Mathematics – Logic
Scientific paper
Sep 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33.1113b&link_type=abstract
Meteoritics & Planetary Science, vol. 33, no. 5, pp. 1113-1122
Mathematics
Logic
32
Scientific paper
In most groups of carbonaceous chondrites minerals occur that are formed due to aqueous alteration in the nebula or/and 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 from 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 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 "pre-accretionary" 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 pre-accretionary aqueous alteration of distinct components in carbonaceous chondrites. In this paper evidence for pre-accretionary aqueous alteration in carbonaceous chondrites and their components taken from previous studies is discussed in detail. The strongest evidence for pre-accretionary 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 pre-accretionary 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 pre-accretionary origin of hydrous phases in Ca,Al-rich inclusions 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.
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