Paul S. Decarli

Formation of Diamond by Explosive Shock

Samples of graphite have been recovered after exposure to explosive shocks of 300,000-atm estimated intensity. X-ray and electron-diffraction examinations prove the existence of diamond in this material. The mechanism proposed for the formation of diamond under these conditions is simple compression in the c-axis direction of the rhombohedral form of graphite.

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 (∼106 km3); 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 ∼250 Ma. Auto-obliteration by volcanism of all craters larger than ∼200 km would explain their anomalous absence on Earth compared with other terrestrial planets in the solar system.

Polymorphic Behavior of Titania under Dynamic Loading

Single‐crystal and polycrystalline specimens (including powder compacts of various densities) of titanium dioxide have been subjected to shock‐wave pressures in the 150–1000–kbar range. Hugoniot measurements have disclosed a phase transition commencing below 200 kbar, and x‐ray diffraction studies of specimens recovered after shocking to various pressures above 150 kbar have shown the presence of an orthorhombic phase with the α‐PbO2 structure. Calculated lattice parameters are a = 4.55 A, b = 5.46 A, and c = 4.92 A, which correspond to a crystal density of 4.34 g/cm3. The orthorhombic phase appears to result on release of pressure from a considerably denser phase of TiO2 (postulated to have approximately 8:4 oxygen‐titanium coordination) that dynamic measurements indicate is formed under shock. Yields of the α‐PbO2 phase as high as about 90% have been obtained from [001]‐ and [111]‐oriented rutile crystals shocked to 450 kbar. At atmospheric pressure this phase can be retained indefinitely at temperature...

Laboratory impact experiments versus natural impact events

Laboratory studies of shock metamorphism have long provided a basis for recognition of the diagnostic features of ancient terrestrial impact craters. However, there are significant differences between the range of parameters accessible in laboratory impact experiments and the conditions of large natural impact events. The basic premise of the laboratory calibrations is that peak pressure is the most significant parameter governing shock metamorphism. To show that other parameters are important, the shock formation of diamond is discussed in detail. Shock-induced phase transitions in silicates are discussed in the framework of current efforts to infer possible kinetic effects. In an effort to encourage critical examination of the literature, we call attention to the characteristics and limitations of experimental techniques. Particular emphasis is placed on the value of thoroughly documenting both the details of shock-loading experiments and the assumptions underlying shock calculations, to permit eventual reassessment of the results in the light of new information.

Design of Uniaxial Strain Shock Recovery Experiments

We present an elementary introduction to the art and science of uniaxial strain-shock recovery experiments. Subjects discussed include generation of planar stress waves, design of sample recovery experiments, stress-gage instrumentation, and temperature effects. The emphasis of the present paper is practical; we hope to provide the neophyte with the basic information needed to design and interpret well-characterized shock recovery experiments.

Shock Wave Synthesis of Diamond and other Phases

Shock wave synthesis of diamond was an unexpected result of experiments designed to explore the effects of shock waves on a variety of materials. The initial announcement in 1959 was controversial; shock synthesis of diamond had been shown to be unlikely, on the basis of kinetic arguments. Jamieson confirmed the identification and suggested a diffusionless mechanism, c-axis compression of rhombohedral graphite. Subsequent work has provided strong evidence that shock wave synthesis of cubic diamond is a conventional thermally activated nucleation and growth process. Thermal inhomogeneities provide the requisite high temperatures; quenching via thermal equilibration is implicit in the process. Shock synthesis of adamantine BN phases appears to be quasi-martensitic; a martensitic mechanism may partially account for the Lonsdaleite (hexagonal diamond) observed in some meteorites and in some artificial shock products. Diamond is also formed as a detonation product in oxygen-deficient explosives. The polycrystalline product of shock synthesis is similar to natural carbonado. The association of carbonado with an ancient giant impact crater is noted.

