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 Monomict brecciaType of breccia whose clasts are composed of a single (mono-) rock type, possibly all from a single rock unit (e.g., L6 with L6). Monomict breccias are rare on the Moon because meteoroid impacts tend to mix different kinds of rocks. The example is a terrestrial granite breccia. Image Source:, cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber.
Purchased in 2003 coordinates not recorded
A single, fresh, partially fusion-crusted stone, composed of two matching fragments with weights of 505.10 g and 596.77 g, was acquired by an American dealer from Rissani, Morocco, in 2003. A later recovery of a 334 g paired stone has been reported. Analysis and classification was completed at Northern Arizona University (T. Bunch and J. Wittke) and NWA 1836 was determined to be an unusual 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 classified as a cumulate, monomict 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. The eucrite clasts, which occur throughout, have highly contorted shapes, and they are surrounded by cataclasticTexture found in metamorphic rocks in which brittle minerals have been broken, crushed, and flattened during shearing. mantles and zones of partially melted to recrystallized material. The shock levelA petrographic assessment, using features observed in minerals grains, of the degree to which a meteorite has undergone shock metamorphism. The highest stage observed in 25% of the indicator grains is used to determine the stage. Also called "shock level". ranges from S1 to S5. The textural appearance of this cumulate eucrite prompted one scientist to give it the nickname ‘Twisted Sister’.
The likely genesis of the HED clan began with the development of a 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 on a chondritic body, the result of heating by short-lived radiogenic elements. This was soon followed by the segregation of an ~75 km radius metallic 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., dated at 4.5661 (±0.0012) b.y. ago based on Hf–W, Al–Mg, and Mn–Cr systematics (Touboul et al., 2008). As the critical melt fraction reached ~15%, 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 staticcrystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. of 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 was initiated within a vigorously convecting magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava.. During this period, gravitational segregation became the predominant force and this resulted in the earliest 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 layer to form—a deep olivine-rich dunite layer ~150 km thick. Following this high-temperature phase, crystallization of an orthopyroxene-rich, ~13-km-thick, cumulate 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 layer ensued. Thereafter, residual liquids which were subjected to 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. (Holzheid and Palme, 2007) were extruded. This period of volcanism produced basaltBasalt is the most common extrusive igneous rock on the terrestrial planets. For example, more than 90% of all volcanic rock on Earth is basalt. The term basalt is applied to most low viscosity dark silicate lavas, regardless of composition. Basalt is a mafic, extrusive and fine grained igneous rock flows that solidified to form a thin 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 ~15 km thick (Mayne et al., 2008). This basaltic crustal rock was buried in turn by continual insulating flows of lavaHot molten or semifluid rock derived from a volcano or surface fissure from a differentiated and magmatically active parent body., resulting in its reheating and metamorphism and eventual formation of the Main Group–Nuevo Laredo trend 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.
The late-stage ascent of a portion of this Main Group magma was contaminated with a crustal partial melt to become the incompatible-element-rich Stannern trend eucrites. Some of the residual liquid, or more likely a separate REE-enriched liquid, was trapped at depths of up to ~10 km and underwent late fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. and re-equilibration processes to produce the cumulate eucrites; this cumulate material has been dated at ~60–100 m.y. after CAISub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they formation based on Hf–W, Sm–Nd, and Lu–Hf systematics (Touboul et al., 2008). Thereafter, surface eucritic material underwent impact gardening to form a 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). , during which time both diogenitic and xenolithic material was intermixed. Ultimately, lithification occurred forming the polymict howarditeOne type of meteorite in the HED (Howardite, Eucrite, Diogenite) achondrite group. Howardites are named after the English chemist Edward Howard (1774-1816), one of the pioneers of meteoritics. Consisting mostly of eucritic and diogenitic clasts and fragments, howardites are polymict breccias. However, they can also contain dark clasts of carbonaceous members of the HED clan. Complete crystallization was achieved within ~20 m.y.
