NWA 1950
Martian ShergottiteIgneous 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
Poikilitic (formerly ‘lherzolitic’ shergottite)
(intermediate, permafic, pyroxene-oikocrystic)
Found January and March 2001
no coordinates recorded A 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 comprising two stones, with weights of 414 g and 398 g, was found in the Atlas Mountains in Morocco by a French team under the organization of Bruno Fectay and Carine Bidaut. The meteorite was classified in collaboration among three French institutions—École Normale Supérieure de Lyon (Gillet), Université d’Angers (Barrat), and Institut Français de Recherche pour l’Exploitation de la Mer (Bohn). An additional 32 g paired stone designated NWA 7721 was purchased by a collector in 2012 (A. Irving and S. Kuehner, UWS). Northwest Africa 1950 is one of a small number of poikilitic 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 found to date, and the first one found outside of Antarctica. The olivine-gabbroic NWA 2646 is considered to be the second hot desert poikilitic shergottite 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.. In addition, poikilitic xenocrysts with similar compositions to this group are present in the shergottite EETA79001 lithology A, while 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 abundances, isotopic signatures, and crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. and CRE ages suggest that the basaltic shergottite NWA 480/1460 might be genetically related to the poikilitic shergottites.
Perhaps key to the classification of NWA 1950 is the martian meteorites RBT 04262 (along with its pairing RBT 04261) and NWA 4468. Both were originally classified as olivine-phyric shergottites, but suggestions have since been made (Mikouchi, 2009) that they might better be considered anomalous members of the poikilitic shergottite group. They are also compositionally very similar to the poikilitic shergottite NWA 2646. RBT 04262 does has a similar crystallization history to that of the other poikilitic shergottites; i.e., formation 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. oikocrysts and their accumulation to form compact poikilitic areas, followed by the crystallization of intercumulus melt to form the non-poikilitic areas. However, it is mineralogically unique in having certain features in common with a 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, such as a high abundance 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 (13.3%, as 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) and chemical zoning in grains within the non-poikilitic areas. These features are consistent with rapid cooling (~0.03–0.09°C/hour) near the surface rather than the slow cooling expected in a 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. RBT 04262 also contains a greater proportion of the non-poikilitic evolved melt component consistent with a basalt. Although 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 in RBT 04262, NWA 4468, and NWA 2646 is more Fe-rich than in other poikilitic shergottites, it is mineralogically more similar to them than it is to other shergottite groups.
These features led Mikouchi et al. (2008) to infer a crystallization for RBT 04262 within a stratagraphic layer closer to the surface than that of the other poikilitic shergottites, but still originating from a common magmatic source region. Based on Ca-zoning, any variation in its crystallization depth could not be resolved to a greater extent than 4–5 m. It was also conjectured that basaltic shergottites with similar young crystallization ages may have formed from this same evolving melt in a stratagraphic layer above that of the poikilitic shergottites.
This martian group has been historically included as a subgroup within the shergottite class, and its members were commonly described as ‘lherzolitic’ shergottites (or shergottitic peridotites) in conformity with the term basaltic shergottites. In actuality, this 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 group does not contain the minimum abundances of olivine or 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 as those established for terrestrial lherzolites. However, since there was no known petrologic relationship existing between the basaltic and ‘lherzolitic’ shergottite subgroups, and these groups are resolved from each other on an O-isotope plot, the use of the term ‘lherzolitic’ shergottite was proposed by Eugster and Polnau (1997) to represent this unique group of martian meteorites.
The discovery of RBT 04262, NWA 2646, and NWA 4468 required further revisions in martian meteorite classification terminology.
The British Geolocical Survey has established a hierarchical classification scheme for terrestrial igneous rocks. The group of igneous rocks that are ultramaficTerm used for silicate minerals with cations predominantly Mg and/or Fe. Mafic minerals are dominated by plagioclase and pyroxene, and also contain smaller amounts of olivine., coarse-grained, crystalline, and have a 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 content >90% are further classified by their content of mafic minerals. Peridotites are distinguished from pyroxenites (at Level 7 of the hierarchy) by containing more than 40% olivine. The peridotites are then divided (at Level 8 of the hierarchy) into the dunite, pyroxene-peridotite, pyroxene–hornblende-peridotite, and hornblende-peridotite groups. The pyroxene-peridotite group is further divided (at Level 9 of the hierarchy) into the harzburgite, lherzolite, and wehrlite groups.
