Asteroid/comet impact clusters, flood basalts and mass extinctions: Significance of isotopic age overlaps

Morgan et al. [J. Phipps Morgan, T.J. Reston, C.R. Ranero. Earth Planet. Sci. Lett. 217 (2004) 263-284.], referring to an overlap between the isotopic ages of volcanic events and four epoch/stage extinction boundaries, suggest a dominant role of Continental Flood Basalts (CFB) and of explosive CO₂-rich volcanic pipes ("Verneshots") as mass extinction triggers. Here I point out that Morgan et al. overlook 3 overlaps between the ages of extraterrestrial impacts, volcanic and mass extinction events, and 3 overlaps between the ages of extraterrestrial impact and volcanic events. These overlaps suggest that both extraterrestrial impacts and volcanism served as extinction triggers separately or in combination. A protracted impact cluster overlaps extinctions at the end-Devonian (∼374-359 Ma) and impact-extinction age overlaps occur in the end-Jurassic (∼145-142 Ma), Aptian (∼125-112 Ma); Cenomanian-Turonian (∼95-94 Ma); K-T boundary (∼65.5 Ma) and mid-Miocene (∼16 Ma) (Table 1). Morgan et al. appear to question the uniqueness of shock metamorphic and geochemical criteria used to identify asteroid/comet impacts. However, shock pressures at 8-35 GPa, indicated by intra-crystalline planar deformation features (PDF), exceed lithospheric and volcanic explosion pressures by an order of magnitude and are not known to be associated with explosive volcanic diatremes, kimberlites or lamproites. These authors make reference to apparent iridium anomalies of volcanic origin. However, platinum group element (PGE) abundance levels, volatile/refractory PGE ratios, and Cr and Os isotopes of meteoritic materials are clearly distinct from those of terrestrial volcanics. Given a Phanerozoic time-integrated oceanic/ continent crustal ratio > 2.5 and the difficulty in identifying oceanic impacts, I suggest the effects of large impacts on thin thermally active oceanic crust-capable of triggering regional to global mafic volcanic events and ensuing environmental effects-provide an essential clue for understanding the relationships between impacts and volcanic events which, separately or in combination, result in deleterious environmental effects, in some instances leading to mass extinctions.