Martian ChassigniteThe group is part of the SNC martian trio and named after the meteorite seen to fall in Chassigny, France, in 1815. Its subsequent recovery led to it being one of the first meteorites to be recognized as a genuine rock from space. Chassigny resembles a terrestrial dunite - a Click on Term to Read More
click on photos for a magnified view Fell October 3, 1815
47° 43′ N., 5° 22′ E. One or more stones fell after sonic booms were heard around 8:00 A.M. near the village of Chassigny, France. Many small fragments were immediately recovered around a small impact hole. The total recovered weight of this first martian meteoriteOver 30 of the meteorites found on Earth almost certainly came from Mars (see http://www.imca.cc/mars/martian-meteorites.htm and http://www2.jpl.nasa.gov/snc/). All but one belongs to the group known as SNC meteorites, which includes the shergottites, nakhlites, and chassignites. SNC meteorites contain minerals that crystallized within the past 1.35 to 0.15 Ga, making them Click on Term to Read More is estimated to be about 4 kg, although only about 800 g is accounted for today. Chassigny, NWA 2737, and NWA 8694 (photo courtesy of L. Labenne) constitute the only known samples of martian dunite.
Chassigny consists of 90 vol% Mg-rich 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 (Fo68), resembling a terrestrial 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 dunite (although more FeO-rich), with the remaining constituents comprising the pyroxenes 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 and augiteHigh-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, that occurs in many igneous rocks, particularly those of basaltic composition. In order to be considered augite, the clinopyroxene must contain 20 to 45 mol % of calcium (Wo20 - 45). An important and unique Martian meteorite is NWA 8159, that has been classified as an augite basalt. Click on Term to Read More (5%), 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 feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. Click on Term to Read More (2%), and chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More (1.4%), along with minor pyrite and both fluorapatite (in melt inclusions) and chlor-fluorapatite (in interstitialTerm applied to ions or atoms occupying sites between lattice points. Click on Term to Read More maskelyniteNatural glass composed of isotropic plagioclase produced during shock metamorphism (not melting) at pressures of ~30 GPa. Maskelynite is commonly found in shergottites though also found in some ordinary chondrites, HED and lunar meteorites. It is also found in association with meteorite impact craters and crater ejecta. Named after British Click on Term to Read More). Chassigny contains noble gasesElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More different from the martian atmosphere, and is presumed to originate from the martian 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 Click on Term to Read More.
In their study of Cl-isotopic compositions in Chassigny apatites, Shearer et al. (2018) ascertained that two distinct Cl reservoirs existed during its formation. One reservoir is represented by fine-grained apatite found in olivine-hosted melt inclusions. Chlorine present in this fine-grained apatite is associated with the primitive mantle and consists of isotopically-light Cl (δ37Cl = –4 to –6‰). The other reservoir is associated with the martian 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, and is represented by late-stage coarse-grained apatite grains that occur within intercumulus regions in Chassigny. This crustal reservoir consists of isotopically-heavy Cl (δ37Cl = >0) likely resulting from the preferential loss of light 35Cl from the martian atmosphere and subsequent exchange processes at the surface. While Chassigny and the nakhlites incorporated an isotopically-heavy crustal component, perhaps through assimilation of a Cl-rich crustal component (0.5–2.0%) or infiltration of a Cl-rich fluid, the NWA 2737 chassignite, which crystallized in a lower stratigraphic sequence, contains only isotopically-light Cl derived from the mantle reservoir (see a schematic illustration below).
