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Vaca Muerta

Mesosiderite, group 1A
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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 such dunites that 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, ultimately resulting in the formation of a thin lower-crustal dunite layer along with more shallow olivine diogenite, diogenite, and cumulate eucrite lithologies. On the other hand, if the dunitic clasts are actually derived from mantle material, a scenario is required to explain how such material was incorporated into the regolith. However, it was argued by Barrat and Yamaguchi (2014) that magma chamber processes are unable to explain the chemical diversity of the diogenites (e.g., the range of heavy-REE ratios in diogenitic orthopyroxenes), and that neither assimilation of wallrock nor incorporation of a trapped melt component can account for this diversity. They contend that the diversity is more likely the result of variability in the respective initial parental melt compositions.

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 have isotopic signatures (e.g., 17O, 54Cr, and 50Ti [Rüfenacht et al., 2018]) that are virtually identical to the HED clan meteorites, many investigators consider that mesosiderites originated on the HED parent body, or at least within similar nebular reservoirs (see diagrams below). 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’

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Diagram credit: Wasson and Göpel, 77th MetSoc, #5446 (2014) 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 are 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 might exist there.

The ~226-km-diameter M-type asteroid, 16 Psyche, has been considered by some to be the mesosiderite parent body. The visible and near-infrared reflectance spectra of 16 Psyche indicate a surface composed of metal and pyroxene consistent with mesosiderites. A study of 16 Psyche conducted by Viikinkoski et al. (2018) has determined that its density (3.99 [±0.26] g/cm–3) is a good match to values calculated by Britt and Consolmagno (2003) for both mesosiderites (~4.25 g/cm–3) and the stony-iron Steinbach (~4.1 g/cm–3), but is inconsistent with that of iron meteorites (~7.8 g/cm–3).

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 border of the 3:1 orbital resonance located at 2.50 AU (a Kirkwood Gap associated with Jupiter), Fieber-Beyer et al. (2010, 2011) found that close similarities exist in the absorption spectra of eleven members (out of twelve members studied) of the Maria asteroid family (total of ~80 members) and the absorption spectra obtained for mesosiderites. The Maria family, which is likely the result of collisional disruption, contains HED-type pyroxenes and exhibits spectral reddening likely caused by FeNi-metal. Fieber-Beyer et al. (2011) ascertained that the near-Earth asteroid 1036 Ganymed is mineralogically similar to diogenites, with pyroxenes identical to those in Johnstown, and they also determined that the spectra of the asteroid is consistent with asteroids from the Maria family. Dynamical models of the Maria family predict that it is a probable source of some or all mesosiderites delivered to Earth (see diagram below). standby for maria asteroid family diagram
Diagram credit: Fieber-Beyer et al., Icarus, vol. 213, #2, p. 534, (2011)
‘The Maria asteroid family: Genetic relationships and a plausible source of mesosiderites near the 3:1 Kirkwood Gap’
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).

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NWA 1961

Mesosiderite, group 3A
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no coordinates recorded Three fragments of a stony-iron meteorite weighing together 288 g were found in Northwest African and sold to a collector in Rissani, Morocco. The meteorite was classified at the University of Münster, Germany (A. Bischoff) as a mesosiderite, while a 3A subtype was assigned to this meteorite by the Natural History Museum of Bern, Switzerland (B. Hoffmann). This mesosiderite has been shocked to stage S2 and is relatively unweathered with a grade of W0/1. Further information about the mesosiderite grouping scheme can be found on the Bondoc page.

The specimen of NWA 1961 shown above is a small 1.2 g end section. The top photo below shows the complete, flattened, egg-shaped, 189.8 g mass from which the specimen above was taken, while the bottom photo below shows a complete slice of NWA 1961 exhibiting an intricate mixture of silicates and FeNi-metal.

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Photos courtesy of JNMC–Zurich

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NWA 1242

Mesosiderite, group 2A
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Found 1985
no coordinates recorded Two fragments with a combined weight of ~7 kg were found near the village of Gillio, Libya by an oil worker. The masses were utilized as bookends until they were purchased from the worker’s daughter by meteorite dealer A. Lang in 1998. A sample was submitted to Rutgers University (Roger Hewins) for analysis and classification, and the name Sahara 85001 chosen to be most appropriate given the lack of an exact find location. However, results were not forthcoming, and four years later, an additional sample was submitted to Northern Arizona University (Ted Bunch). By this time in 2002, the NWA-series had been established by the Meteorite Nomenclature Committee of the Meteoritical Society and the name NWA 1242 was assigned to this mesosiderite; however, since it is a find location in Libya, this meteorite was included within the NWA-series. In the interim a couple of kilos of this meteorite was sold under the name Sahara 85001, and this name is now a synonym for the official name NWA 1242. Northwest Africa 1242 was classified at Northern Arizona University as a member of the small 2A metamorphic subgroup. (see the Bondoc page for further information about the Floran (1978) and Hewins (1984) classification schemes).

Northwest Africa 1242 contains scattered, cm-sized, metal nodules (see photo below), together with lithic clasts consisting of Ca-rich pyroxene, anorthitic plagioclase, pyrrhotite, chromite, and FeNi-metal. This unweathered mesosiderite (W0) is shocked to stage S1. The specimen shown above is an 11.67 g partial slice. The top photo below is a close-up image of a large, etched metal nodule, while the bottom photo shows the main mass of NWA 1242.

