At about 5:00 in the afternoon, numerous stones fell in the West Africa country of Upper Volta (now Burkina Faso) near the villages of Gao and Guénie. Witnesses described an event in which stones bounced off the ground, trees were broken, and henhouses were destroyed. Many stones were recovered, the largest of which weighed 2.5 kg. Although most Gao-Guenie stones are unshocked to very weakly shocked (S1S2) H5 chondrites exhibiting only minor brecciation, a small number of specimens are fragmental breccias and some exhibit features reflecting a high degree (S6) of shock-melting (Schmieder et al., 2016). For example, a 570 g impact-melt stone was found in 1999 in a shipment of Gao-Guenie stones from Burkina Faso purchased by E. Olson, and a second impact-melt stone weighing 88 g was subsequently found in the same lot.
These impact-melt breccia stones contain olivine with the same Fa values (Fa19) as in other Gao-Guenie stones, and it is considered that they are all part of the same meteorite fall. Schmieder et al. (2016) studied one of the impact-melt breccia stones and identified a number of distinct textural zones: (i) chondrite-clast domain, (ii) quench melt domain, (iii) impact melt domain. The chondrite-clast domain comprises petrologic type 5 material from the H-chondrite host lithology. The shock-darkened quench melt domain is found adjacent to the chondrite-clast domain, and exhibits a cryptocrystalline to microcrystalline, fluidal igneous texture. The impact melt domain has a microporphyritic igneous texture and contains relict silicate grains exhibiting shock features such as planar fractures and darkening, along with other shock-generated components including metal/troilite melt spherules, cm-sized vugs, and contraction cracks caused by rapid cooling. Based on petrographic and other evidence, Schmieder et al. (2016) concluded that this impact-melt phase represents an impact-injected melt dike beneath the floor of a crater. The cooling rate of the impact melt was calculated by them to have been ~10°C/s over the temperature interval ~20001100°C, with a slower rate of ~500040,000°C/yr below ~1000°C. In consideration of the cooling rate data and estimates of other parameters involving conductive cooling, they concluded that this dike was likely ~0.55 m in diameter.
Radiometric chronometry was employed to date the melt and clast components of the breccia, establishing an age for the impact-melting event of 303 (±56) m.y. (Swindle et al., 2009). The typical CRE age determined for H chondrites of 510 m.y. could reflect isotopic clock resetting during a more recent impact event, while an even later impact event is attested by a CRE age of ~0.52 m.y. calculated for Gao-Guenie specimens (Schmieder et al., 2016 and references therein). This is the first case in which an impact melt phase was found to be associated with relatively unshocked material within the same fall, and it could provide valuable information on asteroid impact processes. The specimen shown above is a 3.64 g partial slice of the impact-melt phase of Gao-Guenie, which probably contains chondrite-clast domain with adjacent shock-darkened, fluidal quench melt veins. The photo below shows a representative sample (petrographic thin section LPL 1055,1) of the impact melt phase domains in Gao-Guenie .
Photo credit: Schmieder et al., MAPS, vol. 51, #6 (2016)
'The Gao-Guenie impact melt brecciaSampling a rapidly cooled impact melt dike on an H chondrite asteroid?'