Martian The 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 photo for a magnified view Found August 2000 Nine rock fragments constituting a single 611 g stone were found in the Moroccan Sahara by Work 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 hunters under the organization of Bruno Fectay and Carine Bidaut. These black fragments were not recognized as meteoritic until several years later, at which time a sample was submitted for analysis. The importance of this meteorite was soon evident, and multiple analyses were conducted through a collaboration of research groups in France and elsewhere:
- Laboratoire des Sciences de la Terre, (Centre National de la Recherche Scientifique—Unité Mixte de Recherche [CNRS–UMR]), École Normale Supérieure de Lyon: P. Beck, Ph. Gillet, B. Van de Moortele, B. Reynard
- Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer: J-A. Barrat, J. Cotten
- Institut français de recherche pour l’exploitation de la mer (IFREMER), Centre de Brest (Centre national de la recherche scientifique—Unité mixte de recherche [CNRS-UMR]): M. Bohn
- Planetary and Space Science Research Institute, Open University, United Kingdom: I.A. Franchi, R.C. Greenwood
- Johnson Space Center, Houston, Texas
- University of Tokyo, Japan
With the assigned name of NWA 2737, this olivine–Brownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Igneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. was classified as a dunite that exhibits many mineralogical, chemical, and petrographical similarities to the first known martian chassignite that fell in Chassigny, France in 1815. A third chassignite designated NWA 8694 (photo courtesy of L. Labenne) has been classified at the Museum National d’Histoire Naturelle, Paris, France (Hewins et al., 2014, 2015). Northwest Africa 2737 is composed of 89.6 vol% shock-blackened, cumulus forsteritic Group 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, 3.1 vol% chromite, 1.6 vol% sanidine, 1.0 vol% A 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. (High-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., Low-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 Orthorhombic, 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 0.2 vol% phosphate. Carbonates are present in both The 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, with at least some of them showing evidence for a martian origin—shock fractures post-date the Mineral or compound containing carbon and oxygen (i.e. calcium carbonate, CaCO3, calcite). formation (Beck et al., 2006). Both chassignites have similar Often 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. patterns, elements that are carried mostly in their apatite component. A martian origin is attested by the Logarithm of the ratio of two metallic elements in a star relative to their ratio in the Sun. This is used to quantify the relative amounts of individual elements present in a star. For example, the abundance ratio of Mg to Fe, written [Mg/Fe], is: Chemical elements are produced through of Fe/Mn in olivine, the bulk Na/Al ratios, and by the O-isotopic ratios.
Northwest Africa 2737 is one of the least terrestrially altered martian finds as shown by its trace Substance 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 composition (e.g., low Ba and Sr contents) and Element 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. signatures. However, in contrast to Chassigny and the nakhlites, NWA 2737 contains trapped Element 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. more similar to those in Igneous 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, as evidenced by the lack of martian Main 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 Xe, the lack of fission-derived Xe from plutonium, and the enrichment of martian atmospheric Xe (Marty et al., 2006).
While feldspars in Chassigny are mostly composed of Also 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, with lesser amounts of labradorite and sanidine, the feldspars in NWA 2737 only occur as K-rich sanidine or Na-rich analbite. These differences could have been caused by a relatively low Al concentration in the parental melt, resulting in the delayed nucleation of plagioclase with a consequent buildup of Na in the melt (Beck et al., 2006; Papike et al., 2009). The olivine and chromite grains in both chassignites contain melt inclusions with a unique mineralogy, including the occurrence of hydrous kaersutitic (Ti-rich) amphibole, while those in NWA 2737 also include an alkali feldspar-rich glass. Six distinct melt Fragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. species were described in NWA 2737 by He et al. (2010,2013) based on the diverse Inorganic 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 assemblages, including olivine, orthopyroxene, augite, pigeonite, kaersutite, chlorapatite and fluorapatite, biotite, chromite, Iron sulfide group of minerals whose composition ranges widely between its end members pyrrhotite (Fe7S8) whose crystal structure is monoclinic, and troilite (FeS) whose crystal structure is hexagonal. Its general formula is Fe1−xS (where x = 0 to 0.17). The troilite phase is found mainly in meteorites and in the, and feldspathic glass; these minerals occur in melt inclusions that span a wide range of sizes (~5–300 µm). It is thought that some melt inclusions might contain trapped parental Molten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava. at various stages of Concentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process..