In an effort to resolve the discrepencies that exist between the official IUGS definition of lherzolites and the application of that term to the varied group of ‘lherzolitic’ shergottites, Mikouchi (2009) addressed the need for changing the name of the ‘lherzolitic’ shergottites to one that is more consistent and more broadly applicable. Since a texturally-based nomenclature is already employed for some shergottite subgroups, e.g., olivine-phyric, it was suggested that the term ‘pyroxene-oikocrystic’ shergottites would be an appropriate designation to comprise all of the various martian ‘lherzolitic’ shergottites that exist in the worldwide collections. This would include intermediate, enriched, and postulated depleted ‘lherzolitic’ shergottites, as reflected by a geochemical classification scheme.
More recently, in an effort to rectify the discrepencies that exist in martian meteorite nomenclature, the textural term ‘poikilitic’ was proposed by Walton et al. (2012) to apply to those meteorites previously referred to as ‘lherzolitic’ shergottites, which is to be used along with additional descriptive terms for bulk major element compositions (based on a plot of Mg/[Mg + Fe] vs. CaO, where this ratio increases along the sequence from mafic to permafic to ultramafic) and trace element content (based on the enrichment of HREE over LREE, increasing along the sequence from depleted to intermediate to enriched).
| BULK MAJOR ELEMENTS vs. TRACE ELEMENTS (Mg/[Mg + Fe] vs. CaO) |
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|---|---|---|---|---|
| MAFIC | PERMAFIC | ULTRAMAFIC | ||
| HREE/LREE (La/Yb) |
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| ENRICHED | NWA 4468 NWA 7397 NWA 10169 NWA 10618 NWA 10808 RBT 04261/2 |
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| INTERMEDIATE | NWA 1950 NWA 2646 NWA 11065 NWA 11214 |
ALH 77005 GRV 99027 LEW 88516 Y-793605 Y-984028 NWA 4797 NWA 6342 NWA 10697 NWA 11261 NWA 10961 |
||
| DEPLETED | ||||
After Irving et al. (2010), Walton et al. (2012), and Dr. Anthony Irving’s List of Martian Meteorites Poikilitic shergottites are defined as cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber., 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 rocks (e.g., ALH 77005 formed at a depth of ~18 km; Szymanski et al., 2004) which are derived from primary magmas containing >90% mafic minerals. These minerals are composed of >40% olivine (45.3 vol% in NWA 1950), ~>10% low-Ca pyroxene, and ~>10% high-Ca pyroxene (34.5 vol% pyroxenes in NWA 1950). The olivine in poikilitic shergottites is chemically similar to the olivine in the martian dunite Chassigny, but pre-terrestrial Fe redoxOxidation 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 gives olivine in poikilitic shergottites a distinctive brown color.
