Lunar Feldspathic 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 (impact-melt)
Found January 2003 19° 19.9′ N., 54° 47.0′ E. Nine individual stones, having a total combined weight of 245.46 g and exhibiting heterogeneous compositions, were recovered by a German expedition searching in Wadi Quitbit within the Dhofar dense collection area of Oman. The expedition was searching within the strewnfield in which the lunar pairing group of Dhofar 303, 305, 306, 307, 309, 310, 311, 730, 731, 489 (found 24 km away), 908, 909, 911 (comprising nine separate stones), 950, and 1085 was recovered. Terrestrial weathering has produced significant staining from 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 Click on Term to Read More (R. Korotev).
The finder of the individual Dhofar 908 stone, Norbert Classen, adopted the term ‘Rosetta Stone’ to describe this 81 g lunaite due to its having three distinct lithologies that link the diverse finds Dhofar 302, 303, 305, 306, 307, 309, 310, 311, 730, 731, and 489 together; most of these other stones represent only one of the three lithologies. Importantly, Dhofar 908 established a clear pairing relationship among all of these separate finds (see photo below).
Following the analysis at the Institut für Planteologie in Münster, these meteorites were classified as lunar feldspathic breccias, specifically, impact-melt breccias. Interestingly, the lunar feldspathic fragmental breccia that comprises the individual stones Dhofar 081, 280, 910, and 1224, was found in the western half of this same strewnfield, which encompasses an area of 1.4 × 1.2 km—an astounding case of overlapping lunar strewnfields.
Cosmogenic nuclideA nuclear species characterized by Z protons and N neutrons. Click on Term to Read More studies of Dhofar 908 based on 10Be and 26Al have enabled the determination of the excavation depth on the Moon (>6 m), 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 (4 ±1 t.y.), and the terrestrial age (~300 t.y.) (Nishiizumi and Caffee, 2006). Sm–Nd data yield 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 age of 4.31 (±0.07) b.y., and might reflect derivation from magnesian troctolitic-anorthosite precursor material from plutons that intruded into the early ferroan 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 after solidification of the 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 (Nyquist et al., 2010). Gross et al. (2012) have presented evidence found in most feldspathic highlands meteorites that a global lunar 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 ocean did not form, and they lack features of such a scenario; i.e., they contain no ferroan anorthosites, 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, or Mg-suite rocks. Instead, 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 intrusions rise continuously in diapirs, resulting in compositional diversity among the crustal regions.
An Ar–Ar age for the mostly troctoliticTroctolite 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 MorematrixFine 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 material of Dhofar 908 was found to be 4,256 (±20) m.y., taking into consideration evidence for solar windSupersonic flow of high-speed charged particles continuously blowing off a star (mostly e- and p+). When originating from stars other than the Sun, it is sometimes called a "stellar" wind. The solar wind may be viewed as an extension of the corona into interplanetary space. The solar wind emanates radially Ar trapped during residence on the lunar surface. This age may reflect material derived from an old basin within early-formed anorthositic crust on the lunar farside in which low Th and FeO exist (Karouji et al., 2010). The crustal asymmetries that exist between the farside and nearside can be explained by the tilted convectionTransfer of heat energy by moving material. Temperatures increases with depth in planetary objects. Deep hot less-dense material physically rises and cools, releasing heat and becoming denser. The now cooler denser material sinks back into deeper regions, where it will be reheated and rise again. Convection is an important mechanism Click on Term to Read More model. A potential ejection site for the Dhofar pairing group is considered to be the Derichlet-Jackson basin.
Studies of Dhofar 489, a member of the pairing group, revealed the presence of unique magnesian anorthosite clasts and a spinelMg-Al oxide, MgAl2O4, found in CAIs.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 MoreclastA mineral or rock fragment embedded in another rock. Click on Term to Read More. Its bulk analysis is highly depleted in Th (proxy for ITEs) and FeO (Takeda et al., 2007). In addition, studies of the paired stone Dhofar 309 revealed clasts of anorthosite and troctolite composition, considered to be metamorphosed and annealed crystalline rocks associated with an impact-melt pool. A reddish-orange clast found in these samples has a crystalline texture and contains 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 crystals and rounded 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 grains; it is thought to be an impact-melt clast derived from spinel troctolite, but which includes a 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 component derived from 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.
