The Sedimentary Record of Impact Events

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Potassium isotopic composition of Australasian tektitesAbstract-We have analyzed the potassium isotopic composition of four tektites from the Australasian strewn field, spanning a wide diversity of thermal histories, inferred from textures and volatile element contents. Our results indicate no isotopic differences between tektites and terrestrial crustal rocks, placing stringent limits of ≤2% loss of potassium during the brief duration of high temperature heating experienced by these samples. This confirms that the chemical composition of tektites is entirely a reflection of source rock composition and has not been modified by the tektiteforming process for elements less volatile than potassium. Losses of more volatile components, e.g., the halogens and water, are not precluded by the present data. Coupling a radiative cooling temperature-time path with potassium vapor pressure data indicates that tektite melt drops are not likely to develop bulk elemental fractionation during the brief heating episodes of tektites for peak temperatures <2273 K. The extent of K isotopic fractionation is independent of droplet size but dependent on peak heating temperature. The exact peak temperature depends on the choice of vapor pressure data used for K, which need to be better constrained.

“…Tektites have been the subject of much study. For further details on tektites, see the reviews by, e.g., Koeberl (1994Koeberl ( , 2001) and Montanari and Koeberl (2000).…”

Section: Introductionmentioning

confidence: 99%

Potassium isotopic composition of Australasian tektites

Humayun

Koeberl

2004

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“…Extensive studies by Koeberl and other authors (excellent summaries are given by Koeberl [1998Koeberl [ , 2001) have demonstrated that isotopes of the first row transition elements, and also particularly of the platinum-group elements (PGE), can show the presence of even very small quantities of meteoritic material (e.g., Koeberl et al 1994Koeberl et al , 2002. In many of the examples studied to date, there may be no obvious visible sign of impact-derived carrier minerals in hand specimen or petrographic thin section, although textural features may show that high pressure shock processing has occurred.…”

Section: Samples and Analytical Instrumentationmentioning

confidence: 99%

Early fracturing and impact residue emplacement: Can modelling help to predict their location in major craters?

Kearsley

Graham

McDonnell

et al. 2004

Abstract-Understanding the nature and composition of larger extraterrestrial bodies that may collide with the Earth is important. One source of information lies in the record of ancient impact craters, some of which have yielded chemical information as to the impacting body. Many deeply eroded craters have no remaining melt sheet or ejecta yet may contain impactor residue within basement fractures. The emplacement mechanism for fractionated siderophile residues is likely to be gaseous, although, melt droplets and some solid materials may survive. For breccia-and melt-filled fractures to contain extraterrestrial material, they must form very early in the impact process. Most current numerical models do not dwell on the formation and location of early major fractures, although, fractures in and around small craters on brittle glass exposed to hypervelocity impact in low Earth orbit have been successfully simulated. Modelling of fracture development associated with larger craters may help locate impact residues and test the models themselves.

“…Meta-graywacke clasts, comprising up to 42% by volume of the upper impactite interval (Coney et al 2007), also contain common shocked quartz, although not in the same abundance as noted in quartzite. As is characteristic of the Bosumtwi impact breccias (e.g., Koeberl 2003, 2006;Koeberl and Reimold 2005; multiple sources, this issue) and of many other previously described impactite sequences (e.g., French 1998;Koeberl 2001), there is a very heterogeneous distribution of shock levels, both macroscopically and microscopically, within co-occurring lithic and mineral grains in any given interval.…”

Section: Occurrence Of Shocked Quartzmentioning

confidence: 73%

“…impactites analyzed outside of the crater (e.g., Koeberl et al 1998;Koeberl andBoamah 2003, 2006;Koeberl and Reimold 2005; multiple sources, this issue). These differences are primarily in the amount of melt material and in the level of shock metamorphism evidenced by quartz and other minerals, as recorded by abundances of PDFs, diaplectic quartz glass, and ballen quartz.…”

Section: Comparison With Other Bosumtwi Studiesmentioning

confidence: 99%

“…The majority of the reported melt occurs as widely disseminated particles or, more rarely, as bombs within suevite Koeberl andBoamah 2003, 2006;Coney et al 2007;Ferrière et al 2007). Modeling by Kieffer and Simonds (1980) and Melosh (1989) suggests that impact into a water-or volatile-rich porous target would not be expected to produce coherent melt bodies but instead would preferentially favor the development of dispersed melt within heterolithic breccias, i.e., suevites, such as seen in and around the Bosumtwi crater.…”

Section: Target Saturationmentioning

confidence: 99%

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Shock‐metamorphic petrography and microRaman spectroscopy of quartz in upper impactite interval, ICDP drill core LB‐07A, Bosumtwi impact crater, Ghana

Morrow

2007

Abstract-Standard and universal stage optical microscope and microRaman spectroscopic examination of quartz from the upper impactite interval of the International Continental Scientific Drilling Program (ICDP) Lake Bosumtwi crater drill core LB-07A demonstrates widespread but heterogeneous evidence of shock metamorphism. In the upper impactite, which comprises interbedded polymict lithic breccia and suevite from a drilling depth of 333.4-415.7 m, quartz occurs as a major component within metasedimentary lithic clasts and as abundant, isolated, single-crystal grains within matrix. The noted quartz shock-metamorphic features include phenomena related to a) deformation, such as abundant planar microstructures, grain mosaicism, and reduced birefringence; b) phase transformations, such as rare diaplectic quartz glass and very rare coesite; c) melting, such as isolated, colorless to dark, glassy and devitrified vesicular melt grains; and d) secondary, post-shock features such as abundant, variable decoration of planar microstructures and patchy grain toasting. Common to abundant planar deformation features (PDFs) in quartz are dominated by -equivalent crystallographic planes, although significant percentages of and other higher index orientations also occur; notably, c(0001) planes are rare. Significantly, the quartz PDF orientations match most closely those reported elsewhere from strongly shocked, crystalline-target impactites. Barometry estimates based on quartz alteration in the upper impactite indicate that shock pressures in excess of 20 GPa were widely reached; pressures exceeding 40-45 GPa were more rare. The relatively high abundances of decorated planar microstructures and grain toasting in shocked quartz, together with the nature and distribution of melt within suevite, suggest a water-or volatile-rich target for the Bosumtwi impact event.