Meteorite Studies Illuminate Phase Transition Behavior of Minerals under Shock Compression

Some shock wave researchers have long contended that phase transitions of minerals under shock compression occur more rapidly than under comparable static compression conditions. Other researchers argue that phase transition behavior under shock compression does not differ from observations of static high pressure behavior. Many meteorites contain high‐pressure phases that are ascribed to impact. These high‐pressure phases are found within or adjacent to so‐called melt veins, sheets of material that was once molten and was quenched via conduction to surrounding material. Possible mechanisms for melt vein formation on impact include adiabatic shear and jetting. Thermal analysis of melt vein solidification and cooling, together with knowledge of phase stability fields and conditions for metastable survival of high‐pressure phases, constrains the shock conditions and provides evidence that the observed reconstructive phase transitions occurred via the same nucleation and growth mechanisms observed in static ...

Impact Decompression Melting: A Possible Trigger for Impact Induced Volcanism and Mantle Hotspots ?

We examine the potential for decompression melting beneath a large terrestrial impact crater, as a mechanism for generating sufficent quantity of melt to auto-obliterate the crater. Decompression melting of the sub-crater mantle may initiate almost instantaneously, but the effects of such a massive melting event may trigger long-lived mantle up-welling or an impact plume (I-plume) that could potentially resemble a mantle hotspot. 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; therefore the empirical correlation between total melt volume and crater size will no longer apply, but instead be nonlinear above some threshold size, depending strongly on the thermal structure of the lithosphere. We use indicative hydrocode simulations (AUTODYNE-2D) to identify regions of decompression beneath a dynamic large impact crater, (calculated as P-Lithostatic P) using SPH and Lagrangian solvers. The volume of melting due to decompression is then estimated from comparison with experimental phase relations for the upper mantle and depends on the geotherm. We suggest that the volume of melt produced by a 20 km iron projectile travelling at 10 km/s into hot oceanic lithosphere may be comparable to a Large Igneous Province (LIP ~106 km3). The mantle melts will have plume-like geochemical signatures, and rapid mixing of melts from sub-horizontal sub-crater reservoirs to depths where garnet and/or diamond is stable is possible. Direct coupling between impacts and volcanism is therefore a possibility that should be considered with respect to global stratigraphic events in the geological record. Maximum melting would be produced in young oceanic lithosphere and could produce oceanic plateaus, such as the Ontong Java plateau at ~120 Ma. The end-Permian Siberian Traps, are also proposed to be the result of volcanism triggered by a major impact at ~250 Ma, onto continental or oceanic crust. Auto-obliteration by volcanism of all craters larger than ~200 km would explain their anomalous absence on Earth compared with other terrestrial planets in the solar system. This model provides a potential explanation for the formation of komatiites and other high degree partial melts. Impact reprocessing of parts of the upper mantle via impact plumes is consistent with models of planetary accretion after the late heavy bombardment and provides an alternative explanation for most primitive geochemical signatures currently attributed to plumes as originating from the deep mantle or outer core.

Enstatite: Disorder Produced by a Megabar Shock Event

Shocked Bamle enstatite partly transforms to disordered enstatite. Debye-Scherrer patterns of some shocked material are almost identical to those of disordered enstatite from portions of various enstatite achondrites. No disorcdered single crystals have been found.

Evidence for Kinetic Effects on Shock Wave Propagation in Tectosilicates

The question of whether phase transition kinetics can affect shock wave propagation has been around for about 50 years. Some workers have speculated that shock compression is fundamentally different from static compression; others cite evidence that static and dynamic transitions follow the same rules. Metastable high‐pressure phases that are found in large (long‐duration shock) impact craters constrain the post‐shock temperature histories of the neighboring rock. The post‐shock temperature is a function of the area enclosed by loading and unloading paths. We use petrographic evidence to constrain the unloading paths. The presence of metastable phases then serves to constrain possible loading paths. The limited range of possible loading paths indicates that there must be a large kinetic effect on shock wave propagation in tectosilicates.