Studies of the paired Dhofar 307 led to the discovery of magnesian anorthositic granulite clasts, originally derived from impact melts, some of which contain large olivine fragments embedded in a glassy plagioclase matrix (Takeda et al., 2008, 2010). These olivine fragments may represent ejected 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 rock from the massive impact that created the largest impact craterCrater formed by high-speed impact of a meteoroid, asteroid, or comet on a solid surface. Craters are a common feature on most moons (an exception is Io), asteroids, and rocky planets, and range in size from a few cm to over 1,000 km across. There is a general morphological progression Click on Term to Read More in the Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids., the South Pole–Aitken basin, located on the lunar farside. These magnesian anorthositic granulite clasts may be ancient (up to 4.3 b.y.), and are thought to represent basaltic plutons which were emplaced into plagioclase-rich crust following solidification of the lunar magma ocean. The bulk compositions of the anorthositic granulite clasts (FeO ~4.5 wt%; Al2O3 ~28 wt%; Th <1 ppmParts per million (106). Click on Term to Read More) are similar to those calculated for the Feldspathic Highlands Terrane (FHT) on the lunar farside. The magnesian anorthosite clasts represent a distinct geochemical lunar component that is widespread across the lunar surface (Treiman et al., 2010). These clasts contain too much magnesium to be related to the Ferroan Anorthosite Suite (FAN) components, and are too feldspathic and lacking in KREEP to be related to the Mg-Suite components, both of which are typical contaminates in lunar breccias recovered from the nearside of the Moon.
Since Dhofar 908 and its pairing group represent a quickly cooled impact-melt breccia that could have formed at a significant depth beneath a 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, which could potentially have been contaminated with incompatible elements, its low-Th, low-FeO signature taken by itself is not an adequate determinant for either a nearside or a farside origin for this type of lunar 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 (R. Korotev). However, when these analyses are considered together, along with the possible discovery of a norite component, it can be inferred that the origin of this pairing group is most likely the lunar farside—possibly from the area of the South Pole–Aitken basin, or perhaps the FHT at the northern portion of the lunar farside.
An alternative formation scenario has been proposed by Takeda et al. (2008, 2010) in which a large impact into magnesian anorthosites, likely on the northern farside, excavated a basin at least 80 km in diameter and produced an extensive melt sheet. Deep-seated lithologies present in the meteorite were excavated as well. Rapid cooling and subsequent impact gardening within this basin led to the final consolidation of this brecciated rock. A large basin containing many craters in which the Dhofar 908 breccia may have formed is the Dirichlet-Jackson basin, located in the low-Th region of the farside. The mineralogy, chemistry, and petrologyScience dealing with the origin, history, occurrence, chemical composition, structure and classification of rocks. Click on Term to Read More of the various members of this pairing group indicate that they were all derived from a common precursor lithology, one having a spinel troctolite composition consistent with a location at a significant depth (>5 m) within the crust.
The specimen pictured above is a 0.143 g very thin partial slice from 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". Click on Term to Read More of Dhofar 908. The photo below shows the main mass of Dhofar 908, the first stone to be found, weighing 81.43 g. Dhofar 908 was exported by permit from the Ministry of Commerce and Industry, Sultanate of Oman. The bottom photo is a beautiful 0.61 g thin slice from the Dhofar 908 main mass showing three separate lithologies (3.1 cm in longest dimension). Photo courtesy of Norbert Classen click on photo for a magnified view
Three Lithologies: left: IMB clast-poor lithology top: mature regolith w/ dark matrix bottom: IMB clast-rich lithology Photo courtesy of Stephan Kambach
For additional information on the magnesium-rich granulites, read the PSRD article by Linda Martel—‘Unraveling the Origin of the Lunar Highlands Crust’, Sept. 2010.