It was determined experimentally that the parental magma of the chassignites may have resembled a terrestrial, silica-saturated hawaiite magma with a higher than terrestrial Mg# and an aluminum content of ~12 wt% (Filiberto, 2008). It was ascertained that the chassignites likely crystallized after ~30% Physical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. of One 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 phases was attained.
In an effort to estimate the original magma composition of the cumulate chassignites, He et al. (2013) examined the ubiquitous, sub-mm-sized, trapped magmatic inclusions in olivine grains. In consideration of the water Movement 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 coefficient between amphibole and melt, as well as the extent of melt crystallization that occurred prior to amphibole formation (~45%) and other pertinent assumptions, the water content of the primary melt of NWA 2737 was calculated to be ~0.48–0.67 wt%; the water content of the primary melt of the Chassigny meteorite was previously calculated by McCubbin et al. (2010) to be 0.43–0.84 wt%. Utilizing the MELTS program with the known parameters, the research team determined that NWA 2737 was formed under pressure of ~6.8 kbar. Their deduced parental composition for NWA 2737 based on Ca:Al ratios and Mg# is quite similar to that of the martian Basalt 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 Humphrey, measured in situ by the MER rover Spirit in Gusev Bowl-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.. A broad range of Al contents has been found in martian parental source rocks—Al is relatively high in basaltic surface rocks and the chassignites, while it is thought to have been sequestered at depth in Mineral generally found in terrestrial metamorphic rocks, although igneous examples are not uncommon. Garnet is a significant reservoir of Al in the Earth's upper mantle. The garnet structure consists of isolated SiO4 tetrahedra bound to two cation sites. The A site holds relatively large divalent cations (Ca2+, Mg2+, Fe2+, Mn2+); the in the nakhlites and most shergottites; this has implications for the Completely 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 scenario of Mars’ petrogenetic history.
Several differences between Chassigny and NWA 2737 have been identified by Mikouchi (2005): Term applied to ions or atoms occupying sites between lattice points. chromite has a larger grain size in NWA 2737, An 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. occurs as the sodic plagioclase albite in NWA 2737 rather than the calcic plagioclase Rare 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, and all phases of NWA 2737 have a more magnesian composition than those in Chassigny, perhaps reflecting conditions of higher temperature and pressure during A 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. from a less evolved parental melt as suggested by Nekvasil et al. (2005). Petrographic evidence indicates that NWA 2737 was the first cumulate to crystallize at the bottom of the pile, followed by Chassigny next in the sequence, while the order for NWA 8694 remains to be determined (McCubbin et al., 2013). In further studies, 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.
Lorand et al. (2012) studied the sulfide mineralogy and native Element 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 assemblages in NWA 2737 and made comparisons to Chassigny. While the bulk sulfide abundance in NWA 2737 is low, Brass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. (and rare Fe-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) blebs up to tens of µm in size are ubiquitous at primary mineral grain contacts, and as even smaller blebs associated with chromite within melt inclusions. Although the sulfide pyrite is abundant in Chassigny, it was not identified in NWA 2737, and the Cu-sulfide chalcopyrite was virtually nonexistent; both sulfide phases were likely lost during an impact-heating event. Similarly, Fe–Os–Ir alloys that are observed in close association with troilite grains in NWA 2737 are considered to have exsolved from pyrrhotite during subsequent cooling from an impact-heating event. These differences that exist between these two chassignites can be attributed to impact-shock-related Oxidation 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 involving high temperatures, leading to the devolatilization of S and the loss of magnetic properties.
