Lunar Mingled BrecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material Click on Term to Read More (fragmental breccia with clasts of very low-Ti 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 MorebasaltBasalt 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, olivine gabbroWork in progress Coarse-grained igneous rock of basaltic composition that formed at depth and is 90% plagioclase. clinopyroxene, https://www.sandatlas.org/gabbro/ The most important mineral groups that make up this rock type are plagioclase and pyroxene. Plagioclase usually predominates over pyroxene. Plagioclase is sodium-calcium feldspar. It contains more calcium than sodium in gabbro. If there is Click on Term to Read MorecumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. Click on Term to Read More, fragmental breccias, and regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles). Click on Term to Read More breccias)
Found September 2000 ~26° 49′ N., ~12° 49′ W. While visiting the Western Sahara, American 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 collector M. Killgore purchased three meteorite fragments from local nomads. The three fragments, weighing 50 g, 224 g, and 359 g (totaling 633 g), were all recovered in close proximity, and it is apparent that they constitute a single meteorite. The 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 location was reported to be a desert plain near Dchira, Western Sahara. Subsequent paired finds include Anoual [5.92 g], NWA 2700 [31.7 g], NWA 2727, NWA 2977, NWA 3160 [34 g], NWA 3170 [60 g, photo courtesy of S. Ralew], NWA 3333 [33 g], NWA 6950 [1,649 g], NWA 7007 [91 g, photo credit: Kuehner et al., 2012], NWA 8127 [529 g], NWA 10656 [262.5 g, diabase, photo credit: Valencia et al., 2017, #2483], and NWA 10985 [250 g]. The individual meteorites composing this lunar pairing group represent a wide variation in composition including both extrusiveRefers to igneous rocks erupted on a planetary body's surface. Click on Term to Read More and intrusiveRefers to igneous rocks that crystallized underground. Click on Term to Read More lithologies.
Among the twelve currently known members of the NWA 773 clan, Valencia et al. (2017) recognized five distinct lithologies along with a fragmental and/or regolith breccia. Four intrusive lithologies represent a 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 sequence from more magnesian to more ferroan as follows: olivine gabbro → anorthositic gabbro → gabbro → ferroan gabbro; a fifth extrusive lithology is an olivine-phyric basalt. All of these paired stones exhibit a unique chemical signature in that they have the highest Sm/Eu ratios of any other basaltic lunar meteorite or basalt studied from the Apollo collection. For more information and photos of this lunar pairing group see Korotev–WUSL.
Northwest Africa 773 is primarily composed of two distinct lithologies: a light-green magnesian olivine gabbro cumulate, and a dark-colored polymict impact brecciaRock consisting of broken fragments of rock (clasts) cemented by a fine-grained matrix formed from regolith during subsequent impacts. Click on Term to Read More containing a regolith component that comprises gabbroic, basaltic, and volcanicIgneous rock that forms from cooling magma on the surface of a planet or asteroid. elements. The diverse clasts composing the breccia lithology, considered likely derived from a common 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 unit, were grouped into four categories by Fagan et al. (2014): olivine gabbro cumulate (OC); pyroxene–gabbro clasts (PG); symplectite (hedenbergite+fayalite+SiO2) clasts (S); and alkali-phase-ferroan clasts (AF). Boundary textures between the two lithologies in NWA 773 indicate that the cumulate lithology was a large clastA mineral or rock fragment embedded in another rock. Click on Term to Read More within the breccia lithology (Fagan et al., 2003). Several independent analyses of a number of samples have determined a range of 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 for the cumulate portion. It has a modal composition of 48–66% olivine, 26–40% clinopyroxene (comprising both low-Ca 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 high-Ca 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), 8–15% interstitialTerm applied to ions or atoms occupying sites between lattice points. Click on Term to Read MoreplagioclaseAlso 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, 2% 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, and ~1.5% K-feldspar, 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 Click on Term to Read More, and RE-merrillite, along with trace amounts of Ba-rich K-feldspar (hyalophane), Cr-spinel, and troilite–FeNi-metal assemblages. Both the olivine gabbro cumulate and the breccia lithologies are reported to be enriched in incompatible trace elements. A breccia component enriched in incompatible trace elements has also been described in the paired stones NWA 3160 and NWA 3170.