In contrast to the dark olivine present in the 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 NWA 2737, which contains both magnetiteFe oxide, Fe2+Fe3+2O4, containing oxidized iron (Fe3+) found in the matrix of carbonaceous chondrites and as diagnostic component in CK chondrites. In CK chondrites, magnetite is typically chromian, containing several wt. % Cr2O3. and FeNi-metal nanoparticles, it was demonstrated by Kurihara et al. (2009, 2010) that the dark olivine in martian poikilitic shergottites (and in certain other SNCs; Hoffmann et al., 2009) reflects the presence of 200–500 nm-wide parallel bands containing Ni-free hematiteFe-oxide mineral (Fe2O3) that may be the major cause of the red color on Mars. Coarser-grained gray hematite has the same chemical formula as the red variety, but a different crystalline structure. Deposits of gray hematite found in the Terra Meridiani region of Mars may suggest that water once circulated nanoparticles measuring ~20 nm in size. These nanoparticles were attributed to recrystallization following shock meltingComplete melting of target material produced by the shock of a meteoric impact. Different minerals will experience certain shock effects at different pressures and temperatures. For example, dense target rocks like anorthosite will typically experience whole rock melting above 50 to 60 GPa, while chondritic rocks require more than 70 during impact ejection, or possibly during shear stresses. Further studies of NWA 1950 by Mikouchi et al. (2013) verified the presence of abundant 5–100 nm-sized rounded particles, but they also identified rod-shaped Fe-metal particles larger than 100 nm. The nano-particles in NWA 1950 contain a 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. of Fe-metal that formed by 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 of olivine under high temperature annealing (non-melting) conditions, and some are rimmed by magnetite that formed as temperatures decreased under more oxidizingOxidation 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 conditions (Takenouchi et al., 2014). Takenouchi et al. (2015) suggested that the Fe nanoparticles were formed in association with high-pressure polymorphs such as 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, 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 several shergottites including NWA 1950 as well as the chassignite NWA 2737, both of which 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 heterogeneous formation of Fe nanoparticles in olivine during transformation to high-pressure polymorphs such as 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 and wadsleyite. Takenouchi et al. (2017) also determined that subsequent back-transformation occurred under high postshock temperatures of >900K. They ascertained that the impact attained pressures >30 GPa and temperatures of 1500–1700K for a duration of at least ~90 ms; this event probably represents the ejection of this meteorite from Mars.
Poikilitic shergottites also contain a significant amount of feldspathic glass in the form of maskelynite (11.0 vol% in NWA 1950), along with accessory chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. (5.7 vol% in NWA 1950). Other 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 phases include ilmeniteTi-Fe oxide, TiFeO3, found in achondrites, lunar mare basalts, and shergottites. Ilmenite forms as a primary mineral in mafic igneous rocks. It crystallizes relatively early out of a magma before most of the other minerals, and as a result, the heavier crystals of ilmenite precipitate to the bottom of the magma, phosphates (merrillite in NWA 1950), sulfides (pyrrhotiteIron 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 in NWA 1950), and an interstitialTerm applied to ions or atoms occupying sites between lattice points. K-rich glass. Trapped martian atmospheric gases have been identified in maskelynite and melt pockets in some poikilitic shergottites, and this gas is thought to have produced the vesicles present in these specimens through exsolutionSegregation, during cooling, of a homogeneous solid solution into two or more different solids. during decompresion. OxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only isotopic ratios for NWA 1950 are identical to those of other martian poikilitic shergottites.
Two main phases are typically evident in poikilitic shergottites, reflecting two stages of crystallization: 1) a lighter-colored postcumulus phase in which large orthopyroxenes poikilitically enclose cumulus olivine and chromite grains, and 2) a darker-colored non-poikilitic phase consisting of olivine, orthopyroxene, maskelynite, chromite, clinopyroxene, ilmenite, phosphates, and sulfides. This latter phase occurs interstitially to the larger orthopyroxenes and incorporates a significant component of trapped intercumulus melt (Treiman et al., 1994; Mittlefehldt et al., 1997). Olivine and pyroxene in the non-poikilitic areas have experienced a considerable degree of re-equilibration with evolved melts (Mikouchi, 2005).
