A survey of published descriptions of 32 of the largest, least eroded terrestrial impact structures reveals that the amount of melt at craters in crystalline rocks is approximately 2 orders of magnitude greater than at craters in sedimentary rocks. In this paper we present a model for the impact process and examine whether this difference in melt abundance is due to differences in the amount of melt generated in various target materials or due to differences in the fate of the melt during late stages of the impact. The model consists of a theoretical part for the early stages of impact, based on a Birch‐Murnaghan equation of state, a penetration scheme after Shoemaker (1963), and an attenuation model modified from Gault and Heitowit (1963), and a descriptive part for the later stages of impact, based on field observations at the large terrestrial craters. The impacts of iron, stone, permafrost, and ice meteorites 1 km in diameter into crystalline, carbonate, dry sandstone, ice‐saturated sand, and ice targets are modeled for velocities of 6.25, 17, and 24.6 km/s. Tables of calculated crater volume, depth of penetration of the meteorite, equivalent scaled depth of burst, radii to various peak pressure isobars, volume of silicate melt, and volume of water vapor (or, in the case of carbonate, carbon dioxide vapor) are presented. Simple algebraic expressions for pressure attenuation are derived: for the near field, dX/dR = 3Xn/R(1 ‐ n), where X is the pressure normalized to an averaged bulk modulus for the target rocks, R is the radius normalized to the radius of the cavity in which energy is initially deposited, and n is the pressure derivative of the bulk modulus. For the far field the pressure attenuation is given by dX/dR ∼−3X/R. For most materials considered, n = 4–6, and therefore the near‐field attenuation is proportional to R−3.65 ‐ R−4 and the far‐field attenuation is proportional to R−3. The calculations show that the volume of material shocked to pressures sufficient for melting should not be significantly different in sedimentary and crystalline rocks. Hence we conclude that shock melt is formed in the early stages of the cratering process by impacts into rocks rich in volatiles but is destroyed by the cratering process. We propose that the melt is finely dispersed by the great expansion of shocked volatiles upon release from high pressure and that suevite units are the product of this process. The fragmented silicates produced by this process may react penecontemporaneously with the hot volatiles to produce hydrated minerals such as clays. This process may produce hydrothermally altered minerals in planetary regoliths, such as the Martian regolith. The dispersion of shock melt by volatile expansion may also account for the apparent lack of lunarlike melt sheets on the surface of Mars. Because large amounts of volatiles vaporize during impact and are transferred from depth either into space, into the atmosphere, or onto near‐surface ejecta by condensation, repeated impact degasses a planet, depleting some ...
Calculations for the heat transfer between superheated silicate melt and evenly dispersed I-mm cold clasts (a model for impact melt rocks) indicate that (1) most of the thermal gradients are smoothed out in 100 s, (2) the extent to which cold debris is melted is slightly more than proportional to the fractional difference of the equilibration temperature between the solidus and the liquidus of the melt, (3) smaller clasts are preferentially dissolved, (4) the melting of clasts depends on the instantaneous local temperature of the melt in the region of the clasts, and hence clasts can melt during cooling whose liquiduses are higher than the equilibration temperature, and (5) the rate of equilibration is sufficiently high that clasts whose dissolution in the melt is slow may be preserved. Calculations for cooling of the 200-m-thick melt sheet at Manicouagan suggest that complete crystallization takes 35 years at 10 m from the edge and 1600 years at the center. Failure to consider the latent heat of fusion results in estimates of these times which are too short by about a factor of 2. The time to cool to a given temperature is proportional to the square of the distance from the boundary of the melt sheet only when the finite nature of the sheet is unimportant. INTRODUCTION Large meteorite impacts into crystalline igneous and metamorphic rocks on the earth and moon characteristically produce clast-laden impact melt rocks. These rocks, whether from the earth or moon, typically have fine-grained holocrystalline to glassy matrices surrounding up to about 25% mineral clasts 0.1-1.0 mm across and a much smaller volume fraction of lithic fragments ranging in size up to tens of meters. The clasts which are observed for the most part lack the vitrification, planar features, and kink bands characteristic of minerals shocked to pressures over 50 kbar. The specific example discussed here is the 200-m-thick sheet from the 65-km-diameter M anicouagan structure in Quebec. Except for some details, such as the exact thickness of sheet used and the temperature ranges for melting, the calculations are generally applicable to any of the larger terrestrial or lunar craters. The objective of this paper is to model numerically the heat flow both on the scale of the clasts and melt and on the larger scale of loss of heat from the melt sheet as a whole. Such modeling places limits on (1) the time before the viscosity of the melt increases dramatically because the clast-melt thermal interactions trigger nucleation, (2) the temperature of the clasts and melt prior to mixing, and (3) the time scale for crystallization of the melt sheet as a whole.Before the calculations of heat transfer are discussed, it must be emphasized that the processes modeled here take place relatively late in the cratering process, essentially after the excavation has been completed. The cratering process can be divided into a series of steps, as is discussed in a paper in this issue by Simonds et al. [1978] and the papers referenced therein. These steps are as follow...