Cumulate eucrites exist in a wide compositional range, and were formed as a result of fractional crystallization and gravitational settling within an evolved magma which had a composition similar to that of Nuevo Laredo. The resulting cumulate, composed of 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 in equilibrium with a trapped eucritic melt component, remained as a chemically closed systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with within a magma chamber. The incompatible 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 abundances present in cumulate eucrites were contributed by the trapped melt component. Studies of REEOften abbreviated as “REE”, these 16 elements include (preceded by their atomic numbers): 21 scandium (Sc), 39 Yttrium (Y) and the 14 elements that comprise the lanthanides excluding 61 Promethium, an extremely rare and radioactive element. These elements show closely related geochemical behaviors associated with their filled 4f atomic orbital. equilibrium partition coefficients and the chemical effects of interstitialTerm applied to ions or atoms occupying sites between lattice points. melt lead scientists to conclude that parental melts of cumulate eucrites were the same as, or similar to, those of noncumulate eucrites (Barrat, 2004). Final cooling probably occurred at depths of 3–8 km, consistent with intrusion into crustal material.
The asteroid 4 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 has a diameter of ~525 km with an outer basaltic crust thin enough (~10–25 km) to have been completely excavated down to diogenitic material, or deeper still into harzburgitic material, by the impact of a 10–20 km-sized impactor. This event would have produced the 460-km-wide, ~13-km-deep craterBowl-like depression ("crater" means "cup" in Latin) on the surface of a planet, moon, or asteroid. Craters range in size from a few centimeters to over 1,000 km across, and are mostly caused by impact or by volcanic activity, though some are due to cryovolcanism. observed near the south pole. Some of the thousands of identified fragments that were spalled into space by these impacts, appropriately named Vestoids, form a bridge between the orbitThe elliptical path of one body around another, typically the path of a small body around a much larger body. However, depending on the mass distribution of the objects, they may rotate around an empty spot in space • The Moon orbits around the Earth. • The Earth orbits around of Vesta and both the ν6 secular resonance and the 3:1 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. These fragments were eventually perturbed into an Earth-crossing orbit on a time scale of tens of m.y., from which pieces could easily 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. their way to Earth. One Vestoid, 1929 Kollaa, has a spectral signature consistent with a pyroxene composition similar to that of cumulate eucrites.
The Ar–Ar age of NWA 1836 and other eucrites were reset by large impact events at a time closely corresponding to the impact reset ages of lunar meteoritesAchondrite meteorites from the surface of the Moon. Most were found in the hot deserts of northern Africa and Oman and others were found in the cold desert of Antarctica, although one, a 19-gram specimen, was recovered in 1990 from Calcalong Creek, Australia. These stones are of great importance because,, which occurred during the Late Heavy BombardmentPeriod between ~4.0 to 3.8 Ga ago when the Moon and other objects in the Solar System were pounded heavily by wayward asteroids. The evidence for the Late Heavy Bombardment (LHB) includes the lunar maria basins and similar structures elsewhere, such as the Caloris Basin on Mercury and the great period ~3.8–4.1 b.y. ago. Age clusters for eucrites include some that are slightly younger than those representing the Moon; specifically, clusters at ~3.45, ~3.55, and ~3.78 are younger, while other clusters at ~3.90 and 3.98–4.07 b.y. parallel those of lunar samples (Bogard and Garrison, 2009). Both lunar and Vesta chronometer resetting events likely represent the same population of impactors, with impacts on Vesta continuing for a longer time. CRE age distributions of a statistical sampling of HEDs show that at least two major impact events occurred, ~22 and 39 m.y. ago, which delivered samples such as NWA 1836 to Earth.
Northwest Africa 1836 is a member of a relatively small cumulate eucrite group represented in part by the monomict breccias Medanitos, Binda, Talampaya, and Dhofar 007, the unbrecciated eucrites Moama, Moore County, Nagaria, and Serra de Magé, and various other eucrites from the hot and cold deserts of Northwest Africa and Antarctica, respectively. The photo above shows an 8.83 g partial slice of NWA 1836.