Johnstown

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 Click on Term to Read More
OrthopyroxeniteA rock composed primarily of orthopyroxene. Non-terrestrial orthopyoxenites include diogenites and a single martian meteorite, ALH 84001, that was found in the Allan Hills region of Antarctica in 1984. ALH 84001 is a cumulate rock consisting of 97% coarse-grained, Mg-rich orthopyroxene, with small amounts of plagioclase, chromite, and carbonate. It Click on Term to Read More
(≥90 vol% 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 Click on Term to Read More)
click on photo for a magnified view Fell July 6, 1924
40° 21′ N., 104° 54′ W. At 4:20 P.M. in Weld County, Colorado, four explosions were heard followed by the 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 Click on Term to Read More of 28 stones ranging in weight from a few grams to 23.5 kg. A crowd of mourners attending the burial service for John Moore Sr. at the Elwell Cemetery, two miles west of Johnstown, witnessed the fall. A few of the attendees walked across the road to a field and proceeded to dig up a 6.8 kg stone, now on display in the Denver Museum of Natural History. The fragment size distribution of this fall is reversed from the usual, with the larger fragments being found at the beginning of the strewnfield ellipse rather than at the end. This can be explained by the breakup of a large fragment late in its flight. The total collected weight of Johnstown is about 40.3 kg.

Johnstown is a rare calcium-poor 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 Click on Term to Read More consisting of medium green orthopyroxene (hypersthene) crystals broken up in a light green brecciated matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More. It is a 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: derived from a plutonicGeology: 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 Click on Term to Read More rock best classified as a cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. Click on Term to Read More orthopyroxenite with minor eucritic components. Some of these components are transitional to 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 Click on Term to Read More through the Y-75032-type 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 Click on Term to Read More and the Binda-type cumulate eucrites. These two types occupy a compositional gap between 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 crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More in eucrites and orthopyroxene crystallization in diogenites. Accessory FeNi-metal and sulfides (troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More and pentlanditeFe-Ni sulfide, (Fe,Ni)9S8, that is often associated with troilite, and found in the matrix and chondrules of CO, CV, CK and CR chondrites. The color is yellow-bronze with light bronze-brown streak and metallic luster. It typically forms during cooling of magmatic sulfide melts during the evolution of parent silicate melt. The Click on Term to Read More) have been identified by Gounelle and Alard (2009). They argue that the high content of highly siderophile elements (HSE) in diogenites suggests that the production of FeNi-metal did not result from 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 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 Click on Term to Read More or through desulfidation of sulfides, nor was the troilite a result of sulfurization of FeNi-metal. Instead, they believe an HSE-rich magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava. Click on Term to Read More underwent sequential crystallization to produce an FeNi-metal and, from an immiscibleThe property of liquids that are mutually insoluble (won't mix together) such as oil and water or metallic and silicate melts. Click on Term to Read More sulfide melt component, troilite.

Johnstown shows evidence for the presence of a low percentage of trapped interstitialTerm applied to ions or atoms occupying sites between lattice points. Click on Term to Read More melt in the form of 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 Click on Term to Read More and 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 Click on Term to Read More crystals, which occupies the space between orthopyroxene crystals (Barrat, 2004). A wide range of 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 abundances might indicate a trace 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 Click on Term to Read More component within orthopyroxene, while a varied LREE content between orthopyroxene and the bulk rock provides evidence for an LREE-enriched component in Johnstown. Through 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 Click on Term to Read More studies it was determined that the wide range of incompatible elements found among diogenites attests to their crystallization from a diversity of parental magma sources over a relatively short period of time, with the youngest sources experiencing up to ~90% 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. Click on Term to Read More (Schiller et al., 2010). These magma sources were shown to be distinct from those which crystallized basaltic eucrites. The onset of diogenite formation was dated at 0.7–1.3 m.y. after CAIsSub-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 Click on Term to Read More, which predates the crystallization of basaltic eucrites.

