Chondritic meteorites secondary processing

Krot, A. N., Hutcheon, I. D., Brearley, A. J. et al. (2006) Timescales and setting for alteration of chondritic meteorites. In Meteorites and the Early Solar System II, eds. Lauretta, D. S. and McSween, H. Y., Jr. Tucson University of Arizona Press, pp. 525-553. A good review of what is known about the timing and nature of secondary processing in chondrites. 

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. 

Of the secondary processes that have affected chondritic meteorites, aqueous alteration is among the most widespread. Evidence of varying degrees of aqueous alteration is present in all the major chondrite groups, with the exception of the enstatite chondrites. This alteration is typically indicated by the presence of hydrous phyllosilicates (principally serpentines and smectite clays), often associated with carbonates, sulfates, oxides (magnetite), and secondary sulfides. The variable alteration assemblages present in different chondrite groups are principally the result of alteration under different conditions (P, T, fo, water/rock ratio) (e.g., Zolensky et al., 1993). 

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. 

Turner G, Gilmonr JD, Whitby JA (2000) High iodine-129/iodine-127 ratios in primary and secondary minerals Chronology orflnid processes. Meteoritic Planet Sci 35 A160-A161 Wasson JT (1985) Meteorites Their record of early solar system history. W. H. Freeman, New York. Wasson JT, Wang S (1991) The histories of ordinary chondrite parent bodies U,Th-He age distributions. Meteoritics 26 161-167... 

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