Within the moderately eroded Manicouagan structure a sheet of clast‐laden impact melt 230 m thick and 55 km in diameter forms an annular plateau surrounding an uplift of shocked anorthosite. The melt sheet is divided into three vertically gradational units based on decreasing clast abundance and coarsening of the melt above the base. A very fine‐grained lower unit, rich in millimeter‐ and centimeter‐sized inclusions, thickens radially outward but is overlapped and replaced toward the center by a coarser middle unit containing fewer, larger inclusions. The upper unit is medium grained, virtually clast free, and texturally the most homogeneous of the three melt units. Within the lower and middle units a variety of textures are present. Textural development is a function of the cooling rate determined by stratigraphic position and the degree of supercooling determined by initial clast content. The mineralogy of the melt rocks is similar in all units and consists of zoned plagioclase, sanidine, Ca‐poor pyroxene, augite, quartz (rare in the lower unit), magnetite‐ilmenite intergrowths, smectite, and apatite, Pseudomorphs after olivine, and pigeonite in various stages of inversion to hypersthene, are widespread only in the upper unit, while minor biotite and hornblende are confined to the lower and middle units. Replacement of olivine and much of the Ca‐poor pyroxene by smectite, and alteration of iron oxides occurred during late stage crystallization and subsolidus cooling. The melt rocks as a group are chemically homogeneous, with a bulk composition similar to that of latite. No statistically significant regional chemical variations were found as a function of vertical, lateral, or radial position in the melt sheet. A local mafic variant represented by two samples with poikilitic texture indicates that the melt is not completely chemically homogeneous. The poikilitic rocks texturally resemble some Apollo 17 impact melt rocks and are inferred to have had a similar origin and thermal history.
Abstract. The 65‐km‐diameter Manicouagan impact structure has an eroded 230‐m‐thick sheet of clast‐laden, impact melt rock with an estimated preerosional volume of >270 km3. All samples are characterized by mineral and lithic clasts or their incompletely digested remnants. Drawing upon previous theoretical studies of shock waves, we suggest that the Manicouagan melt formed in 1 or 2 s in a 5‐km‐radius hemisphere near the point of impact. The melt accelerated to a few kilometers per second, and the melt and the less shocked debris surrounding it flowed downward and outward for a few minutes until the melt formed a lining of a 5‐ to 8‐km‐deep, 15‐ to 22‐km‐radius cavity. Extremely turbulent flow thoroughly homogenized the melt and promoted the incorporation and progressive digestion of slower moving, less shocked, cooler debris surrounding the melt. This debris had been finely fragmented, but not melted, to grain sizes of less than 1 mm by the passage of the shock waves. Because of the fine grain size, the melt and fragmented debris equilibrated thermally in about 100 s. During thermal equilibration, virtually all clastic debris (i.e. alkali feldspar, biotite, hornblende, garnet, and scapolite), other than highly refractory quartz and plagioclase as well as many of the centimeter size lithic clasts other than anorthosite, were digested. The preservation of quartz and plagioclase mineral clasts implies that the clasts and melt equilibrated to temperatures near but not above the liquidus. Plagioclase nucleation was initiated by the drop in temperature and possibly by direct nucleation on undigested debris. The initiation of crystallization vastly increased the melt viscosity, preventing settling of 10‐mm clasts of basement. Flow of melt through basement fractures is evidence that readjustment of the crater floor took place during the period of clast‐melt thermal equilibration.
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