The polymict brecciaGeneral term for all breccias that are neither monomict nor dimict. Modified from image source: http://www.saharamet.com/meteorite/gallery/HED/index.html. portion of NWA 773 has a fragmental texture that contains dispersed fragments of both magnesian and ferroan olivine gabbro, Ti-poor (VLT) olivine-phyric mareBroad low plains surrounded by basin-forming mountains, originally thought to be a sea (pl. maria). This term is applied to the basalt-filled impact basins common on the face of the Moon visible from Earth. Click on Term to Read More basalt, and Fe-rich lithic clasts comprising fayalitePure* iron end-member (Fe2SiO4) of the olivine solid solution series and an important mineral in meteorites. When iron (Fe) is completely substituted by magnesium, it yields the the pure Mg-olivine end-member, forsterite (Mg2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) Click on Term to Read More gabbros and fayalite granites, with both of the latter representing late-stage differentiates. Other evolved clasts present in the breccia include FeO-rich symplectites (fayalite+hedenbergite+silicaSilicon dioxide, SiO2.) that can be formed by two different processes: 1) breakdown of pyroxferroite (a pyroxenoid formed by rapid cooling of a Mg-depleted melt), and 2) quenching of a 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 liquid. The second formation pathway of direct quenching of a melt is favored for NWA 773 due to the presence of feldspathic components in the silica (Fagan et al., 2003). Other unusual clast types have been identified, one of which contains fayalite, hyalophane, silica, and plagioclase, while another containsg silica, K-feldspar, plagioclase, troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More, baddeleyiteA rare zirconium oxide (ZrO2) mineral, often formed as a shock-induced breakdown product of zircon. This mineral can be found in some lunar and martian meteorites. Click on Term to Read More, and RE-merrillite. Mineral fragments present include fayalite, silica glass, agglutinateCommon particle type in lunar regolith (photograph below). Agglutinates are small glassy breccias formed when micrometeorites (< 1 mm in diameter) strike the lunar regolith. During micrometeorite impacts, some of the regolith melts and some doesn't, so the final product is a glass with entrained mineral and rock fragments. The Click on Term to Read More glass, and hedenbergitic 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. The partial conversion of 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 to 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 in NWA 773 reflects a low 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 S2, and the presence of some minor calcite filling fractures indicates only minor alteration consistent with a weathering grade of W1.
Although variable mineral proportions observed in studied samples initially indicated that the cumulate portion was a noriteIgneous rock composed of 90% plagioclase, 95% orthopyroxene (low-Ca pyroxene) and less than 10% olivine. Norite is most commonly found in the lunar (highlands) meteorites but has also been found in about a dozen diogenites, a few shergottites, and a very small number of other achondrite types. Gabbro is very Click on Term to Read More or gabbronorite, the dominance of clinopyroxene over orthopyroxene in the plagioclase–orthopyroxene–clinopyroxene ternary diagram (Stöffler et al., 1980) suggests that this lithology is actually a gabbro. In a similar way, when the high proportion of olivine in this cumulate lithology is entered on the plagioclase–pyroxene–olivine ternary diagram (Stöffler et al., 1980), it indicates that this is an olivine gabbro. The plagioclase content of 14.2 vol% differentiates this rock from a peridotite (requiring <10 vol% plagioclase).
The olivine and pyroxene clasts within the breccia component exhibit a wide range of Fe contents constituting a magmatic sequence progressing from the magnesian olivine gabbro cumulate to an extreme enrichment in Fe# in the ferroan symplectite and silica-bearing FeO-alkali clasts; these ferroan clasts have been compared by some researchers to terrestrial 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 tholeiitic lithologies (Fagan, et al., 2002, 2013). The more ferroan olivine gabbro clasts contain no anorthositeA phaneritic, intrusive igneous rock made with a modal composition (i.e. volume%) > 90% plagioclase feldspar of undefined composition (anorthitic to albitic, or combination thereof), and a small mafic component between 0 - 10% such as pyroxene, ilmenite, magnetite, and olivine 1. The name anorthosite is derived from the calcium-rich Click on Term to Read More (i.e., no highlands component) and have an LREE-enriched pattern with a strong Eu depletion—an unusual composition for VLT basalts, but one which exhibits some similarities to Apollo 14 basalts.
