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Groupe de l'événement « Vernissage Chemin Land Art 2022 »

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Hermann Konovalov
Hermann Konovalov

Achondrite



An achondrite[1] is a stony meteorite that does not contain chondrules.[2][3] It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies.[4][5] As a result, achondrites have distinct textures and mineralogies indicative of igneous processes.[6]




achondrite



Primitive achondrites, also called PAC group, are so-called because their chemical composition is primitive in the sense that it is similar to the composition of chondrites, but their texture is igneous, indicative of melting processes. To this group belong:[8]


Asteroidal achondrites, also called evolved achondrites, are so-called because they have been differentiated on a parent body. This means that their mineralogical and chemical composition was changed by melting and crystallization processes. They are divided several groups:[8]


Only 6.9% of stony meteorites are achondrites. The achondrites include eucrites (35%), ureilites (14%), diogenites (13%), howardites (10%), lunar (10%), martian (6%), and some rare achondrites (together, 12%). In the absence of a fusion crust, neither you nor I can identify a rock as an igneous achondrite simply by visual examination and I think no one else can either. Achondrites are made of the same minerals as terrestrial rocks. The igneous (mostly basaltic) achondrites (some eucrites, some diogenites, some lunar, and most martian) look just like their terrestrial counterparts. They are all formed by the same processes. Many brecciated achondrites (lunar, eucrites, howardites) look like some kinds of terrestrial sedimentary rocks and I have known experienced scientists and collectors who have mistaken terrestrial rocks and brecciated eucrites for lunar breccias.


If it is a breccia that contains iron-nickel metal, then it is a meteorite. Some brecciated achondrites contain iron-nickel metal, but not much, although in a few (e.g., NWA 5000) metal grains can be seen on a sawn face or as rusty spots. I suspect that only a few brecciated achondrites contain enough metal for the rock to attract a magnet.


Any meteorite that is not a chondrite is technically an 'achondrite'. The prefix "a" will typically indicate "without" in scientific terms. So a meteorite with the designation of 'achondrite' signifies a meteorite that has no chondrules. This is important to note when trying to get a handle on differentiation because it is this very process of differentiation that creates the achondrites. And conversely if an asteroid never gets big enough to undergo the process of differentiation, it remains chondritic.


If we accept the idea that the entire Solar System was created or 'accreted' from the same giant molecular cloud of gas and dust particles, then we can make the leap to say that until any given accreted body in our Solar System grows large enough to undergo differentiation, it remains a potential chondritic meteorite parent body. When one of these same chondritic bodies in our Solar System grows large enough to undergo the process of differentiation, it can no longer produce chondritic meteorites, transforming instead into a potential achondrite parent body.


When ejecta from the surface of a larger asteroid or planet that has undergone differentiation hits the Earth, it will have had most if its metal removed and chondrules eliminated, making it an achondrite.


An achondrite that fell in Sioux County, Nebraska. This eucrite has been broken open, revealing the distinction between its black, shiny, and smooth fusion crust and its light-colored interior. The specimen is about 7 cm from left to right. Photo by J. Kurtzmen.


Achondrites contain mostly one or more of the minerals plagioclase, pyroxene, and olivine, and generally, but not always, lack the small rounded inclusions known as chondrules that are typical of chondrites. Most achondrites are chemically similar to basalts and are thought to be the product of melting on large asteroids, moons, and planets. Soon after these worlds formed, they were heated from within and partially melted. Although this process is still active on Earth, it ended about 4.4 billion years ago on asteroids, 2.9 billion years ago on the Moon, and perhaps one billion years ago on Mars. Heating of the primordial mixture of stony minerals, metals, and sulfides (of which chondrites are made) produced liquids, the densest of which sank to become planetary or asteroidal cores. Lighter stony minerals rose and solidified to become basaltic rocks, fragments of which were subsequently broken off by impacts and hurled into space. More than 200 of these evolved achondrites have been found, covering a wide range of compositions and origins.


The so-called HED group includes the howardites, eucrites and diogenites, which appear to share the same parent body, believed to be the asteroid Vesta. Other evolved achondrites that seem to have come from partially differentiated asteroids other than Vesta have been mostly assigned to two distinct groups known as the angrites and the aubrites.


Although the majority of achondrites are of asteroidal origin, some are known to have come from the highland regions of the Moon's farside (see lunar meteorites) and from Mars (see Mars meteorites). NWA011, a meteorite found in the Sahara in 1999, is suspected of having originated on Mercury.


As well as the evolved achondrites, there's an entire group of primitive achondrites whose members are similar to chondrites in composition and have an age similar to that of the primordial chondrites. Primitive achondrites, which belong to the so-called PAC group, seem to have derived from small chondritic parent bodies that only partially melted and differentiated through accretion processes or from impact events, and then rapidly cooled. Following an initial heating phase, they quickly cooled to become geologically inactive.


