Lunar 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 BasaltBasalt is the most common extrusive igneous rock on the terrestrial planets. For example, more than 90% of all volcanic rock on Earth is basalt. The term basalt is applied to most low viscosity dark silicate lavas, regardless of composition. Basalt is a mafic, extrusive and fine grained igneous rock Click on Term to Read More
no coordinates recorded A single fusion-crusted stone weighing 137 g was found in the Sahara Desert. It was sold by the finder to German dealer S. Ralew in 2007, and a sample was submitted to the Museum für Naturkunde (A. Greshake) in Berlin, Germany for analysis and classification under the designation NWA 4898.
This coarse-grained (gabbroic), magmatic rock is composed primarily of phenocrysts 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 Click on Term to Read More
(Ti-rich pigeoniteLow-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, found as a major mineral in eucrites and shergottites. In order to be considered pigeonite, the clinopyroxene must contain 5 to 20 mol % of calcium (Wo5 - 20). Chondrites of petrologic types 4 and below contain significant low-Ca clinopyroxene. During metamorphism to higher temperatures, all existing
and augiteHigh-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, that occurs in many igneous rocks, particularly those of basaltic composition. In order to be considered augite, the clinopyroxene must contain 20 to 45 mol % of calcium (Wo20 - 45). An important and unique Martian meteorite is NWA 8159, that has been classified as an augite Click on Term to Read More
, 50 vol%) and Ca-rich 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
(32 vol%, all of which is converted 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
), with lesser amounts of Fe-zoned 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
megacrysts, needle-shaped grains of 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
, high-silica glass, and chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More
, along with minor FeNi-metal and FeS. The matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More
texture reveals radiating, densely branched, polycrystalline sprays (called ‘spherulites’ because of their large-scale average spherical shape) of clinopyroxene and plagioclase, attesting to rapid quenching of the 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
during eruption to the surface. It has been demonstrated that this spherulitic texture forms when lunar basalts cool at ~20–60°C/hour. Besides the presence of maskelynite, other features that reflect moderate shock levels include the occurrence of planar fractures and mosaisicm in silicates, twinning in ilmenite, partial melt veining, and localized melt pockets with embedded FeS droplets.
Northwest Africa 4898 is a low-Ti, low-Fe, high-Al basalt compositionally similar to Apollo 14 mare basalts. However, NWA 4898 is significantly younger than Apollo 14 samples, having a Rb–Sr-based 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 3,578 (±40) m.y. compared to the calculated age range of 3,950–4,330 m.y. for Apollo 14 samples. Certain elemental ratios also reflect significant differences between the NWA 4898 and Apollo 14 basalt 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
sources. Moreover, NWA 4898 is derived from a highly evolved mantle source, and has the highest Sm/Nd ratio known among lunar mare basalts, reflecting the highest 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
depletion of any lunar mantle source studied thus far.
Spacecraft have found that basalts are not present in all topographic low areas, but instead, reservoirs of basaltic magma are confined at great depth with eruptions being contingent on a combination of factors such as the crustal thickness, the concentration of heat-producing elements (K–U–Th), and the extent of the underlying magma columns. Based on spectral data, it was determined that many different basalt units exist within individual mariaBroad 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
, these representing a wide range of crystallization ages, from ~4.35 b.y. (components of the lunar 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
Kalahari 009) to as young as ~1.3 b.y. 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
counting methods indicate that some maria could have formed as recently as 1 b.y. ago (G. T. Taylor, 2007). With a crystallization age (Rb–Sr) of 3.578 b.y., NWA 4898 is somewhat older than the mare basalt NWA 032, the latter crystallizing 2.852 b.y. ago (Rb–Sr) and considered to be one of the youngest mare basalts in our collections. A look at the ages determined for the 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
reveals that none of them have a bulk rock age older than ~3.85 b.y., upholding the lunar cataclysm hypothesis.
Further information detailing the formation of mare basalts can be found on the NWA 032
page of this website. In-depth information about lunar meteorites in general, and NWA 4898 in particular, can be found on the lunar meteorite website
of the Department of Earth Sciences, Washington University. The specimen of NWA 4898 pictured above is a crusted partial slice weighing 0.207 g. The Washington University website presents a high-resolution close-up photo of the pyroxene–plagioclase sprays
which were formed in the matrix during quenching of this rock.