Although the olivine gabbro lithology lacks solar 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, the breccia lithology exhibits an enhancement in the solar gas content indicating that only the breccia resided at the lunar surface for any significant time prior to ejection. The CRE ages of these two lithologies support this finding: 5.2 (±0.8) m.y. for the olivine gabbro and 68 m.y. for the breccia lithology. This age for the breccia is equivalent to a surface residence of 136 m.y. Lorenzetti et al. (2005) argue that after the breccia lithology was exposed near the surface, it was mixed with the cumulate lithology during an impact event and was subsequently buried at a depth of a few meters for the relatively short span of 100 (±20) m.y. The Moon–Earth 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. time is considered to have been <30 t.y., and according to Nishiizumi and Caffee (2010) such a short transit time corresponds to a launch event from a more shallow depth of <1–4.7 m.
A detailed petrogenetic model for mare basalts was presented by J. Day and L. Taylor (2007), a synopsis for which can be found on the NWA 032 page. This model, which demonstrates that NWA 032/479 could be launch paired with the Antarctic LaPaz (LAP) pairing group, was then expounded upon to explore the possibilities that the NWA 773 pairing group might also be derived from the same differentiated stratigraphic magma unit as the NWA and LAP samples (Hallis et al., 2007). Based on chemical compositions, mineralogies, textures, cooling rates, and crystallization and CRE ages, it was argued that the lunar pairing group of NWA 773 may represent the more rapidly cooled cumulate-rich base of this magma unit, whereas the olivine basalt component, well represented in NWA 3160, derives from the lowermost layer adjacent to local pre-existing rock. The uniformly slow-cooled LAP samples are proposed to have crystallized in the middle of the flow, while the more rapidly cooled NWA 032 is consistent with crystallization at the upper margin. The NWA 6950 pairing member has a crystallization age that is ~100 m.y. older than NWA 773, inferring a possible faster-cooling position in the cumulate pile for this rock (Shaulis et al., 2012). In-depth studies of the NWA 032/NWA 4734/LAP pairing group mare basalts conducted by Elardo et al. (2014) led them to conclude that these meteorites were formed in a non-KREEPy source reservoir as opposed to the KREEPy source of the NWA 773 pairing group, ruling out any close relationship between them. However, it remains a possibility that the KREEP-rich component present in members of the NWA 773 clan is a late addition incorporating ejectaFractured and/or molten rocky debris thrown out of a crater during a meteorite impact event, or, alternatively, material, including ash, lapilli, and bombs, erupted from a volcano. Click on Term to Read More from a more distant impact.
The K–Ar chronometer associated with NWA 773 and other pairings reflects an age of 2.75 (±0.3) b.y. (2.865 ±0.031 b.y. calculated from Sm–Nd isochron), possibly indicating a late crystallization age (Burgess et al., 2007). The weighted average Pb–Pb age derived from baddeleyite grains from NWA 773 and other pairings, reflecting primary crystallization of the various pairings of this clan, was determined to be 3.1156 (±0.0068) b.y. (Shaulis et al., 2012, 2013, 2017). The matching Pb–Pb ages for both magnesian and ferroan olivine gabbro cumulate components indicate that they formed during the same time period, while the comparative ages of the olivine gabbro lithologies and the polymict breccia attest to the fact that the constituents of the latter were derived from the former. Furthermore, the chronological dataset on the whole is consistent with all members of the NWA 773 clan being magmatically related (Shaulis et al., 2013, 2017). The NWA 773 clan lunaites have one of the youngest ages measured for a lunar rock with only a few exceptions, such as the low-Ti mare basalt LAP 02205 dated at 2.991 (±0.014) b.y.; Rankenburg et al., 2007).