The HREE-enriched parental magmas that crystallized some diogenites such as Tatahouine are consistent with derivation from a melt composed of a harzburgitic cumulate at low pressure located below the eucritic 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 Click on Term to Read More. A scenario for the formation of Johnstown begins with crystallization of an orthopyroxene-rich cumulate pile within a magma 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 Click on Term to Read More or shallow layered intrusion within the eucritic crust, or alternatively, between a crustal layer and a convecting 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 Click on Term to Read More, on its 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. Click on Term to Read More. Remelting within this cumulate layer followed, causing a buoyant condition and subsequent intrusion into the overlying crust as diapirs (Barrat et al., 2008). In a similar case, formation of some HREE-enriched diogenites may have involved remelting of the residue from a source that sustained a high degree of melting (Stolper, 1977). Consequently, the previous theory surmising a sequential crystallization of diogenites and eucrites from a common, evolving parental melt is no longer favored. Based on elemental ratios, it is now believed that eucrites crystallized from parental melts which were not genetically related to those of diogenites. In any case, subsequent brecciationThe formation of a breccia through a process by which rock fragments of of various types are recemented or fused together. Click on Term to Read More and thermal metamorphism of surface lavas occurred, resulting in the formation of ordinary eucrites and howardites. Variable thermal histories also affected diogenites, some experiencing multiple reheating events, with the result that both equilibrated (elemental homogeneity) and unequilibrated (igneously zoned) diogenites were formed (Yamaguchi et al., 2010). The Johnstown diogenite was ultimately excavated at depth at high temperatures and ejected from the asteroid.

It is still under debate whether the source for diogenites was 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, or if the petrographic evidence is more indicative of a different parent body (Irving et al., 2014). Geological mapping of Vesta by the Hubble space telescopeA 2.4-meter reflecting telescope, which was deployed in low-Earth orbit (600 kilometers) by the crew of the space shuttle Discovery (STS-31) on 25 April 1990. The Hubble Space Telescope is a cooperative program of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA). HST's current complement Click on Term to Read More does show the sort of layered impact cratering that could serve as the source of these meteorites. Visible, near-infrared, and mid-infrared spectral studies of Vesta and HED 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 Click on Term to Read More samples have been conducted, which demonstrate that a compositional fit could exist between Vesta and the spectrum obtained for the mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More abundances in laboratory spectra for Johnstown, as well as for 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 Click on Term to Read More and 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 Click on Term to Read More samples (Donaldson Hanna and Sprague, 2008, 2009). While it may be true that some investigators 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. Click on Term to Read More the laboratory models consistent with the surface of Vesta, as a heterogeneous mixture composed primarily of howardite material at specific longitudes (–155° to 0°) and 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 Click on Term to Read More material at others (90° to –130°), others do not yet find convincing evidence for Vesta being the source for diogenites.

Based on 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. Click on Term to Read More patterns, the diogenites Johnstown, Bilanga, Roda, A-881548, and Dhofar 700 exhibit similar very low Eu/Eu* values and significant light REE depletions, and they may constitute a subgroup (Barrat et al., 2010). The investigators argue that these diogenites with very low Eu/Eu* ratios were contaminated by <10% of a low degree (<5%) eucritic crustal melt phase associated with a very large Eu anomaly. This scenario would be consistent with diogenites forming concurrent with or later than the formation of eucrites.

To see an alternative classification 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 for the diogenites based on mineralogical and petrographical features, proposed by Beck and McSween (2010) and modified by Wittke et al. (2011), click here. The photo above shows the cut face of a 7.0 g specimen. Below is the reverse side with minor fusion crustMelted exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with the air will raise a meteorite’s surface temperature upwards of  4800 K (8180 °F) and will melt (ablate) the surface minerals and flow backwards over the surface as shown in the Lafayette meteorite photograph below. Click on Term to Read More visible on the top edge, and the bottom image is an excellent petrographic thin sectionThin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope. micrograph of Johnstown, shown courtesy of Peter Marmet.

click on image for a magnified view
Photo courtesy of Peter Marmet