Mesosiderite, group 1A
standby for vaca muerta photo standby for vaca muerta photo
Found during or before 1861
25° 45' S., 70° 30' W. approx.

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, in a landscape described as vegetation-free, sand and gravel covered open plains, crossed by dry river beds. Possibly many tons of Vaca Muerta were found and subsequently broken apart by 19th century miners to retrieve the metal-rich portion for smelting.

In 1985, a mining engineering student named Edmundo Martinez explored the Atacama Desert region from which Vaca Muerta was originally found by miners a century earlier (Pedersen et al., 1992). Field work by Martinez and his associates led to the characterization of many sites in the strewn field and the recovery of both undisturbed and disturbed masses of this mesosiderite, in sizes of 13 g–312 kg and 2 g–850 kg, respectively. In addition, they found a large pile of Vaca Muerta igneous clasts discarded by 19th century miners who were seeking the more metal-rich stones for smelting. Almost 4,000 kg of this meteorite has been accounted for, while the pre-atmospheric mass is estimated to have been over 6,000 kg. Calculations indicate that the Vaca Muerta meteoroid had a pre-atmospheric diameter of ~100–140 cm (Reedy et al., 1994). The extensive strewnfield has dimensions of 11.5 × 2.1 km.

A two-stage irradiation history has been proposed for mesosiderites 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. Subsequent to differentiation of the mesosiderite parent body, a low velocity collision of a large (~50–150 km diameter) iron projectile and a large (~200–400 km diameter), still molten iron planetesimal occurred ~4.4 b.y. ago, melting and mixing the cool silicate layer of the planetesimal with the molten Fe-metal of the projectile into complex breccias. The partial or total collisional disruption and gravitational reassembly of the target body is considered a strong likelihood by some (Haack et al., 1996). Others favor a scenario in which the molten metal that became mixed with cooler silicates was derived from the differentiated core of the mesosiderite planetesimal itself through a severe impact, without a catastrophic disruption and reassembly (Scott et al., 2001).

Based on in-depth mineralogical and textural studies of numerous samples of a massive >80 kg mesosiderite, portions of which having been described and classified by several independent labs, Bunch et al. (2014) have proposed an alternate petrogenetic history for the mesosiderites. Because they did not find any eucritic inclusions in any of the fragments from this very large mesosiderite, and the O- and Cr-isotopic compositions of the orthopyroxene and plagioclase in this mesosiderite are virtually identical to those of diogenites, they developed a scenario in which a small, previously cooled iron-rich asteroid collided with a still warm (~800°C), previously differentiated diogenitic asteroid at a low angle and relatively low velocity (<6 km/s). The frictional shear forces created in this oblique impact produced small rotating molten metal spherules which mechanically incorporated diogenite fragments, resulting in a minimally-shocked, fragmental stony-iron layer of debris.

Each of these scenarios envision a brief period of rapid cooling due to the mixing of warm and cold material, followed by very slow cooling and annealing consistent with deep burial of the mesosiderite precursor material under an extensive debris blanket and/or subsequent lava flows. Other evolutionary stages considered to have occurred include reduction processes and episodic impact events, the latter resulting in re-melting, metal–silicate mixing and brecciation, formation of quench textures, mixing of deep silicates and near-surface silicates, regolith gardening, thermal metamorphism, and degassing which reset the Ar–Ar age chronometer to reflect an age of ~3.6–3.9 b.y. Thereafter, the impact excavation and ejection from depth of the mesosiderite meteoroid (~138 m.y. ago for Vaca Muerta) initiated a second stage of irradiation (Bajo and Nagao, 2011). Other calculated CRE ages of mesosiderites reflect different excavations occurring over the past 10–150 m.y. A terrestrial age of <2000 years was calculated for Vaca Muerta (Jull et al., 2009).

