Shock effects in meteorites comprise two maj or phenomena: (1) Shock metamorphism defined as the mechanical deformation and transformation of rocks below or above the solidus by shock compression and (2) breccia fo rmation which involves ballistic or non-ballistic transport and the relative movement of rock fragments and melts by displacement from the primary location in the parent target bodies. Various collision scenarios lead to specific combinations of shock metamorphism and breccia fo rmation if the relative sizes and velocities of the colliding bodies and the specific impact energy are freely variable above a certain threshold value of the impact velocity. In the low ve10city regime "accretionary breccias" can be fo rmed by catastrophic disruption and reaccre tion of the fragmented bodies. These breccias may lack distinct shock-induced fe atures of their constituents. In the impact cratering regime (impact velocities >0.5 to 1 km/s) shocked rocks and impact melts are fo rmed and incorporated into crater deposits or in the crater basement of asteroidal surface-subsurface units in which various types of "impact breccias" can be recognized: monomict breccias and polymict breccias such as regolith breccias, fragmental breccias, impact melt breccias and granulitic breccias. Shocked and brecciated meteorites may aiso directly evolve from high velocity catastrophic fragmentation of the colliding bodies when the fragments exceed the escape velocity. Shock effects have been observed in a11 major groups of meteorites. They affect not only the petrographic characteristics but also the chemical and isotopic properties and the ages ofthe primordial meteoritic material. Progressive stages of shock metamorphism in the range of", 5 to 80 GPa have been observed in ordinary chondrites. Localized melting (veins, melt pockets) is a feature in the 10-80 GPa range. Ordinary chondrite breccias are fo rmed by shock lithification of clastic asteroidal surface debris or by accretion of disrupted parent body material. Enstatite and carbonaceous chondrites were also affected by shock metamorphism. Fragmental and re golith breccias are known for the enstatite and carbonaceous chondrite groups. It appears that accretionary breccias occur among carbonaceous chondrites. Basaltic achondrites are affected by a11 stages of shock metamorphism but the shock-induced melts occur only as clasts within polymict breccias. Ureilites have been shocked up to about 60 GPa and form rarely polymict breccias, whereas aubrites display no severe shock «20 GPa) but polymict breccias are quite common. The SNC-meteorites are never breccias but either shocked or unshocked rocks. The lunar meteorites are clearly derived from the regolith or subregolith rocks. The regolith or fragmental breccias are exclusively polymict and show variable degrees of shock metamorphism; the basaltic lunar meteorites appear to be volcanic rock fragments. Iron meteorites range from unshocked to shock melted and display variable degrees of static thermal annealing. It appears that col...

Abstract-In most groups of carbonaceous chondrites, minerals occur that are formed due to aqueous alteration in the nebula and/or within meteorite parent bodies. For determining the evolution of materials in the early solar system, it is of significant importance to clearly identify evidence for either nebular or planetary aqueous alteration. Therefore, results fi-om the study of chondrites have fundamental implications for ideas concerning nebular dynamics, gas-solid interactions in the nebula, and accretionary processes.Considering the topic of this review, it is important to define nebular and parent body processes. The solar nebula activity should include condensation processes of high-and low-temperature components as well as processes of chemical fractionation and grain-size sorting, mixing of solids and gas, and interactions between early formed solids with the remaining gas; but it should exclude processes that occurred in small uncompacted protoplanetary objects that may have been totally destroyed again before accretion of the final meteorite parent bodies. Therefore, the term "preaccretionary" instead of "nebula" is used in this paper to include all these processes that may have occurred in small precursor planetesimals.Currently, there is no doubt that parent body aqueous alteration is a fundamental process in the evolution of several groups of carbonaceous chondrites. However, due to textural and mineralogical observations and chemical analyses, strong arguments have been found also indicating preaccretionary aqueous alteration of distinct components in carbonaceous chondrites. In this paper, evidence for preaccretionary aqueous alteration in carbonaceous chondrites and their components taken from previous studies is discussed in detail.The strongest evidence for preaccretionary alteration comes from studies of CM chondrites. The survival of highly unequilibrated mineral assemblages in accretionary rims, and sharp contacts between chondrule glass and surrounding phyllosilicates, are only two important arguments for preaccretionary alteration features among many others discussed in the paper. Similar observations were also made in CR, CH and related chondrites. Due to the small abundance of water-bearing phases in CO and CV chondrites, the origin of phyllosilicates in these groups is less clear. A preaccretionary origin of hydrous phases in Ca-Al-rich inclusions (CAIs) has also been suggested by several scientists. In CI chondrites, no strong indications for such processes have been found, mainly due to heavy brecciation and severe parent body alteration.