A possible scenario for the formation of the olivine gabbro begins with the formation of a lunar 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 (LMO) to a depth of 400–500 km, and possibly as deep as 1,000 km (Ranen and Jacobsen, 2004). As cooling proceeded, differences in densityMass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much Click on Term to Read More began to show their effect. After 75% crystallization of the magma ocean, buoyant plagioclase-rich rock began to crystallize and float to the surface to form the original anorthite-rich plagioclase 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 constituting the uppermost ~5–30 km of the magma ocean. At the same time, an increasingly more dense (i.e., a gradual decreasing ratio of Mg to Fe) and 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, compositionally-zoned lower crust was accumulated at depths of ~25–55 km. Thereafter, an unstable configuration resulted as rock of a higher density lay above rock of a lower density, which resulted in convective overturn. This event in turn initiated the pressure-release remelting of early magma ocean olivine- and orthopyroxene-rich cumulates.
An anomalous KREEP-rich region of limited extent (16% of the surface), known as the Procellarum KREEPLunar igneous rock rich in potassium (K), rare-earth elements (REE), phosphorus (P), thorium, and other incompatible elements. These elements are not incorporated into common rock-forming minerals during magma crystallization, and become enriched in the residual magma and the rocks that ultimately crystallize from it. Click on Term to Read More Terrane (PKT), was formed during the final phase of LMO solidification through extreme (>99.5%) 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 which occurred 4.492 (±0.061) b.y. ago. (Korotev 2005). This region is also thought to be the source of an 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. Click on Term to Read More upper 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 melt that was the parent magma of the Mg-suite rocks. Through studies of Sm–Nd data from lunar samples, it was determined that solidification of the LMO was complete in 60–200 m.y. after its onset, at least by 4.417 (±0.006) b.y. ago (Boyet and Carlson, 2007; Grange et al., 2009). Based on Pb–Pb dating of zirconOrthosilicate mineral, Zr(SiO4), observed in all terrestrial rocks type and in ordinary chondrites, eucrites, mesosiderites, and lunar rocks. crystals, this LMO crystallization interval has been refined to 100 m.y. (Nemchin et al., 2009). Nemchin et al. (2009) determined that the formation of an anorthosite crust could not begin until 80–85% of the magma ocean had crystallized, which would allow relatively rapid cooling over a time interval of ~50 m.y. The final 25% of crystallization would have taken place under an insulating anorthosite crust over a similar time interval of ~50 m.y. Based on zircon crystallization studies, it was determined by Grange et al. (2009) that all magmatic activity in at least some locations (e.g., Serenitatis) had completely ended by 4.2 b.y. ago. According to Crow et al. (2011), the Pb–Pb ages of Apollo zircons show a peak at ~4.33 b.y. A somewhat younger Pb–Pb age of 3.953 (±0.018) b.y. was found for a large zircon grain located in the breccia lithology of NWA 773; this attests to the incorporation of some older material located in close proximity to the 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 event (Shaulis et al., 2017).
It is possible that the parent magma assimilated material containing a high-K KREEP composition and a high LREE/HREE ratio, or alternatively, material with a high RE-merrillite composition. This assimilation produced the high incompatible elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of Click on Term to Read More abundances present in the later-formed rocks. In contrast to this assimilation scenario, it was suggested by Shearer et al. (2005) that the observed compositional diversity of KREEP-rich magmas is more consistent with the addition of the KREEP component to the basaltic magma during the melting phase, prior to olivine crystallization. In support of the source mixing model, Borg et al. (2005) concluded that the 87Rb–86Sr ratio of NWA 773 would require 22% KREEP assimilation by the parental magma compared to only a 2% KREEP addition to the source magma. The low Fe content and high Mg# determined for NWA 773 are more consistent with a lower proportion of KREEP as predicted by the source mixing model. Nevertheless, it is now being considered by some that the KREEP component was incorporated into those specific lunar samples through a late impact into the Procellarum KREEP Terrane.
