Impact induced melting and the development of large igneous provinces

Abstract

We use hydrodynamic modelling combined with known data on mantle melting behaviour to examine the potential for decompression melting of lithosphere beneath a large terrestrial impact crater. This mechanism may generate sufficient quantity of melt to auto-obliterate the crater. Melting would initiate almost instantaneously, but the effects of such massive mantle melting may trigger long-lived mantle up-welling that could potentially resemble a mantle hotspot. Decompression melting is well understood; it is the main method advocated by geophysicists for melting on Earth, whether caused by thinned lithosphere or hot rising mantle plumes. The energy released is largely derived from gravitational energy and is outside (but additive to) the conventional calculations of impact modelling, where energy is derived solely from the kinetic energy of the impacting projectile, be it comet or asteroid. The empirical correlation between total melt volume and crater size will no longer apply, but instead there will be a discontinuity above some threshold size, depending primarily on the thermal structure of the lithosphere. We estimate that the volume of melt produced by a 20 km diameter iron impactor travelling at 10 km/s may be comparable to the volume of melt characteristic of terrestrial large igneous provinces (V10 6 km 3 ); similar melting of the mantle beneath an oceanic impact was also modelled by Roddy et al. [Int. J. Impact Eng. 5 (1987) 525]. The mantle melts will have plume-like geochemical signatures, and rapid mixing of melts from sub-horizontal sub-crater reservoirs is likely. Direct coupling between impacts and volcanism is therefore a real possibility that should be considered with respect to global stratigraphic events in the geological record. We suggest that the end-Permian Siberian Traps should be reconsidered as the result of a major impact at V250 Ma. Auto-obliteration by volcanism of all craters larger than V200 km would explain their anomalous absence on Earth compared with other terrestrial planets in the solar system. ß

On the ages of flood basalt events

“…And there are a number of reasonably large and welldated craters that correspond to no extinction or other geologically recorded bio-event. A recent paper by Jones et al [89] proposes a model of impact-induced decompression melting as a possible mechanism for CFB generation. Based on hydrodynamic modeling, the authors propose instantaneous melting followed by long-lived mantle upwelling, and suggest that the endPermian Siberian traps should be reconsidered as the result of a major impact.…”

“…The paper unfortunately fails to quote a wealth of field, geochronologic, paleomagnetic and other published evidence on (i) the duration of LIP volcanism, (ii) the detailed dating of the Siberian traps and Meishan sections, (iii) the fact that the K/T Ir level is within the reversed chron 29R in the Deccan, hundreds of thousands of years after the onset of volcanism, (iv) the fact that field evidence for an impact at P/T time has been at best equivocal, attempts to replicate the results being unsuccessful... The discussion and conclusion of Jones et al [89] are largely unsubstantiated, as can readily be seen from the data and observations recalled in the present review.…”

Origin and evolution of magmas on the Ontong Java Plateau

“…Rogers 1982;Jones et al 2002;Ingle & Coffin 2004), but there is no evidence in the Aptian sedimentary record for an impact of the required magnitude. Furthermore, the impact would normally be expected to produce magma with the chemical and isotopic characteristics of N-MORB as it is the upper mantle that would be melted.…”

Evaluation of different models for the origin of the Siberian Traps

“…Two varieties of the impact model can be discussed in relation to the Siberian Traps FBP; the impact site was somewhere within the Siberian Traps FBP (Jones et al, 2002) and the impact site was antipodal to the Siberian Traps FBP (Hagstrum, 2005). The former model has related Cu-NiPt mineralization in the Siberian Traps and impactor material.…”