Thermal processing chondritic meteorites

Meteorites are divided into two broad categories chondrites, which retain some record of processes in the solar nebula and achondrites, which experienced melting and planetary differentiation. The nebular record of all chondritic meteorites is obscured to varying degrees by alteration processes on their parent asteroids. Some meteorites, such as the Cl, CM, and CR chondrites, experienced aqueous alteration when ice particles that co-accreted with the silicate and metallic material melted and altered the primary nebular phases. Other samples, such as the ordinary and enstatite chondrites, experienced dry thermal metamorphism, reaching temperatures ranging from about 570 to 1200 K. In order to understand the processes that occurred in the protoplanetary disk, we seek out the least-altered samples that best preserve the record of processes in the solar nebula. The CV, CO,... 

In order to consider the processes of dust coagulation in the early Solar System, we first review the characteristics of this material. Of considerable importance is the fact that these samples - represented principally by chondritic meteorites, but also by IDPs and by samples from Comet Wild 2 collected by the Stardust mission - all come from parent bodies of different kinds. As a result, even the most primitive of these materials has been processed, both physically and chemically, to different degrees. The processes that affected Solar System dust may have occurred in different environments such as the solar nebula (e.g. evaporation/condensation, annealing) and asteroidal parent bodies (aqueous alteration and/or thermal processing, mild compaction to extensive lithihcation). A major challenge is to understand the effects of this secondary processing. 

Meteorites provide perhaps the best record of the chemical evolution of small bodies in the Solar System, and this record is supplemented by asteroidal spectroscopy. Meteorites show progressive degrees of thermal processing on their parent asteroids, from primitive carbonaceous chondrites that contain percent-level quantities of water, through ordinary chondrites that show a wide range of degree of thermal metamorphism, to the achondrites that have been melted and differentiated. 

The most primitive of the chondritic meteorites are thought to represent the "raw material" from which the Earth and other terrestrial planets were accreted. These most primitive chondrites, the Cl - carbonaceous chondrites (see Section 2.3.3.1), are identified by their high volatile content (they may contain up to 30 wt% of HaO, S, and C). They lack evidence of thermal processing after their accretion and so can be treated as the least altered condensates of the solar nebula. This view is supported by a plot of element concentrations in Cl chondrites against concentrations in the solar photosphere. There is a strong 1 1 correlation for all elements except the gaseous elements (H, He, N, O, and the... 

Chemical changes involving loss of a constituent, like carbon or water in chondrites, require an open system other changes in Table II could occur in open or closed systems. It should be emphasized that thermal metamorphism can only affect secondary (parent body) characteristics - those listed horizontally in Table II - not primary ones. Postaccretionary processes by which H chondrite-like material can form from L or vice versa are unknown. Achondrites, from melted and differentiated parent bodies - including planets (e.g. Martian meteorites like Nakhla) - have petrologies indicative of igneous processing at temperatures 1000 C. 

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