The parent magma underwent differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities. When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including Click on Term to Read More by fractional crystallization, and a Ti-containing cumulus ilmenite was gravitationally extracted. The magma eventually formed plutons of Mg-suite material (petrogenetically distinct from the Mg-suite material from the PKT region), which then intruded the lower crust in some regions of the Feldspathic Highlands Terrane. This Fe-enriched magma may have been part of a shallow layered intrusive, or possibly a thick differentiated 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. Crystallization occurred as the melt cooled from ~1200°C to 1050°C. The upper mantle, composed of ultramafic, olivine-rich dunite or harzburgite, may have contributed melt material to the crystallization process of the Mg-suite rocks. The crystallization sequence from the base of the crust upwards was inferred as follows: dunite (>90 vol% olivine) → troctoliteTroctolite is an intrusive igneous rock consisting of plagioclase feldspar and olivine. It is a member of gabbroic rocks family. It is compositionally similar to gabbro. The main difference is that it does not contain pyroxene or contains very little while it is a major mineral in gabbro. It can Click on Term to Read More (magnesian olivine + 10–60 vol% plagioclase) → norite (low-Ca orthopyroxene + 10–60 vol% plagioclase) → gabbro (high-Ca clinopyroxene + 10–60 vol% plagioclase) → anorthosite (>90 vol% plagioclase). This lunar crystallization sequence is unlike that of any terrestrial oceanic or continental basalt; in the terrestrial case, gabbroic high-Ca pyroxene crystallizes before noritic low-Ca pyroxene. The International Union of Geological Sciences—Subcommission on the Systematics of Igneous Rocks, having established a Working Party on the classification of lunar rocks, has adopted a Classification System for Lunar Rocks.
Northwest Africa 773 is probably derived from an upper mantle or lower-crustal parent melt that was mixed with a KREEP-rich melt component, possibly within a deep 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. This mixture later intruded into upper crustal rock where crystallization under relatively rapid cooling conditions occurred within a 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 or sill (a hypabyssal rocktype), or possibly within a thick lava pool. Elemental distributions within the cumulate lithology provide evidence of cooling rates consistent with this scenario. Ultimately, differentiation of the melt produced the various components that are incorporated into the NWA 773 breccia lithology.
Jolliff et al. (2003) have found that close compositional similarities exist between NWA 773 and Apollo 14 Green Glass B, Type 1, and they suggest that NWA 773 may have originated from a parent melt in proximity to the source region of these picritic green volcanic glasses—located within the Procellarum KREEP Terrane (PKT). It was proposed that the high KREEP concentration was incorporated as the melt transited from the mantle to the surface. As the melt cooled near the surface, crystallization of olivine and Ca-rich pyroxene was initiated and a trapped melt component of 15–25 vol% was incorporated. Various surface volcanic basaltic lithologies were mixed at this stage to produce the NWA 773 impact-breccia lithology. The high concentration of the heat-producing elements Th, U and K present in the PKT region could have permitted an extended period of melting and mixing that is consistent with the young age of NWA 773; however, VLT material has not been identified in significant amounts in this region.
In the PSRD article ‘Damp Moon Rising’ by G. Jeffrey Taylor (July 2010), it was described how studies at the Carnegie Institute of Washington (McCubbin et al., 2010) and Okayama University in Japan (Yamashita et al., 2010) employed a technique called ‘hydrogenLightest and most common element in the universe (~92% by atoms; ~75% by mass). Hydrogen's isotopes are: • 1H (99.9885 %) • 2H (0.0115 %), also called deuterium. • 3H, also called Tritium, is a radioactive (t½ = 12.32 y) by-product of atmospheric thermonuclear tests in Earth's hydrosphere and atmosphere. Click on Term to Read More manometry’ to construct an OH– calibration curve from apatite standards. This curve was then employed to determine the amount of water present in specific lunar meteoritesAchondrite meteorites from the surface of the Moon. Most were found in the hot deserts of northern Africa and Oman and others were found in the cold desert of Antarctica, although one, a 19-gram specimen, was recovered in 1990 from Calcalong Creek, Australia. These stones are of great importance because, Click on Term to Read More such as NWA 773. Fluorapatite in NWA 773 and pairings contains 0.4 to 0.7 wt% (4,000–7,000 ppmParts per million (106). Click on Term to Read More) water, significantly more than previously supposed. This converts to a minimum of 0.7 to 1.7 wt% (7,000–17,000 ppm) water in the KREEP-bearing, late-stage magma from which NWA 773 and pairings were derived. In their study of apatite grains in NWA 773, Tartèse et al. (2014) found significantly higher water contents, and determined that the apatites could be resolved into two distinct groups: one characterized by moderate amounts of H2O in the range of 700–2,500 ppm, and another having high to extremely-high amounts of H2O in the range of 5,400–16,700 ppm. The extraordinarily high water content in some apatite grains of the brecciated lithology of NWA 773 has been conjectured to be related to silicate liquid immiscibility, possibly as a result of depletion of F and Cl in the Si-rich melt fraction.
