated between the Pilbara Craton of Western Australia (WA) and the Kaapvaal Craton of South Africa (SA), to assess temporal and spatial changes in spherule size distribution as a means to evaluate to relative distance between the cratons and the history of tectonic spreading between them. Thin sections of 55 samples from the nine spherule layers associated with these four impacts were photographed in their entirety and all the spherules counted and measured for short and long axis to calculate a representative diameter. It is found that the average difference in spherule diameter increased through time (3.47 Ga event: 26 µm, 2.63 Ga event: 66 µm, 2.57 Ga event: 123 µm, 2.49 Ga event: 207 µm). Spherule size distribution graphs yield similar findings, and each correlated spherule layer for an impact event has a range within 200 µm (3.47 Ga Western Australia: 1338 µm, 3.47 Ga South Africa: 1141 µm, 2.63 Ga WA: 1510 µm, 2.63 Ga SA: 1409 µm, 2.63 Ga WA: 1365 µm, 2.57 Ga WA: 911 µm, 2.57 Ga SA: 966 µm, 2.47 Ga WA: 1203 µm, 2.47 Ga SA: 1206 µm). Other potential influences on spherule diameter, such as petrologic type, depositional environment, bed thickness, and impactor size, have also been ruled out as controlling this difference in spherule diameter. These observed changes through time thus implicate a corresponding change in distance from the impact site, and using this as a proxy for relative distance suggests that the Pilbara and Kaapvaal Cratons were progressively spreading from 3.47-2.49 Ga. This study concludes that the Pilbara and Kaapvaal Cratons were oriented as a “Vaalbara” supercontinent during the Paleoarchean, but had spread into individual landmasses by the Neoarchean, and provides invaluable evidence for plate tectonic activity during this phase of Earth’s history.