click on image for a magnified view
Diagram credit: Shearer et al., GCA, vol. 234, p. 32 (2018)
‘Distinct chlorine isotopic reservoirs on Mars. Implications for character, extent and relative timing of crustal
interactions with mantle-derived magmas, evolution of the martian atmosphere, and the building blocks of an early Mars’
(http://dx.doi.org/10.1016/j.gca.2018.04.034) McCubbin et al. (2007) posited that the Cl-enrichment of the interstitial material developed as a result of the upward migration of a high-temperature, Cl-rich fluid, which then lost Cl and gained water as it percolated through the cumulus pile along interstitial pathways. McCubbin and Lindsley (2006) inferred from the structural formula of apatite that the water content was likely very low (<0.4 wt%) during 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 of Chassigny; apatites from basaltic shergottitesIgneous stony meteorite with a Martian origin consisting mainly of plagioclase (or a shocked glass of plagioclase composition) and pyroxene. They are the most abundant type of SNC meteorites and the type member is the Shergotty meteorite, which fell in India in 1865. Shergottites are igneous rocks of volcanic or Click on Term to Read More are more water-rich. In a contrasting study, McCubbin et al. (2009) found that kaersutite and Ti-biotite in Chassigny melt inclusions contain higher abundances of water than previously measured, a value which correlates to a parental source 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 water content between ~460 and 840 ppmParts per million (106). Click on Term to Read More (0.5–0.8 wt%), while lower abundances of Cl and F were observed.
In support of the conclusion that martian parental magmas were water-poor, results of analyses by Filiberto and Treiman (2009) of both apatite and kaersutite in martian meteorites are consistent with a parental melt volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). content that was Cl- and S-rich and water-poor, properties that would affect 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 compositions and crystallization temperatures similar to the way in which a hydrous parental magma would. They cite the Cl-rich surface rocks revealed in analyses by the Mars Exploration Rovers in support of their theory. Employing volatile partitioningThe tendency of elements to prefer one mineral phase relative to another or to preferentially enter the solid or remain in the liquid during crystallization. Click on Term to Read More models, Giesting and Filiberto (2013) calculated the halogenReactive nonmetal that is in Group 17 (VIIA) of the periodic table: F, Cl, Br, I and At. All of halogen elements are strongly electronegative. Click on Term to Read More content of the magma that was precursor to the chassignitesThe group is part of the SNC martian trio and named after the meteorite seen to fall in Chassigny, France, in 1815. Its subsequent recovery led to it being one of the first meteorites to be recognized as a genuine rock from space. Chassigny resembles a terrestrial dunite - a Click on Term to Read More based on the Cl/Fl ratios in existing hydrous minerals (apatite) and in magmatic melt inclusions hosted by olivine (apatite, kaersutite, and biotite). A crystallization sequence of kaesutite ⇒ apatite was found to be consitent with the data, while biotite is thought likely to have crystallized from a late-stage, halogen-rich metasomatic fluid; the latter is also considered to be a factor in the composition of ferromagnesian minerals, as well as an influence upon the acidic nature of the martian environment.
Three different types of glass-bearing inclusions (up to 0.2 mm) are present in Chassigny: 1) multi-crystalline or multiphase (glass + several mineral inclusions); 2) crystal + glass or monocrystal (glass + a single mineral inclusionFragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. Click on Term to Read More); and 3) pure glass (Monkawa et al., 2003; Varela and Zinner, 2015). Multiphase inclusions typically contain low- and high-Ca 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. Click on Term to Read More, feldspar- and Si-rich glasses, sometimes low-H, Ti-rich kaersutite, and more rarely, chlorapatite, FeS, chromite, anorthiteRare compositional variety of plagioclase and the calcium end-member of the plagioclase feldspar mineral series with the formula CaAl2Si2O8. Anorthite is found in mafic igneous rocks such as anorthosite. Anorthite is abundant on the Moon and in lunar meteorites. However, anorthite is very rare on Earth since it weathers rapidly Click on Term to Read More and albite. Mineralogical evidence suggests that these inclusions could have formed in the martian mantle under more reducingOxidation 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 conditions than those of the Chassigny parent magma (Monkawa et al., 2006). Similar magmatic inclusions are present in nakhlites, shergottites, and lherzolites, which all typically contain Al–Ti augite. Monkawa et al. (2003) argue that both the reverse zoning present in some augites in Chassigny inclusions, and the absence of Ti-rich phases other than kaersutite, provide evidence for a late impact-shock event on Chassigny. This impact created more reducing conditions (causing reverse zoning) and higher temperatures and pressures (causing Ti-rich phases other than kaersutite to melt). Rapid cooling of the magmatic inclusions ensued.
