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 strewnfield has dimensions of 11.5 × 2.1 km.

A two-stage irradiation history has been proposed by Hidaka and Yoneda (2011). In the first stage, occurring <4.4 b.y. ago, the silicates were irradiated near the surface prior to mesosiderite formation. Subsequently, an iron meteorite collided with the silicate layer after differentiation, producing the mesosiderite precursor material. Ensuing episodic impact events on this asteroid produced early melting, metal–silicate mixing, quench textures, regolith gardening, mixing of deep and near-surface (eucritic pebbles) silicates, thermal metamorphism, and degassing, including the possible partial or total collisional disruption and gravitational reassembly of the parent body ~3.9 b.y. ago. This episode was followed by slow cooling until the impact excavation and ejection from depth of the Vaca Muerta mesosiderite meteoroid 138 m.y. ago, producing the second stage of irradiation (Bajo and Nagao, 2011).

Present throughout the 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 were formed during a very early magmatism on the mesosiderite parent body, ~4.563 (±0.015) b.y. ago. The Vaca Muerta silicate phase, which consists of gabbroic pebbles along with a basaltic component, has a CRE age based on Cl–Ar and Sm–Gd of 138 (±11) m.y., which reflects only irradiation during its transit to Earth (Albrecht et al., 2000; Hidaka and Yoneda, 2011). The eucritic pebbles have an additional Kr–Kr-based cosmogenic age of >60 m.y., which reflects the additional exposure received by the precursor material during residence at shallow depth on the parent asteroid. Studies of cosmogenic 131Xe which was produced while the meteoroid was in transit to Earth indicates that the progenitor of Vaca Muerta had been located deep within the source object (Bajo et al., 2011). In addition, large clasts of olivine have been recovered in Vaca Muerta which likely represent mantle material. The olivine has O-isotopes consistent with an origin on the mesosiderite parent body rather than a xenolithic origin (Greenwood et al., 2009). A terrestrial age of <2000 years was calculated for this mesosiderite (Jull et al., 2009).

Because mesosiderites have an O-isotopic signature almost identical to that of the HED clan meteorites, it has been conjectured by some 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 suggests 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 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 parent body, the HED parent body shows no evidence of a crustal melting episode, or a metal–silicate mixing event (Rubin and Mittlefehldt, 1993). In another study of the highly siderophile element (HSE) contents of mesosiderites, Xu et al. (2011) found that the HSE patterns are very similar to those in FeNi-metal from H-group chondrites, and they suggest a genetic link may exist there.

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 members of the Merxia and Agnia families, may be similar to the metal–pyroxene-rich mesosiderites (Sunshine et al., 2004). In a similar way, utilizing near-IR spectrography of asteroids located near the 3:1 orbital resonance (Kirkwood gap of Jupiter), Fieber-Beyer et al. (2010) found that close similarities exist in the absorption spectra of eleven members of the Maria genetic asteroid family and that of mesosiderites. The Maria asteroid family, which likely experienced collisional disruption, contain HED-type pyroxenes and exhibit spectral reddening likely caused by FeNi-metal. Dynamical models of the Maria family predict that it is a probable source of mesosiderites to Earth.

Based on silicate matrix textures, Vaca Muerta has been placed into group 1A. (see the Bondoc page for further information about the grouping scheme). It had a pre-atmospheric diameter of ~100–140 cm (Reedy et al., 1994). The photo above shows a 9.1 g end section of a stony-iron portion of Vaca Muerta on the left, and a 51.6 g end section of a naturally faceted, eucritic pebble inclusion on the right.

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