Tenoumer Crater, Mauritania: Age and petrologic evidence for origin by meteorite impact

Abstract

Inclusions of granitic basement rocks, collected from the sparse ‘lava’ exposed at the 1.8‐km‐diameter Tenoumer crater, Mauritania, exhibit distinctive petrographic features considered indicative of shock‐wave action and diagnostic for meteorite impact. The observed effects include (1) distinctive planar features in quartz, oriented parallel to ω{ 101¯3 } and to other specific planes; (2) intense selective vesiculation of quartz and felspar grains; (3) presence of lechatelierite apparently formed by fusion of quartz grains; and (4) partial fusion and decomposition of biotite grains. These effects are quite distinct from those produced from conventional fusion of granitic xenoliths in lavas. The evidence from these inclusions supports the theory of a meteorite impact origin for the Tenoumer crater. The lava, which is chemically similar to the basement rocks and which has apparently been rapidly quenched, is interpreted as an impact melt formed by fusion of the basement rocks at the time of impact and injected into fractures in the crater wall, carrying with it highly shocked inclusions of basement rock. Values of Sr87/Sr86 determined for the lava are approximately 0.720; they are almost identical with values determined for basement rock samples and are too high to represent primary melt derived from the mantle. Potassium‐argon age determinations on the melt rock establish the age of the Tenoumer crater as 2.5±0.5 m.y.

Petrographic studies of “fallout” suevite from outside the Bosumtwi impact structure, Ghana

“…6c and 6d) was found in some of the thin sections. Similarly textured ballen quartz has been found in impact glasses and crystalline impact melts of many impact craters (e.g., French et al 1970;Engelhardt 1972;Carstens 1975;Dressler et al 1997). This ballen quartz has been interpreted to represent pseudomorphs after cristobalite, which, in turn, is thought to have replaced lechatelierite, originally formed by shock-induced melting of quartz (Carstens 1975), or to represent recrystallized diaplectic quartz glass that underwent transition to cristobalite and then α-quartz (Bischoff and Stöffler 1981;1984).…”

The nature of the Moon's surface: evidence from shock metamorphism in Apollo 11 and 12 samples

“…Almost identical rock types have been found at some terrestrial impact structures. At the Tenoumer structure in Mauritania, inclusions of shocked granite are held in what has been interpreted (French et al, 1970) as an impact, melt, formed by fusion of the target rock and injected along with the fragments into crater wall fractures. Cooling of this melt-fragmental mixture produced a texture (Fig.…”

Chemical compositions of impact melt breccias and target rocks from the Tenoumer impact crater, Mauritania

“…On the basis of the occurrence of small dikes, which were supposed to be rhyodacitic lava, Monod and Pomerol (1966) suggested a possible volcanic origin for the structure. Finally, definitive evidence for the impact origin of this crater was supplied by French et al (1970), who found up to eight sets of planar deformation features (PDFs) in quartz grains in inclusions of granitic basement scattered throughout the above-mentioned "lava." Following this discovery the nature of this "lava" was reevaluated by the same authors and interpreted as rapidly quenched impact melt rock.…”