In another study of the melt inclusions in Chassigny, Filiberto et al. (2004) concluded that formation of Chassigny is more consistent with 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. Click on Term to Read More from an alkalic, silica-saturated parental liquid under elevated pressure, rather than from a low-pressure fractionation of an olivine tholeiite. In support of this scenario, it was experimentally demonstrated by Nekvasil et al. (2007) that a magma compositionally analogous to terrestrial tholeiite could be the parental liquid of the chassignites. They found that a tholeiitic magma can undergo crystallization at >4.3 kbar (equivalent to martian depths of ~35 km) and >0.4 wt% water to produce an alkalic, silica-saturated hawaiitic melt. This melt can evolve to form the polyphase melt inclusions in Chassigny olivines, including the highly evolved alkali-rich rhyolitic glass component. By a similar process, but involving a less evolved, mildly alkalic 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 Click on Term to Read More liquid, the trapped melt component found in the chassignite NWA 2737 could be formed. Crystallization at this significant depth would be followed by ascent to a near-surface location where impact excavation can occur.
Martian tholeiites have been identified by NASA’s Mars Exploration Rover Spirit, examples of which are the rocks Adirondack, Mazatzal, and Humphrey; Spirit has also identified several hawaiitic rocks, including the silica-saturated Backstay, Wishstone, and Irvine. In a study by Elardo et al. (2008) the martian hawaiitic Backstay rock, which was studied by the MER Spirit at Columbia Hills of Gusev 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. Click on Term to Read More, was utilized as a model for the Chassigny parental melt. It was demonstrated that the accepted crystallization sequence for Chassigny is consistent with a parental magma water content of between 1.5 wt% and 2.6 wt% (Nekvasil et al., 2009). Models utilizing a Backstay-like rock as a parental melt were utilized to reveal that similar phases which are known to exist in Chassigny, both cumulus phases and melt inclusions, would be produced during crystallization and loss of residual liquids at the base of a 50–70 km thick crust (6.8–9.3 kbar) given a bulk water content of 2.6 wt% (1.5 wt% Cl, 0.3 wt% S, 0.4 wt% water; Ustunisik and Nekvasil, 2010).
Other experiments were conducted to determine the actual composition of the parental magma of the chassignites (Filiberto, 2008). The results revealed that previous estimates of the parental magma composition based on the melt inclusions fail to produce some of the phases that are observed in the chassignites, and therefore this parental magma must instead have been more magnesium- and aluminum-rich than in previous estimates. In addition, a formation depth equivalent to ~9.3 kbar was found to be most reasonable, with the chassignite assemblages crystallizing only after 8–30% crystallization of maficOne of the two broad categories of silicate minerals, the other being felsic, based on its magnesium (Mg) and/or iron (Fe) content. Mafic indicates silicate minerals that are predominantly comprised of Mg and/or Fe.The term is derived from those major constituents: Magnesium + Ferrum (Latin for iron) + ic (having Click on Term to Read More phases was achieved. This newly calculated composition for the chassignite parental liquid is similar in composition to the Gusev tholeiitic basalt rock named Humphrey that was analyzed in situ on the martian surface.
Although it has been generally accepted that Chassigny and its glassy melt inclusions have a magmatic origin, studies by Varela et al. (2000) suggest that the composition of these melt inclusions is more consistent with having been trapped at sub-igneous temperatures concurrent with the formation of the olivine host. As the olivine precipitated from a chondritic fluid phase, the solid glass precursors became trapped within the crystallizing 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, some serving as nucleation sites for other mineral phases. The non-equilibrium condition of the various components within polyphase inclusions supports such a non-igneous scenario (Varela et al., 2007). In still another study of trapped melt inclusions in Chassigny, Varela and Zinner (2015) argue that the non-homogeneous composition of the inclusions’ glass, the chemical (both 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 Click on Term to Read More) variability among the three types of inclusions, and the lack of 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 Click on Term to Read More that exists both among crystalline phases within inclusions and between these crystalline phases and the host glass, are most consistent with open-system behavior. This significant variability is thought to be the result of post-entrapment modification. One such modification process likely involves metasomatic fluid conductionTransfer of heat as a result of collisions between molecules; when one end of an object is heated or excited, the molecules vibrate faster and their energy is transferred sequentially to their neighbors. Click on Term to Read More through radial fractures associated with the melt inclusions. Therefore, these melt inclusions might not represent the primary parental magma.
