Rare-earth abundances in chondritic meteorites

Petrographic variations among carbonaceous chondrites of the Vigarano type

The MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths in kimberlite

According to the results of the present analysis, carbonaceous chondrites are obviously not pristine samples of proto-solar-system condensates. In terms of chemistry, however, these chondrites may represent the most primitive solar system materials known. It appears that the alterations experienced by carbonaceous chondrites were isochemical, or nearly so, so that their bulk compositions have remained practically unchanged, provided that the analyses are representative of large portions of the meteorites.

Are carbonaceous chondrites primitive or processed? A review

(Received 1996 August 30; accepted in revised form 1997 January 30) Abstract-Imaging of asteroids Gaspra and Ida and laboratory studies of asteroidal meteorites show that im- pacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks. However, in recent years, impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids, such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events. The latter were proposed to explain the origin of ureilites, aubrites, mesosiderites, the Eagle Station pallasites, acapulcoites, lodranites, and the IAB, IIICD, and IIE irons. We considered fundamental observations from terrestrial impact craters, combined with results from laboratory shock experiments and theoretical consider- ations, to evaluate the efficiency of impact heating and melting of asteroids. Studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of aster- oids will produce two types of impact melts: (1) large-scale whole rock melts (total melts, not partial melts) at high shock pressure and (2) localized melts formed at the scale of the mineral constituents (mineral specific or grain boundary melting) at intermediate shock pressures. The localized melts form minuscule amounts of melt that quench and solidify in sifu, thus preventing them from pooling into larger melt bodies. Partial melting as defined in petrology has not been observed in natural and experimental shock metamor- phism and is thermodynamically impossible in a shock wave-induced transient compression of rocks. The total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater. Internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids, and impact melt rocks are usually clast-laden. Thermal metamorphism of country rocks by impact is extremely minor. Experimental and theoretical considerations suggest that (1) single dis- ruptive impacts cannot raise the average global temperature of strength- or gravity-dominated asteroids by more than a few degrees; (2) cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter; (3) small crater size, low gravity, and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small (-0.014.1%) fraction of the total displaced crater volume; (4) total impact melt volume formed during the typical lifetime of an asteroid is a small fraction (<0.001) of the volume of impact-generated debris; and (5) much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and, thus, not retained on the asteroid. The inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above, and we must turn to processes other than impact, such as decay of short-lived radionuclides or electromagnetic induction during an early T-tauri phase of the Sun to explain heating and melting of the parent bodies of these meteorites.

Constraints on the role of impact heating and melting in asteroids

Carbonaceous and non-carbonaceous lithic fragments in the Plainview, Texas, chondrite: origin and history☆

Five lithic fragments with poikilitic textures in five LL-group chondrites are examined by microscope and electron microprobe to determine whether the textures have resulted from processes related to impact events, such as thermal metamorphism or partial melting. The bulk and pyroxene compositions of the fragments are determined. The compositional characteristics of minerals in certain fragments are found to indicate an apparent melt origin. It is concluded that impact processes produced the poikilitic textures and that complete melting, partial melting, and solid-state recrystallization all have had some part in producing the textures.

Implications of poikilitic textures in LL-group chondrites

Petrology of volcanic rocks from an aseismic rise: Implications for the origin of the Rio Grande rise, South Atlantic Ocean

A komatiite-like lithic fragment with spinifex texture in the Eva meteorite - Origin from a supercooled impact-melt of chondritic parentage

The Oro Grande, New Mexico, U.S.A., chondrite was found in 1971. Electron microprobe analyses and microscopic examination show the following mineralogy: olivine (Fa 19.3 mole %), orthopyroxene (Fs 16.2 mole %), diopside, feldspar (An 13.6 mole %), chlorapatite, whitlockite, kamacite, taenite, troilite, chromite, and an iron-bearing terrestrial weathering product. A bulk chemical analysis of the meteorite shows the following results (weight %): Fe 0.84, Ni 1.46, Co 0.07, FeS 3.62, SiO2 34.18, TiO2 0.14, Al2O3 1.83, Cr2O3 0.55, Fe2O3 21.25, FeO 9.13, MnO 0.31, MgO 21.52, CaO 1.72, Na2O 0.70, K2O 0.08, P2O5 0.25, H2O(+) 2.14, H2O(-) 0.40, C 0.22, sum 100.41. On the basis of composition and texture the Oro Grande meteorite is classified as an H5 chondrite.

R. V. Fodor

Papers

Carbonaceous and non-carbonaceous lithic fragments in the Plainview, Texas, chondrite: origin and history☆

Implications of poikilitic textures in LL-group chondrites

Petrology of volcanic rocks from an aseismic rise: Implications for the origin of the Rio Grande rise, South Atlantic Ocean

A komatiite-like lithic fragment with spinifex texture in the Eva meteorite - Origin from a supercooled impact-melt of chondritic parentage

The oro grande, new mexico, chondrite and its lithic inclusion