Variation in the intensity of shock metamorphismMetamorphism 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 is apparent in olivine and pyroxene, which ranges from mosaicism and planar deformation features, shock-induced veining, high-pressure polymorphs of olivine, partial meltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different melting and recrystallization, and culminating in localized sub-mm- to mm-sized melt veins and melt pockets which constitute 1.8 vol% of the meteorite. Microporphyritic textures including euhedral and dendritic host rock crystallites, chromite stringers, sulfide globules, and vesiculated maskelynite with flow textures are all associated with the melt pockets (Walton and Herd, 2007). StishoviteDense, high-pressure phase of quartz; so far identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters. Stishovite was synthesized in 1961 before it was discovered at Meteor Crater, Arizona. Its structure consists of parallel chains of single SiO6 octahedra. The octahedra are on their sides, sharing opposing edges. Image has been identified in the maskelynite near melt zones by Raman spectra (Gillet et al., 2005). These shock features, along with the content of trapped 40Ar (Terribilini et al., 1998), are indicative of very strong shock pressures equivalent to ~35–45 GPa (S5), and a post-shock temperature of ~200°C, conditions similar to those experienced by LEW 88516 and Y-793605. Compared to the degree of shock observed in NWA 1950, the poikilitic shergottites GRV 99027 and ALH 77005 experienced significantly higher degrees of shock, up to at least 55 GPa, reflecting post-shock temperatures of 1000°C. These shock levels are manifest in the interconnected melt pockets and other shock melt components constituting up to 29 vol% of the bulk of ALH 77005.
Based on several radiometric chronometers, a young isotopic age was determined for the igneous crystallization of poikilitic shergottites, as well as for many of the basaltic shergottites; e.g., ~150 to ~225 m.y. for the enriched poikilitic group (Combs et al., 2018). An older age of 382 (±36) m.y. based on Ar–Ar was ascertained for the intermediate poikilitic NWA 1950 by Walton et al. (2008). Yamato 793605 has a much shorter terrestrial age of 35 (±35) t.y. compared to 190 (±70) t.y. for ALH 77005, while its terrestrial age is within the range of error of 21 (±1) t.y. calculated for LEW 88516. An ejection age (CRE age + terrestrial age) of ~4 m.y was ascertained for the three poikilitic shergottites Y-793605, ALH 77005 and LEW 88516, which is about 1 m.y. earlier than the ejection event calculated for many of the basaltic shergottites. The CRE age for NWA 1950, based on various rare gas chronometers, shows a range of between 2.3 (±1.0) m.y. and 5.3 (±3.0) m.y., which is similar to the ranges determined for the other martian poikilitic shergottites. In addition, they all share similar chemical compositions (including 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. abundances; Hsu et al., 2004) and petrologies, and therefore it is presumed that they experienced a simultaneous ejection from a common lithological unit on Mars.
Cosmic ray exposure ages have now been determined for many martian meteorites, and Mahajan (2015) compiled a chart based on the reported CRE ages for 53 of them. He concluded that together these 53 meteorites represent 10 distinct impact events which occurred 0.92 m.y., 2.12 m.y., 2.77 m.y., 4.05 m.y., 7.3 m.y., 9.6 m.y., 11.07 m.y., 12.27 m.y., 15 m.y., and 16.73 m.y.—see his chart here. It was argued that NWA 1950 was launched from Mars during the 4.05 m.y.-old impact event. In a subsequent review based on multiple criteria, Irving et al. (2017 [#2068]) made a new determination of the number of separate launch events associated with the known (101 at the time of their study) martian meteorites. They speculate that the number could be as few as twenty, and suggest that NWA 1950 and at least 14 other intermediate poikilitic shergottites were ejected in a common impact event unique from the others.
Shock pressure comparisons indicate that NWA 1950 was in a shallower position within the magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava. unit than ALH 77005—compare shock pressures of 30–44 GPa for NWA 1950 to those of 45–55 GPa for ALH 77005. It is apparent that some of these martian samples existed as separate meteoroids during their journey to Earth. Northwest Africa 1950 has sustained only slight terrestrial weathering effects. The specimen shown above is a 0.44 g partial slice with a small amount of 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.. The top photo below shows the poikilitic shergottite ALH 77005, which has an olivine composition and re-equilibration stage very similar to that of NWA 1950. The three photos below that are views of the main massLargest fragment of a meteorite, typically at the time of recovery. Meteorites are commonly cut, sliced or sometimes broken thus reducing the size of the main mass and the resulting largest specimen is called the "largest known mass". of NWA 1950.
ALH 77005—NASA photo #S78-37989
Photos courtesy of B. Fectay and C. Bidaut—Meteorite.fr