While the overall shock effects in Chassigny are only moderate (S4), portions are present that must have experienced much higher shock pressures (45–55 GPa; S5–S6) consistent with the presence of planar deformation features and localized melting. By contrast, the olivine in NWA 2737 exhibits exceptional darkening which is thought to reflect the disordered Regularly spaced array of points that represents the structure of a crystal. Crystals are composed of groups of atoms repeated at regular interval in three dimensions with the same orientation. The smallest division of the lattice which can still be used to represent the entire structure is called the unit state of an incomplete transformation to olivine high pressure polymorphs, as well as the presence of nanophase FeNi-metal particles within the olivine (Reynard et al., 2006). These Fe nanoparticles are thought to have formed through subsolidus reduction of olivine during the high temperature phase (~1300°C) of a shock event corresponding to a minimum A 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". of S5. A partial cause of the darkening (brown color) in olivines as described by Treiman et al. (2006) derives from the conversion of Fe+2 to Fe+3 in an Oxidation 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 environment, resulting from the shock heating loss of H+ that was previously dissolved in a hydrous magma. Only a low Parts per million (106). abundance of Fe+3 would be required to cause the observed darkening. Continued investigations utilizing spectral reflectance techniques, including Mössbauer Technique of splitting electromagnetic radiation (light) into its constituent wavelengths (a spectrum), in much the same way as a prism splits light into a rainbow of colors. Spectra are not smooth but punctuated by 'lines' of absorption or emission caused by interaction with matter. The energy levels of electrons in, indicate that the darkening is primarily the result of shock disseminated nanophase metallic iron particles (Pieters et al., 2007). Other deformed olivine grains in NWA 2737 have been recrystallized to a visually colorless phase. Takenouchi et al. (2015) suggested that the Fe nanoparticles were formed in association with high-pressure polymorphs such as High 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, during a high-pressure, high-temperature shock event. Ultimately, the wadsleyite was back-transformed to olivine during a period of high post-shock temperatures, and this olivine now appears brown due to the presence of the Fe nanoparticles. Subsequent studies were conducted by Takenouchi and Mikouchi (2016) of NWA 2737 and several shergottites that contain darkened olivine associated with shock-melt phases. They found higher Fe+3 ratios in both brown (or brownish) olivine compared to adjacent colorless olivine located within the same grain, attesting to the formation of Fe nanoparticles by olivine Oxidation 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 and reduction during transformation to high-pressure polymorphs such as High-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 and wadsleyite.
Cooling rate estimates for both Chassigny and NWA 2737 have been set at 28–30°C/year, consistent with a formation within a thick Hot molten or semifluid rock derived from a volcano or surface fissure from a differentiated and magmatically active parent body. flow or a shallow Planar, 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. or sill. The Sm–Nd data for both plot on a similar isochron—1.416 (±0.057) b.y. for NWA 2737, and 1.380 (±0.030) b.y. for Chassigny. Both of these chassignites, as well as the nakhlites, also have similar CRE ages of 10–11 m.y. based on 3He and 21Ne. Therefore, it may be argued that both of these chassignites, as well as the nakhlites, were ejected during a common impact event from the same igneous region on Mars. Based on Ar–Ar data, it was proposed by Bogard and Garrison (2008) that NWA 2737 may have experienced an intense impact event resulting in its burial ~1–20 m deep under a warm Generally symmetrical apron of ejecta surrounding a crater; it is thick at the crater's rim and thin to discontinuous at the blanket's outer edge., while a subsequent, less-intense impact event was responsible for its ejection from Mars 10–11 m.y. ago.
Differences have been noted in the gas retention ages of Chassigny and NWA 2737. After correction for terrestrial contamination, a more accurate determination of the K–Ar crystallization age for NWA 2737 was made, and this revised K–Ar age of 376 (±168) m.y. is significantly younger than that of Chassigny and the nakhlites, and more like that of shergottites (Marty et al., 2005; 2006). However, this discordant age has been attributed to the higher Metamorphism produced by hypervelocity impact between objects of substantial size moving at cosmic velocity (at least several kilometers per second). Kinetic energy is converted into seismic and heat energy almost instantaneously, yielding pressures and temperatures far in excess those in normal terrestrial metamorphism. On planetary bodies with no atmosphere, smaller experienced by NWA 2737. The specimen of NWA 2737 shown above is a 0.48 g cut end fragment. The photo below is an excellent petrographic Thin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope. micrograph of NWA 2737, shown courtesy of Peter Marmet.
click on image for a magnified view
Photo courtesy of Peter Marmet