For their calculations, Yamashita et al. (2010) presumed that apatite would have formed from such an evolved magma only after 90–95% crystallization, so they argue that the original basaltic magma would have proportionally contained 360–850 ppm water. Moreover, given the reasonable scenario in which 10% 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 Click on Term to Read More of the lunar interior occurred, the unmelted interior source region which hosted NWA 773 and pairings would have contained 7–17 ppm water. Although no longer thought to be dry, the Moon certainly contains much less water than the 500–1,000 ppm incorporated in the Earth.
Through remote sensing technology aboard the Clementine spacecraft, utilizing multispectral reflectance imaging, measurements of the mafic minerals olivine, pyroxene, and plagioclase feldspar, and indirectly, anorthosite, have been performed across nearly the entire lunar surface. Notwithstanding the proposal by Jolliff et al. suggesting that the PKT region is a possible source location for NWA 773, a different gabbro-containing site located on the far side of the Moon, in the South Pole–Aitken (SPA) basin, has been identified by Clementine. This 2,500 km-diameter impact structure has had its upper crust completely removed, and a homogeneous melt sheet was formed. This large basin is thought to preserve ancient crustal rock that is mostly uncontaminated by subsequent basin ejecta transported over the Moon’s surface (Petro and Pieters (2008). Subsequent impact events onto the SPA basin, such as those which formed the 64-km-diameter Bhabha 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 and the 505-km-diameter Apollo basin, have excavated lower-crustal material from depths greater than 20 km. Although SPA is predominantly noritic in composition, a small rise known as ‘Olivine Hill’ is interpreted to be an olivine gabbro lithology. Volcanic activity associated with small mare ponds that occurred after basin formation is consistent with the presence of the VLT basaltic component identified in the NWA 773 breccia.
Calzada-Diaz et al. (2015) compared compositional and age data from a large number of lunar meteorites with elemental remote sensing data obtained by the Lunar Prospector gamma rayMost energetic form of radiation, similar to x-rays and light, except with shorter wavelengths (<0.01 nm) and higher energies (>105 eV). Despite their high energies and penetrating power, g-rays from cosmic sources are absorbed by the atmosphere. In nuclear reactions, decay by g-ray emission permits an excited nucleus drop lower Click on Term to Read More spectrometer, primarily for Fe, Ti, and Th, to better constrain the meteorite’s source regions. For the basaltic breccia NWA 773, plausible ejection sites were identified in Mare Serenitatis, Mare Crisium, and the western boundary regions of Oceanus Procellarum, while Mare Fecunditatis was found to be inconsistent with the age data (see image below). Image credit: A. Calzada-Diaz et al. MAPS, vol. 50, #2, p. 219 (2015) ‘Constraining the source regions of lunar meteorites using orbital geochemical data’ (http://dx.doi.org/10.1111/maps.12412) The lunar meteorites Y-793274, QUE 94281, and EET 87521/96008 share many compositional characteristics with NWA 773 and may have experienced a similar petrogenesis. With the recovery of NWA 773, representing lower-crustal olivine gabbro, our ability to understand the Moon’s early history has been greatly enhanced. The top photo above shows both the cumulate lithology and the breccia which constitute the NWA 773 meteorite. The specimen on the left is a 0.094 g cut fragment of the dark-colored polymict breccia component, pervaded by fragments of the cumulate lithology and other diverse mineral and lithic clasts, while the specimen on the right is a 0.085 g cut fragment of the olivine gabbro cumulate component consisting of green to tan olivine crystals within pyroxene, interspersed with black chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More grains and transected by a small shock-melt vein. An enlarged photo of this cumulate specimen is shown as well.