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 3A/4A
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 fireballA fireball is another term for a very bright meteor, generally brighter than magnitude -4, which is about the same magnitude of the planet Venus as seen in the morning or evening sky. A bolide is a special type of fireball which explodes in a bright terminal flash at its end, often with visible fragmentation. 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 brecciaGeneral term for all breccias that are neither monomict nor dimict. Modified from image source: http://www.saharamet.com/meteorite/gallery/HED/index.html. includes iron inclusions together with large areas of silicates including 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, 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., 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. Mineralogical studies have determined that matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. 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 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. has been explained through several competing theories. A recent model based on smoothed-particle hydrodynamicsStudy of fluid in motion. calls for the disruption and re-accretion of a 200–400 km differentiated asteroid with a molten 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.. 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 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 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 accretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the, melting, and crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. 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 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). 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 crustOutermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the of a molten parent body founder and sink through the 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 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 ageTime interval that a meteoroid was an independent body in space. In other words, the time between when a meteoroid was broken off its parent body and its arrival on Earth as a meteorite - also known simply as the "exposure age." It can be estimated from the observed effects 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.