This superior chunk of achondrite has gone from prime to sublime through a process you don't understand and aren't going to worry about. What's important is that it promises an impressive yield from aetherial reduction.


An achondrite is a stony meteorite that is made of material similar to terrestrial basalts or plutonic rocks. Compared to the chondrites, they have all been differentiated and reprocessed to a lesser or greater degree from the effects to melting and recrystallization on or within meteorite parent bodies.


The unique achondrite meteorite Northwest Africa (NWA) 6704 and its paired samples are fragments of an unknown parent asteroid that experienced large-scale igneous melting early in our solar system's history. The geochemistry and mineralogy of NWA 6704 show that its parent asteroid has affinities with carbonaceous chondrites and that the precursor materials were relatively oxidized. While large-scale melting has affected the meteorite, there is no evidence for equilibration with a metallic melt. NWA 6704 paired meteorites therefore provide insights into the evolution of planetesimals and bodies that accreted from source materials, possibly in the ice-rich outer solar system. Currently, we lack an understanding of the distribution of potential parent asteroids of the NWA 6704 meteorites. We have undertaken a detailed multiwavelength (0.35-25 μm) spectroscopic and geochemical investigation of NWA 6704 to provide constraints on the potential parent asteroids of these enigmatic meteorites. In comparison with asteroid spectra, NWA 6704 is similar to the S(VI) subtypes of the S-asteroid complex. By using the Bus-DeMeo Taxonomic Classifier, we determine that NWA 6704 has affinities toward V-type (Vesta type) asteroids. We have determined that the parent asteroid of NWA 6704 would be a V-type asteroid that is not dynamically linked to Vesta and also fall in the S(VI) subtype of the Band I center versus Band area ratio diagram. A search in the literature for potential parent bodies yielded one asteroid, (34698) 2001 OD22, as a possible candidate.


Accretion and differentiation of planetesimals are critical steps of planetary formation. By studying achondrite meteorites, we can bring further geochemical and chronological constraints on the source and composition of materials and their evolution throughout the protoplanetary disk. Major and trace element analyses for mineral and whole-rock compositions and high-precision isotopic compositions for the short- and long-lived radiochronometers 53Mn-53Cr, 147,146Sm-143,142Nd and 176Lu-176Hf are reported for two eucrites, two diogenites, and three ungrouped achondrites. Of these, NWA 12338 is a newly classified ungrouped achondrite with an anomalously higher Δ17O than mean value of eucrites, and distinguished by the presence of igneous olivine. e142Ndi of carbonaceous achondrites is slightly lower than non-carbonaceous achondrites by 15 ppm, indicating a s-process nucleosynthetic deficit in the formation reservoir of the former ones. Results on NWA 11001 eucrite suggest that a protracted magmatism on the eucrite parent body persisted for at least 25 Ma after Solar System formation.


Achondrite meteorites are extra-terrestrial magmatic rocks which were ejected from the outer silicate-rich layers of planetesimals that formed more than 4.5 billion years ago in the inner Solar System. They can tell us about the first stages of planetary melting and crust-mantle formation, and also about the distinct reservoirs where planetary embryos formed in the protoplanetary disk well before being mixed together in the asteroid belt. Based on isotopic fingerprints such as mass-independent oxygen and chromium isotopic variations, cosmochemists have recently identified two isolated nebular reservoirs which have been named carbonaceous chondrite (CC) and non-carbonaceous chondrite (NCC) reservoirs. We here investigate achondrite meteorites which may be related to these two groups (based on their O-, Cr-isotopic compositions and/or classification) to compare their elemental and isotopic compositions and chronology records. We report their elemental compositions at the mineral and whole-rock scales (e.g., trace elements, via EPMA, LA-ICP-MS and Q-ICP-MS) and the short-lived and long-lived radiogenic and stable isotopic systematics for seven achondrites (53Mn-53Cr, 147,146Sm-143,142Nd and 176Lu-176Hf). Their geochemistry, such as Rare-Earth Element (abbreviated as REE) concentrations normalized to CI chondrites provide information about their igneous history, and the degree of oxidation of their source reservoirs; the radiogenic isotopes tell us about variable timescales of planetary differentiation on different parent bodies; while the stable isotopes (e.g., 178Hf, 145Nd and 54Cr mass-independent isotopic variations) can be used as tracers of planetary reservoirs and irradiation histories. Coupled with literature data on other achondrites, we find that achondrite meteorites preserve isotopic heterogeneities consistent with isolated planetary formation reservoirs with different oxidizing conditions. Additionally, one of our samples, NWA 11001, supports a protracted magmatic history in the eucrite parent body over 25 million years after formation of the Solar System. 041b061a72


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