The use of heavy mineral correlation for determining the source of impact ejecta: A Manicouagan distal ejecta case study

Due to the nature of distal impact ejecta layers on Earth, preserved deposits are rare and are commonly altered by diagenetic/weathering processes. To establish the source of ejecta, trace element signatures of impact glasses have typically been used to establish the source crater of a deposit. However, in many cases, impact glasses in ejecta deposits are devitrified and altered to clays resulting in loss of original melt and original trace element composition. This is the case for the Late Triassic ejecta deposit of SW Britain where impact melt spherules have been completely altered to clay. This Late Triassic ejecta deposit was originally believed to be derived from two possible sources, the Rochechouart or Manicouagan impact structures. To accurately establish the source of this ejecta deposit an alternative correlation technique was developed using garnet major oxide data and radiometric whole grain fusion Ar-Ar dating of shocked biotite (U-Pb dating of zircon was also used but proved inconclusive). Radiogenic dating of shocked biotites (observed exclusively in the ejecta deposit) yielded ages consistent with the Grenvillian target rocks at Manicouagan and excluded Rochechouart as a potential source. Garnet major oxide compositions of garnets in the ejecta deposit were directly compared to garnets from the Manicouagan target rocks and impactite rocks. A strong garnet composition signature correlation between the samples from Manicouagan and the ejecta deposit provides convincing evidence that Manicouagan is the source of the SW Britain ejecta deposit. Furthermore, we suggest that heavy mineral correlation techniques should be considered in future studies as a correlation tool for establishing the source of ejecta.