VACA MUERTA

The first of many large masses of this polymict, metal–silicate breccia was found by a prospector in Atacama, Chile, ~100 miles SE of Taltal. The discovery was first reported by the Chilean geologist Ignacy Domeyko in 1862. Possibly many tons of Vaca Muerta was found and subsequently broken apart by 19th century miners in order to retrieve the metal-rich portion for smelting. About 4,000 kg of this meteorite have been recovered, and the pre-atmospheric mass is estimated to have been 6,000 kg. The extensive strewn field has dimensions of 11.5 x 2.1 km.

Present throughout this mesosiderite are eucritic inclusions, characterized as monogenic basalts with gabbroic textures. The major and trace element compositions of these inclusions are most similar to the cumulate eucrites, and they were formed during a very early melting event on the mesosiderite parent body, ~4.56 b.y. ago. Episodic impact events produced melting, quench textures, and thermal metamorphism, until the collisional disruption and gravitational reassembly of the parent body occurred ~3.9 b.y. ago.

Because mesosiderites have an O-isotopic signature almost identical to that of the HED clan meteorites, it has been conjectured that mesosiderites originated on the HED parent body, or at least that a genetic link exists (Greenwood et al., 2006). However, studies of the Eu/Sm systematics suggest otherwise. The silicates in mesosiderites have a Eu/Sm ratio higher than CI and so have a positive Eu anomaly. The howardites of the HED clan have a Eu/Sm ratio less than CI and so have a negative Eu anomaly. Further evidence suggesting separate parent bodies rather than distinct regions on a common parent body includes the fact that the CRE ages are not correlated; most howardites are established into two clusters of ~21 and ~38 m.y., while the mesosiderites have widely ranging ages from <10 to 340 m.y. Moreover, in contrast to the mesosiderite PB, the HED parent body shows no evidence of a crustal melting episode, or a metal–silicate mixing event (Rubin and Mittlefehldt, 1993).

The 264 km, M-type asteroid, 16 Psyche, has been considered by some to be the mesosiderite parent body. A recent detailed spectroscopic survey of main-belt and near-Earth asteroids has identified certain differentiated asteroids having high concentrations of high-Ca pyroxene, abundant plagioclase, minor olivine, and a significant metal component. These asteroids, including the S-type asteroids 17 Thetis and the members of the Merxia and Agnia families, may be similar to the metal–pyroxene-rich mesosiderites (Sunshine et al., 2004).

Vaca Muerta has a cosmic-ray exposure age of ~133 m.y., and 14C-derived terrestrial age of 3,500 (±1,300) years; however, estimates based on a more accurate estimation of its initial size suggest a lower terrestrial age. Based on silicate matrix textures, it has been placed into group 1A. (see the Bondoc page for further information about the grouping scheme). The above photo shows a 9.1 g end section of the stony-iron mass (left) with a 51.6 g end section of a naturally faceted, eucritic pebble inclusion (right).

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