In a study of pyroxene lamellae widths, Monkawa et al. (2004) determined a cooling rate for Chassigny. The widths of augite exsolutionSegregation, during cooling, of a homogeneous solid solution into two or more different solids. Click on Term to Read More lamellae of pigeonite in Chassigny was compared to those widths present in Zagami, which is considered to be from a thick lavaHot molten or semifluid rock derived from a volcano or surface fissure from a differentiated and magmatically active parent body. Click on Term to Read More flow or shallow dikePlanar, blade-like, intrusive igneous body that cuts across preexisting layers usually at a high-angle to near-vertical orientation. By definition, a dike is always younger than the rocks that contain it. Terrestrial dikes are typically 0.5 to 3 m wide and extend for a few 10s of kilometers. Click on Term to Read More. Chassigny is consistent with a cooling rate of 35–43°C/yr, which is 4–5 times slower than that of Zagami, and corresponds to a burial depth of ~15 m. Furthermore, they studied the Ca zoning profiles in Chassigny olivine that resulted from atomic diffusionMovement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). Diffusion, the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. Because diffusion is thermally activated, coefficients for diffusion Click on Term to Read More after cooling, and determined a cooling rate of 28°C/yr. This suggests a relatively shallow burial depth of ~15 m, consistent with that indicated by the pyroxene lamellae.
On a Sm–Nd isochron plot reflecting the igneous crystallization age, Chassigny, the nakhlites, and the shergottites DaG 476, and QUE 94201 all show a linear match of ~1.4 b.y. The Ar–Ar age of 1.4 b.y. calculated for Chassigny is consistent as well. Other similarities which suggest a petrogenetic link between Chassigny and the nakhlites include identical Sr isotopic ratios, similar whole rock 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 abundance patterns that are LREE-enriched, and similar REE compositions of their respective parent melts. In support of this data, it was recently discovered that cumulus 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 in the form of enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). Click on Term to Read More is present in Chassigny, further evidence 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 Click on Term to Read More was derived from a shergottite-like parent composition.
In a study by Fritz et al. (2005), they utilized numerical simulations (Artemieva and Ivanov, 2004) to determine the extent of the source region of martian meteorites. They found that oblique impacts are required to eject material, and that such material is confined to an area between 1 and 3 radii of the projectile to a depth of 0.2 radii. Mosaicism of olivine and maskelynization of feldspar, along with planar fractures and shear stresses are evidence of one and possibly two moderate shock events generating pressures of 26–32 GPa and a post-shock temperature increase of 40–60°C. A shock pressure of this magnitude leading to escape velocityVelocity that an object needs to escape the primary gravitational influence of a more massive object: where, m = the object's mass, r = distance from object's center, and G = gravitational constant of the larger object. Click on Term to Read More (>5.4 km/s) would require that the target material be situated relatively close to the surface (e.g., within 20 m of the surface for a 200 m impactor). Several high-pressure phases, including the olivine high-pressure polymorph wadsleyiteHigh pressure polymorph of olivine, β-Mg2SiO4, found on Earth and in some meteorites. It is thought to make up 50% or more of Earth's mantle between depths of 400 and 525 km. Wadsleyite transforms into ringwoodite at high pressure, but the exact pressure depends strongly on composition. At lower pressures, and ringwooditeHigh-pressure olivine polymorph with a spinel structure that is found in highly shocked meteorites (above ~50 GPa, shock level > S5) and the Earth's transition zone mantle (~13 GPa). Under even higher pressure in the lower mantle (~24 GPa), ringwoodite decomposes into perovskite (Mg,Fe)SiO3, and magnesiowüstite (Mg,Fe)O, whose properties are Click on Term to Read More, have been identified in melt pockets and along grain boundaries (Greshake and Fritz, 2009). Fritz et al. (2005) discovered that a correlation exists between the Mars-to-Earth transfer time and the shock stageA 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". Click on Term to Read More of the material; i.e., fragments having a higher degree of shock also have a faster 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 an Earth-crossing 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 Click on Term to Read More. Therefore, the absence in our collections of certain highly shocked martian samples (such as nakhlites) might be reconciled by consideration of their short lifetimes on Earth.
