Meteorites formation conditions

Cameron (1973) speculated that grains from stellar sources survive in the interstellar medium, become incorporated into bodies of the Solar System, and may be found in meteorites, because some meteorites represent nearly unprocessed material from the time of Solar System formation. These grains may be identified by unusual isotopic abundance ratios of some elements, since material from nuclear burning zones is mixed at the end of the life of stars into the matter from which dust is formed. Indeed, these presolar dust grains3 were found in the late 1980s in meteorites (and later also in other types of primitive Solar System matter) and they contain rich information on their formation conditions and on nucleosynthetic processes in stars (see Section 2.2). By identifying such grains in primitive Solar System matter it is possible to study the nature and composition of at least some components of the interstellar dust mixture in the laboratory. 

Keays, R. R., Ganapathy, R., Laul, J. C., Anders, E., Herzog, G. F. Jeffery, P. M. 1970. Trace elements and radioactivity in lunar rocks implications for meteorite infall, solar-wind flux, and formation conditions of Moon. Science, 167, 490-493. 

This chapter briefly introduces the chemistry in circumstellar envelopes (CSE) around old, mass-losing stars. The focus is on stars with initial masses of one to eight solar masses that evolve into red giant stars with a few hundred times the solar radius, and which develop circumstellar shells several hundred times their stellar radii. The chemistry in the innermost circumstellar shell adjacent to the photosphere is dominated by thermochemistry, whereas photochemistry driven by interstellar UV radiation dominates in the outer shell. The conditions in the CSE allow mineral condensation within a few stellar radii, and these grains are important sources of interstellar dust. Micron-sized dust grains that formed in the CSE of red giant stars have been isolated from certain meteorites and their elemental and isotopic chemistry provides detailed insights into nucleosynthesis processes and dust formation conditions of their parent stars, which died before the solar system was bom 4.56 Ga ago. 

Genuine star dust is preserved in meteorites. Most of the presolar grains comes from RG and from 0-rich and C-rich AGB stars. Dust from supemovae and novae has also been found. Elemental, isotopic, and structural analyses of this star dust gives details on stellar nucleosynthesis and dust formation conditions in the circumstellar environments. 

Busfield A, Gilmour JD, Whitby JA, Turner G (2004) Iodine-xenon analysis of ordinary chondrite halide implications for early solar system water. Geochim Cosmochim Acta 68 195-202 Busso M, Gallino R, Wasserburg GJ (1999) Nucleosynthesis in asymptotic giant branch stars relevance for galactic enrichment and solar system formation. Annu Rev Astronom Astrophys 37 239-309 Cameron AGW (1969) Physical conditions in the primitive solar nebula. In Meteorite Research. Millman PM (ed) Reidel, Dordrecht, p 7-12... 

The geochemistry of angrites is characterized by strong silica undersaturation, by which we mean that there is not enough SiC>2 to combine with various cations to form common silicate minerals. The result is the formation of silica-poor minerals like kirschsteinite and nepheline. These meteorites also show strong depletions in moderately volatile elements. They are thought to have formed as partial melts of a chondritic source under oxidizing conditions. 

Cosmochemislry places important constraints on models for the origin of the solar nebula and the formation and evolution of planets. We explore nebula constraints by defining the thermal conditions under which meteorite components formed and examine the isotopic evidence for interaction of the nebula with the ISM and a nearby supernova. We consider how planetary bulk compositions are estimated and how they are used to understand the formation of the terrestrial and giant planets from nebular materials. We review the differentiation of planets, focusing especially on the Earth. We also consider how orbital and collisional evolution has redistributed materials formed in different thermal and compositional regimes within the solar system. 

El Goresy and Kullerud12 correlated the physicochemical conditions of mineral formations in meteorites to the Fe—Cr—S system. The Cr—S subsystem is displayed in Fig. 3. 

It appears that conditions in the solar nebula were appropriate for the FTT but not the Miller-Urey reaction. Kinetic calculations (Lewis and Prinn, 1980) as well as observations on comets (Delsemme, 1977) show that CO and COj, not CH, were the principal forms of carbon. And the dust-laden solar nebula was opaque to UV, precluding any photochemical reactions. It seems best, however, to approach the problem empirically, by examining the meteoritic organic compounds themselves for clues to their formation. We shall review these compounds class by class, looking for the signatures of the FTT or Miller-Urey reactions. 

We note, however, that this classification parameter does not generally reflect the oxidation state of individual chondritic components, e.g., CAIs in all chondrite groups formed under highly reducing conditions (see Chapter 1.08), and magnesian (type-I) and ferrous (type-II) chon-drules (both of which occur in the same meteorites) require formation under different redox conditions (see Chapter 1.07). In addition, the role of nebular and asteroidal processes in establishing of the oxidation states of chondrites remains controversial (e.g., Krot et aL, 2000a). 

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