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Photos courtesy of Alan Lang—R. A. Langheinrich Meteorites

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Mesosiderite, group 3A
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Fell March 12, 1935
52° 0′ N., 19° 55′ E. At 12:52 A.M. near Lowicz, Poland, a shower of stones was seen and heard to fall over an area of 9.2 km² along a trajectory from west to east. However, in contrast to most other fall distributions, the smallest fragments were recovered in the farthest part of the strewnfield near Seligow, while the larger pieces were found in the closest part in Krepa. This reversed distribution could be the result of fragmentation low in the atmosphere. At least 83 and perhaps over 100 stones with a total weight of over 60 kg were eventually recovered, but many were subsequently lost during World War II.

Mesosiderites have been divided into different groups (Floran, 1978 and Hewins, 1984) based on their composition and degree of thermal metamorphism, with Lowicz falling into group 3A (see the Bondoc page for further information about the grouping scheme). The formation of mesosiderites has been examined in the context of several different models. See the Estherville page for details concerning the most current models. Lowicz has a CRE age of ~53 m.y. The above specimen is a very thin partial slice approximately 19 mm × 11 mm × 1 mm. It weighs 0.9 g and shows abundant metal mixed with the silicates.

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Mesosiderite, group 3A/4A
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Fell May 1879
43° 25′ N., 94° 50′ W. At 5:00 P.M. on May 10, in Estherville, Iowa, several large masses and hundreds of small iron nodules fell after a fireball was seen and sonic booms heard. Over 700 pounds of material was recovered, including one mass of ~437 pounds and one of 151 pounds. The largest mass was divided among the London, Paris, and Vienna Museums while the location of the smaller mass is unknown. Hundreds of the atmospherically ablated iron nodules are preserved at Yale’s Peabody Museum.

This polymict breccia includes iron inclusions together with large areas of silicates including olivine, pyroxene, and plagioclase. Mineralogical studies have determined that matrix olivines and olivine clasts are most likely xenoliths from separate parent bodies, which were assimilated together onto the mesosiderite planetesimal during impact late events (Hassanzadeh et al., 1990). Lithic clasts of eucritic and diogenitic material are present.

The formation of mesosiderites on their parent body has been explained through several competing theories. A recent model based on smoothed-particle hydrodynamics calls for the disruption and re-accretion of a 200–400 km differentiated asteroid with a molten core. The impactor is calculated to have been a 50–150 km body with an impact speed of 5 km/s. This event initially caused rapid cooling (~0.1°C/y.) from high temperature equilibration, followed by very slow cooling (~0.5°C/m.y.) as the brecciated material was deeply covered by a massive debris blanket. The relatively young Ar–Ar ages of mesosiderites of 3.7–4.1 b.y. reflect this period of very slow cooling. Weakly shocked olivine was sequestered into the core at the time of the catastrophic impact, as molten metal was mixed with cold crustal fragments during re-accretion. Recent dating of zircons in Estherville by Haba et al. (2014) places the formation age of this mesosiderite (i.e., metal-silicate mixing and crustal remelting) at 4.520 (±027) b.y. Cooling rate studies conducted by Sugiura and Kimura (2015) on a number of mesosiderite samples indicate that Estherville, Vaca Muerta, NWA 2924, and Dong Ujimqin Qi experienced rapid cooling from peak temperatures down to intermediate temperatures, while others including NWA 1242, NWA 1878, Crab Orchard, ALH 77219, and A-882023 cooled much more slowly over the same temperature range.

A more conventional theory calls for the accretion, melting, and crystallization of the large parent body ~4.56 b.y. ago. A period of impact-melting and metamorphism ensued until 3.9 b.y. ago, by which time the brecciated nature of the mesosiderite parent body had been established. It was at this time, 3.9 b.y. ago, that a major thermal event occurred, raising temperatures to as high as 500°C. A likely cause for this event is the collisional disruption and gravitational reassembly of the asteroid. The surface breccias were buried under a deep regolith where slow cooling and annealing proceeded. Subsequent impacts excavated this deeply buried material and some of it was ejected into space, establishing a range of cosmic-ray exposure ages for mesosiderites of ~10–340 m.y. Estherville has a Sm–Gd-based CRE age of 70 (±7) m.y. (Albrecht et al., 2000).

A more outdated theory has the basaltic crust of a molten parent body founder and sink through the mantle to the metallic core where mixing occurred. Subsequent collisions exposed this stony-iron layer and delivered fragments to Earth. It is notable that the O-isotopic values of the mesosiderites are almost identical to those of the HED suite of meteorites, implying that a genetic link exists between these disparate groups (Greenwood et al., 2006). Conversely, multiple line of evidence indicate that separate parent bodies were probably involved.

In the classification scheme of Floran, 1978 and Hewins, 1984, Estherville was assigned as a transitional member to group 3A and 4A (see the Bondoc page for further information about the grouping scheme). Calculations based on cosmogenic radionuclides show that Estherville had a pre-atmospheric diameter of at least 62 cm. Its cosmic-ray exposure age of ~70 m.y. is similar to that of Crab Orchard and Chinguetti, suggesting a common ejection event for these three mesosiderites. The Estherville specimen shown above is an 18.6 g complete slice, which exhibits a stony matrix containing iron inclusions and numerous olivine crystals.