Mesosiderites of subgroup 1 have experienced the least thermal metamorphism and are more commonly richer in eucritic enclaves (Ikeda et al., 1990), some of which have escaped significant recrystallization. Vaca Muerta eucritic enclaves or clasts, also known as "eucritic pebbles", are mostly characterized as monogenic basalts with granular to gabbroic textures, composed primarily of silicate (93–98 wt% as plagioclase and pigeonite) with minor amounts of tridymite (derived from both the primary eucritic material and through reduction processes sustained by phosphorus in FeNi-metal), phosphates, oxides, and various opaques. The major and trace element compositions of the Vaca Muerta clasts are most commonly similar to cumulate eucrites having a more magnesian composition (ilmenite-free; Mg# ≥0.47), whereas other mesosiderites might contain more ferroan clasts (ilmenite-bearing; Mg# ≤0.46) and show similarities to non-cumulate eucrite, diogenite, and dunite material. The eucritic clasts were formed during the very early stages of magmatism on the mesosiderite parent body ~4.563 (±0.015) b.y. ago, representing both magmatic rocks (ilmenite-bearing) and cumulate and/or residual rocks (ilmenite-free). The significant petrographic differences that exist between these mesosiderite eucritic clasts and the HED meteorites may indicate they originated on separate parent bodies.

Both the Vaca Muerta eucritic clasts (see example 1, example 2, example 3, and right photo above) and the metallic-melt host phase have CRE ages based on Cl–Ar and Sm–Gd of 138 (±11) m.y., which reflects only irradiation received during 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 prior to impact ejection. Studies of cosmogenic 131Xe that 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, rare examples (0–6 vol%; Prinz et al., 1980) of large, coarse-grained, magnesian olivine (dunite) clasts have been found in Vaca Muerta and some other mesosiderites, but it is still unresolved whether they represent higher-level cumulates or mantle material (Greenwood et al., 2015). Mandler and Elkins-Tanton (2013) proposed a formation scenario for the mesosiderite dunites which involves a two-stage crystallization process: first, an equilibrium crystallization process from the late-stage liquid after 60–70% solidification of the global magma ocean; second, a fractional crystallization process within an ascended, high-level (crustal) pluton, composed of the former extracted residual melt. On the other hand, if the dunitic clasts are derived from mantle material, a scenario is required to explain how such material was incorporated into the regolith.

The diverse suite of silicate clasts (e.g., basalts, gabbros, orthopyroxenites, dunites) present in Vaca Muerta and other mesosiderites have O-isotopes consistent with an origin on the mesosiderite parent body rather than a xenolithic origin (Greenwood et al., 2009, 2013, 2015). Because mesosiderites do have an O-isotopic signature virtually identical to that of the HED clan meteorites, many investigators consider that mesosiderites originated on the HED parent body, or at least that a genetic link exists. Further information regarding the origin of the dunitic clasts in our collections can be found on the NWA 2968 page.

standby for o-isotopic diagram
Diagram credit: Greenwood et al., 2017
For an explanation of the diagram components see the open access article in Chemie der Erde, vol. 77, p. 25 (2017)
'Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies'

However, studies of the Eu/Sm systematics suggest that the HED clan meteorites and the mesosiderites did not originate on a common parent body. The silicates in mesosiderites have a Eu/Sm ratio higher than CI and so exhibit a positive Eu anomaly, while the howardites of the HED clan have a Eu/Sm ratio less than CI and show a negative Eu anomaly. Further evidence supporting separate parent bodies rather than distinct regions on a common parent body includes the fact that the CRE ages are not correlated; most howardites fall 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 accord with this viewpoint, it was shown by D. Mittlefehldt (2014) that mesosiderites contain basaltic clasts that have a wide range of anomalous characteristics, while howardites do not. He cited these anomalies as evidence for the fact that clast formation succeeded metal–silicate mixing on the parent body, and he also noted that petrologic data is indicative of clast formation preceding impact gardening. Therefore, the apparent lack of such anomalies in components of howardites is inconsistent with a common parent body for both HED and mesosiderite meteorites. 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, contains 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 is theorized by one investigative team that Vesta and the Vestoids, along with the isotopically similar mesosiderites and IIAB irons, were themselves the products of a catastrophic breakup of an even larger parent object which they have named ‘Opis’ (Irving et al., 2009). In Greek mythology, Opis was the wife of Saturn, whose children were Jupiter, Neptune, Pluto, Juno, Ceres, and Vesta. The left photo above shows a 9.1 g end section of a metallic-melt host phase of Vaca Muerta, while the right shows a 51.6 g end section of a naturally faceted, eucritic inclusion likely formed from a localized impact melt of an existing cumulate eucrite lithology (photos not to the same scale).