Allende, Mexico, Meteorite Shower

Abstract

The Allende meteorite fell near Parral, Chihuahua, Mexico, between 0105 and 0110 Central Standard Time on Saturday, 8 February 1969. The fireball approached from the south-southwest (S37?W), and broke up in the atmosphere, producing thousands of fusion-crusted meteoritic stones. The smallest individuals were recovered 4 km east of Rancho Polanco (26?432 N, 105?282 W), and the largest near, Rancho El Cairo (27?062 N, 105?122 W), some 50 km to the north-northeast across the Parral-Jimin?z highway. Specimen size increases generally as one moves to the north-northeast within the field, and many large specimens (5-15 kg) were recovered in and around the area enclosed by Pueblito de Allende, San Juan, Rancho Blanco, and Santa Ana. At least two tons of meteoritic stones have been recovered, with crusted individuals ranging in weight from approximately 1 g to one individual of 110 kg. Specimen shapes are mainly fragmental, due to one major disruption of the parent body, followed by minor subsequent fragmentation. Individual stones have primary and secondary fusion crust, and some fresh fracture surfaces due to late-stage breaking. A small percentage of stones shows strong ablative shaping due to oriented flight. The elongate strewnfield possibly exceeds 300 km2 in area, making Allende the largest recorded stony meteorite fall both in its areal extent and in total weight of recovered meteorites. Allende fell near the sites of find of two major iron meteorites, Morito and Chupaderos. Chemical and mineralogical compositions establish that Allende is a Type III carbonaceous chondrite. Three distinct components can be recognized: finegrained black matrix (<"60%), chondrules (<"30%), and irregular white aggregates (<"10%). The matrix consists almost entirely of iron-rich olivine (average 50% Fe2SiO4), with minor amounts of troilite, pentlandite, and taenite, rendered opaque by dispersed carbonaceous material. Most of the chondrules are magnesium-rich, and consist of olivine (average 9% Fe2SiO4) with minor amounts of clinoenstatite and some glass; a few chondrules are rich in calcium and aluminum, and are made up largely of anorthite, gehlenite, augite, and spinel. The irregular aggregates are also rich in calcium and aluminum, and contain anorthite, gehlenite, augite, spinel, nepheline, grossular, and sodalite (the last two minerals have not previously been recorded from meteorites). Complete chemical analyses have been made of the bulk meteorite, a dark inclusion, the matrix, a chondrule concentrate, two individual chondrules, and a single aggregate.