Large springs in the United States

Abstract

The texture and modal composition of fourteen carbonaceous chondrites of the CM group were studied in detail to unravel the origin of their various components and the evolution of their parent body(ies). The analysis and “mapping” of complete thin sections by scanning electron and optical microscopy revealed textural units in most samples in which all coarse-grained “primordial” chondritic components (chondrules, chondrule fragments, refractory inclusions, particles rich in hydrous silicates (PCP), and various mineral fragments) are coated by fine-grained, serpentine- and tochilinite-rich dust. The dust-mantled particles have spheroidal shapes and form a special lithology in which they are densely aggregated and compacted without any interstitial clastic material. The dust mantles termed “accretionary dust mantles” and their host lithology called “primary accretionary rock” are interpreted as products of accretion processes in the solar nebula. An origin by processes on the parent body is excluded for textural and mineralogical reasons. Most CM chondrites are affected by secondary impact-induced brecciation of variable intensity except for Yamato 791198, which is composed entirely of a primary accretionary rock, obviously unaltered by secondary parent body processes such as aqueous alteration or brecciation. The CM chondrites Y74662, Haripura, Cold Bokkeveld, Kivesvaara, Mighei, Pollen, Nogoya, Y793321, ALH 83100, Murray, and Murchison consist of clasts of primary accretionary rock embedded in a fine-grained clastic matrix derived from primary rock material. The volume percentage of the clastic matrix ranges from 5% ( Y74662) to 76% (Murchison). Bells and Essebi are brecciated and affected by aqueous alteration to an extent that fragments of primary rock are not preserved. The evaluation of literature data indicates that solar wind—implanted noble gases are absent in the primary accretionary rock component of CM chondrites. They are only present in brecciated samples. It must be concluded from the relation between the content of trapped solar wind and degree of brecciation that ( 1 ) the solar noble gases are residing exclusively in the clastic matrix and ( 2 ) some CM chondrites are regolith breccias and others are fragmental or monomict breccias. Both types were formed on the parent body although the latter were never exposed to the solar wind. Based on the results of textural, chemical, and mineralogical investigations, we conclude that a major portion of the water-bearing phases in CM chondrites were produced by the hydration of refractory phases prior to the formation of dust mantles and prior to the formation of the recent CM parent body. This process requires special physicochemical conditions which allow the crystallization of OH-bearing silicates, e.g., higher density of the nebula gas or special precursor planetesimals of the CM chondrite parent body.