In the international quarterly Meteorite, vol. 7, nos. 3 and 4 (2001), Kevin Kichinka published a two-part article which presents an exhaustive review of the record concerning many aspects of the Chassigny meteorite. By permission, his article is presented here in its entirety.
As with the shergottites and nakhlites, Chassigny has a young crystallization age of 1.3 b.y. The 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 Click on Term to Read More (and ejection age) based on Ar systematics of Chassigny (10.51 ±0.18 m.y.) is indistinguishable from that of Nakhla (10.35 ±0.06 m.y.) and the other nakhlites, supporting the likelihood that they all formed within a common cumulate pile. The chassignites crystallized first at the bottom of the magma chamber, and the nakhlites crystallized thereafter, some possibly following the eruption and emplacement of a lava flow. Mikouchi et al. (2016) found that significant ambiguities exist among the three known chassignites. For example, although each of the chassignites exhibit a similar cooling rate (0.003–0.1 °C/hr), olivine compositions between them show large variations: NWA 8694 is Fa46, Chassigny is Fa31, and NWA 2737 is Fa21; moreover, each chassignite exhibits a distinct shock history. Therefore, they suggest that each of the chassignites is more likely associated with a separate flow or lobe (possibly within a common extensive igneous unit) rather than a single sequential accumulation. See the Nakhla page for further details on the stratographic sequence for nakhlites and chassignites.
The chassignites and nakhlites were launched toward Earth in a common ejection event. Recently, thermal emission spectrometry performed by the Mars Global Surveyor along with data from the Mars Odyssey THEMIS have led to the identification of craters with favorable characteristics in a few specific regions, particularly Nili Fossae in the Syrtis Major volcanicIgneous rock that forms from cooling magma on the surface of a planet or asteroid. complex. Here there exists a basement basaltic unit that shows evidence for episodic occurrences of aqueous alteration, which is overlaid by an olivine-rich basalt unit containing Mg-rich olivine similar to that found in the chassignites (Amador and Bandfield, 2015). The presence of olivine in the Nili Fossae region is indicative of persistent dry conditions ever since the olivine was exposed through post-impact faulting probably over 3 b.y. ago (T. Hoefen, USGS ). Importantly, in an adjacent region in eastern Syrtis Major, Fe-rich olivine and high-Ca pyroxene similar to nakhliteOne of the Martian SNC meteorites, nakhlites are basaltic cumulate clinopyroxenite rocks, and most all are comprised mainly of sub-calcic augite with approximately 10% Fe-rich olivine (giving the augite a green color) that are set in a very fine-grained matrix (mesostasis) comprised of plagioclase, K-feldspar, clinopyroxene, Fe-Ti oxide (Ti-magnetite), sulfide, Click on Term to Read More compositions have been identified (Harvey and Hamilton, 2005). It is conceivable that lava flowing from Syrtis Major onto the Nili Fossae region would form the kind of terrain from which both chassignites and nakhlites could have been launched in a single impact event. Tornabene et al. (2006) have recognized the 3.3 km-diameter rayed crater Zumba, located in Daedalia Planum south of Tharsis volcano, which reflects late Hesperian age volcanic terrain (spanning a period ~3.5 to ~1.8 b.y. ago). They consider this crater as a possible source of the 1.3 b.y. old chassignites and nakhlites.
The specimen of Chassigny shown above is a 0.76 g cut fragment with 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 along one edge. A large 312.406 g specimen of Chassigny is curated at the Muséum National d’Histoire de Paris and displayed on their website. Shown below is an unlabeled 3.02 g fragment of Chassigny that was ‘rediscovered’ in 2011 by Fabien Kuntz at the Muséum Georges Cuvier in the city of Montbéliard, France.
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