GAO-GUENIE

Many stones fell in Upper Volta at about 5pm in the afternoon; witnesses described an event in which stones bounced off the ground, trees were broken, and henhouses were destroyed. The largest recovered stone weighed 2.5 kg. Although most Gao-Guenie stones are unshocked H5 chondrites, some have been shock-melted. For instance, a 570 g, impact-melted stone was found in December 1999 in a shipment of Gao-Guenie stones from Burkina Faso purchased by E. Olson. A second stone found within the same lot, which weighed 88 g, may also be an impact melt.

Studies conducted to date on the impact-melted stone have determined that it is an impact melt breccia having a Fa content of 17.9—very similar to that of other Gao-Guenie stones. The melt matrix consists of a microcrystalline texture with shock features in the form of cm-sized vugs, FeNi-metal forming melt spheres and streaming patterns, and tension cracks within melt regions, the latter likely caused by contraction cooling. Small components of the H-chondrite host material still exist within the melt phase.

Radiometric chronometry techniques conducted on the melt and clast components of the breccia, reflecting an age of 303 (±56) m.y., perhaps indicating the time of the last major thermal event (Swindle et al., 2009). Alternatively, the widespread CRE age that is found for H chondrites of 5–10 m.y. may reflect resetting during a more recent disruption event. The specimen of NWA 2058 shown above is a 0.19 g part slice.This is the first case in which an impact melt phase was found to be associated with relatively unshocked material from the same fall, and this 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.

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