HOW CAN WE DISTINGUISH BETWEEN IMPACT EJECTA DEPOSITS AND VOLCANIC FLOWS IN PLANETARY DATASETS AND SAMPLES?

Hypervelocity impact events generate pressures and temperatures that can vaporize, melt, shock metamorphose, and/or deform a substantial volume of the target sequence. The transport and mixing of impact-metamorphosed materials during the formation of impact craters produces a wide variety of distinctive “impactites”. The impact process is a fundamental geological process that has had a major influence on the origin and evolution of the Moon, Mars, asteroids and many other planetary bodies in the Solar System. On many of these bodies, volcanism has also been prevalent. How then does one distinguish between an impact versus a volcanic origin for an outcrop or sample from an object other than Earth?

While some rocks may be obviously generated by impact events – e.g., clast-laden impact melt rocks with shatter-coned clasts – many impactites share close similarities with volcanic rocks. This contribution will highlight the study of impact craters on Earth, which show that impact-generated melts will flow just like volcanic flows (with the textures to prove it) and cool to form columnar joints. At the hand specimen scale, many impact melts lack visible clasts and can display many textures that are typical of volcanic flows, such as vesicles, flow textures, quench textures, and so on.

When samples can be studied on Earth, either delivered as meteorites or from the various human and robotic sample return missions, the search for shock metamorphic effects and geochemical tracers of the projectile can at times provide quick confirmation of an impact origin. However, this contribution will demonstrate that some products of impact events (e.g., clast-free impact melt rocks) contain no unequivocal proof that they formed from hypervelocity impact – beyond the context of being in a proven meteorite impact crater. Examples from several Canadian craters are provided. While largely a discussion consigned to the history books of the 20^th century, it is notable that many of these Canadian craters (e.g., Clearwater, Mistastin, Sudbury) were first reported as being of volcanic origin. How ironic then would it be if studies of samples from these originally so-called crypto-volcanic structures hold the key to finding criteria that could unequivocally distinguish between impact ejecta deposits and volcanic flows throughout the Solar System?