The interstellar material from which the solar system formed has been modified by many processes: evaporation and condensation in the solar nebula, accretion into protoplanetary bodies and post-accretion processes within these bodies. Meteorites provide a record of these events and their chronology. Carbonaceous CI chondrites are among the most primitive, undifferentiated meteorites, but nevertheless show evidence of post-accretion alteration; they contain carbonates that are believed to have formed by reactions between anhydrous CI precursor materials and circulating fluids in the meteorite parent body (or bodies), yet little is known about the nature of these reactions or the timescale on which they occurred. Here we report measurements of excess 53Cr--formed by the decay of short-lived 53Mn--in five carbonate fragments from the CI chondrites Orgueil and Ivuna. Our results show that aqueous alteration on small protoplanetary bodies must have begun less than 20 Myr after the time of formation of the oldest known solar-nebula condensates (Allende refractory inclusions). This upper limit is much shorter than that of 50 Myr inferred from previous studies, and clearly establishes aqueous alteration as one of the earliest processes in the chemical evolution of the solar system.

Samples from the North Ray crater ejecta blanket, Apollo 16, were investigated by petrographic microscope, electron microprobe, instrumental neutron activation and Xray fluorescence analyses, and 40Ar‐39Ar and Rb‐Sr dating techniques. Nine major groups of monomict and polymict breccias were defined on the basis of microscopic texture and these were further subdivided into chemical subgroups on the basis of characteristic elements such as Al, Mg, Fe, Cr, REE, Ni, and Co. The polymict breccias — fragmental breccias, granulitic breccias, and impact melt breccias — are the result of multiple impact‐induced mechanical mixing and melting, and of thermal and impact metamorphism of rock and mineral clasts derived from primordial igneous crustal rocks. For calculations of mixing models it was found that end‐members consisting of the pristine igneous rock components present as discrete samples at the Apollo 16 site and supplemented by KREEP, dunite, and a meteoritic component yield the best fits for the composition of polymict breccias. The end‐member rocks a re: ferroan anorthosite, various magnesian gabbronorites including “sodic ferrogabbro” and “feldspathic lherzolite,” and spinei troctolite. The following model is proposed for the composition and stratigraphy of the target for North Ray crater. The lower section of the stratigraphy is composed of a megabreccia with clasts of highly feldspathic polymict breccias (KREEP‐free “Old Eastern Highland Rock Suite”) interpreted as Nectaris ejecta (Descartes formation). The top section contains KREEP‐bearing polymict breccias (KREEP‐bearing “Young Western Highland Rock Suite”) and appears to be similar to the lithologies found in the Cayley plains. This material interpreted as Cayley formation may be a distant facies of Imbrium basin ejecta deposits of the Imbrium basin. The petrographic differences between these two major selenographic units (the Descartes and the Cayley formations) in the Apollo 16 area are more distinct than the chemical differences. The petrographic and chemical composition of the primordial igneous upper crust in the regions of the Nectaris and Imbrium basins has been calculated by subtraction of the KREEP and meteoritic components from the bulk composition of the Descartes and Cayley materials. The Nectaris, crust which is better constrained, consists of 86‐87% a northosite, 4% sodic ferrogabbro, 0.5‐1.3% feldspathic lherzolite, 6‐8% regular magnesian gabbronorite, 1.8‐2.8% dunite, and 0.1% spinel troctolite. A model for the evolution of the upper lunar crust in the Descartes highlands is proposed on the basis of isotope ages and clast‐matrix relationships of polymict breccias. Essential features of this model in sequenial order are: (1) development of multiple layers of KREEP‐free “early feldspathic fragmental megabreccias” and impact melt sheets on the primordial crust in the time period from 4.4 aeons to the time of the Nectaris impact, which could have occurred as late as 3.85 aeons ago, (2) excavation of these megabreccias by the Nectaris event and d...

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