Vaca Muerta

MesosideriteOne of two main types of stony-iron meteorite, the other being pallasites. Mesosiderites are a mixture of approximately 50% basaltic, gabbroic and orthopyroxenitic silicates and 50% Ni-Fe metal and sulfides. The name derives from the Greek "mesos" meaning "middle" or "half" and "sideros" for "iron;" hence "half-iron". The silicates are, group 1A

Found during or before 1861
25° 45′ S., 70° 30′ W. approx. The first of many large masses of this polymict, metal–silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the brecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material 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 fieldArea on the surface containing meteorites and fragments from a single fall. Also applied to the area covered by tektites, which are produced by large meteorite impacts. Strewnfields are often oval-shaped with the largest specimens found at one end. Given that the largest specimens go the greatest distance, a meteoroid's 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 meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and has been accounted for, while the pre-atmospheric mass is estimated to have been over 6,000 kg. Calculations indicate that the Vaca Muerta meteoroidSmall rocky or metallic object in orbit around the Sun (or another star). 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 differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities.   When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including of the mesosiderite parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today., 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 metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn that became mixed with cooler silicates was derived from the differentiated coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. 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 findMeteorite not seen to fall, but recovered at some later date. For example, many finds from Antarctica fell 10,000 to 700,000 years ago. any eucritic inclusions in any of the fragments from this very large mesosiderite, and the O- and Cr-isotopic compositions of the orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur and plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite in this mesosiderite are virtually identical to those of diogenitesDiogenites belong to the evolved achondrite HED group that also includes howardites and eucrites. They are named after the Greek philosopher Diogenes of Apollonia, of the 5th century BCE, who was the first to suggest that meteorites come from outer space (a realization forgotten for over 2,000 years). They are, 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 diogeniteDiogenites belong to the evolved achondrite HED group that also includes howardites and eucrites. They are named after the Greek philosopher Diogenes of Apollonia, of the 5th century BCE, who was the first to suggest that meteorites come from outer space (a realization forgotten for over 2,000 years). They are 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 lavaHot molten or semifluid rock derived from a volcano or surface fissure from a differentiated and magmatically active parent body. flows. Other evolutionary stages considered to have occurred include reductionOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants processes and episodic impact events, the latter resulting in re-melting, metal–silicate mixing and brecciationThe formation of a breccia through a process by which rock fragments of of various types are recemented or fused together., formation of quench textures, mixing of deep silicates and near-surface silicates, regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles).   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 pigeoniteLow-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, found as a major mineral in eucrites and shergottites. In order to be considered pigeonite, the clinopyroxene must contain 5 to 20 mol % of calcium (Wo5 - 20). Chondrites of petrologic types 4 and below contain significant low-Ca clinopyroxene. During metamorphism to higher temperatures, all existing) with minor amounts of tridymiteSilica group mineral in which the tetrahedra occur in sheets. Tetrahedra alternately point up or down to share oxygen with tetrahedra of other sheets, forming six-sided rings perpendicular the sheets. Tridymite has a fairly open structure and accommodates Na+, K+ and Ca2+; charge balance is achieved by Al3+ ↔ Si4+. (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 elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of compositions of the Vaca Muerta clasts are most commonly similar to cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. eucritesMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after 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 eucriteMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after, diogeniteDiogenites belong to the evolved achondrite HED group that also includes howardites and eucrites. They are named after the Greek philosopher Diogenes of Apollonia, of the 5th century BCE, who was the first to suggest that meteorites come from outer space (a realization forgotten for over 2,000 years). They are, 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 transitWhen a small celestial body moves in front of a much larger one (as when Mercury or Venus appears in silhouette against the solar disk or when a satellite passes in front of Jupiter or Saturn). The shadow of a satellite may also transit the disk of its primary. 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 mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in material (Greenwood et al., 2015).

Mandler and Elkins-Tanton (2013) proposed a formation scenario for such dunites that involves a two-stage crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. process: first, an equilibriumTerm used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static equilibrium occurs when the components of forces and torques acting in one direction are balanced by components of forces and torques acting in the opposite direction. A system in static crystallization process from the late-stage liquid after 60–70% solidification of the global magma oceanCompletely molten surfaces of terrestrial planets or moons that formed soon after accretion. Samples returned by the Apollo missions provide evidence of a lunar magma ocean, crystallization of which produced a stratified Moon with a low-density crust formed by accumulation of the mineral plagioclase overlying a higher density mantle of; second, a fractional crystallizationA crystallization process in which minerals crystallizing from a magma are isolated from contact with the liquid. It is a key process in the formation of igneous rocks during the process of magmatic differentiation. Also known as crystal fractionation. process within an ascended, high-level (crustal) plutonGeology: Igneous intrusive body that forms when magma is injected into host rocks and solidifies. Plutons occur in the crust of asteroids undergoing differentiation or planets. Named after Pluto, the Roman god of the underworld. Plutonic rocks are the rocks found within a pluton. Astronomy: Category of planet including all composed of the former extracted residual melt, ultimately resulting in the formation of a thin lower-crustal dunite layer along with more shallow olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion diogenite, diogenite, and cumulate eucriteMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after 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 magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava. 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.
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 oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: ¹H, ²H (deuterium), and ³H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. studies’
(http://dx.doi.org/10.1016/j.chemer.2016.09.005)

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 fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If 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 clastA mineral or rock fragment embedded in another rock. 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 elementLiterally, "iron-loving" element that tends to be concentrated in Fe-Ni metal rather than in silicate; these are Fe, Co, Ni, Mo, Re, Au, and PGE. These elements are relatively common in undifferentiated meteorites, and, in differentiated asteroids and planets, are found in the metal-rich cores and, consequently, extremely rare on (HSE) contents of mesosiderites, Xu et al. (2011) found that the HSE patterns are very similar to those in FeNi-metal from H-group chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such, 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 pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. consistent with mesosiderites. A study of 16 Psyche conducted by Viikinkoski et al. (2018) has determined that its densityMass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much (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 resonanceDynamic relationship between bodies in heliocentric orbits in which a small body has an orbital period that is a simple fraction of a nearby larger body such as Asteroid Belt objects and Jupiter. The periodic gravitational tug of the large body induces changes in the orbit of the smaller body located at 2.50 AUThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The (a Kirkwood GapLack of asteroids in regions within the asteroid belt (between Mars and Jupiter) coinciding with orbital periods that are simple fractions of Jupiter’s own orbital period. The absence of asteroids in these gaps is due to resonance with Jupiter’s gravitational influence.         associated with Jupiter), Fieber-Beyer et al. (2010, 2011) found that close similarities exist in the absorptionTransfer of energy to a medium as a particle or electromagnetic radiation passes through it. Absorption of electromagnetic radiation is the combined result of Compton scattering, σ, and photoelectric absorption, τ. It may be quantified: where, t = thickness, ρ = density, and μ = mass absorption coefficient, which combines Compton and photoelectric effects (μ = σ + τ). spectra of eleven members (out of twelve members studied) of the MariaBroad low plains surrounded by basin-forming mountains, originally thought to be a sea (pl. maria). This term is applied to the basalt-filled impact basins common on the face of the Moon visible from Earth.   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 asteroidAsteroids with orbits that bring them within 1.3 AU (195 million km) of the Sun. NEAs are a dynamically young population whose orbits evolve on 100-million-year time scales because of collisions and gravitational interactions with the Sun and the terrestrial planets. These asteroids are probably ejected from the main belt 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).
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’
(https://doi.org/10.1016/j.icarus.2011.03.009) Based on silicate matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. 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 VestaThird largest and fourth brightest asteroid; it was discovered in 1807 by Heinrich Olbers and named for the ancient Roman goddess of the hearth. 4 Vesta has a basaltic surface composition and an average density not much less than that of Mars. Evidently lava once flowed here indicating that the 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 inclusionFragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. likely formed from a localized impact melt of an existing cumulate eucrite lithology (photos not to the same scale).