Carbonaceous chondrites aqueous alteration

EUer and Kitchen (2004) have re-evaluated the hydrogen isotope composition of water-rich carbonaceous chondrites by stepped-heating analysis of very small amounts of separated water-rich materials. Their special aim has been to deduce the origin of the water with which the meteorites have reacted. They observed a decrease in 5D with increasing extent of aqueous alteration from 0%c (least altered, most volatile rich) to —200%c (most altered, least volatile rich). 

The aqueous fluids formed by melting of ices in asteroids reacted with minerals to produce a host of secondary phases. Laboratory studies provide information on the identities of these phases. They include hydrated minerals such as serpentines and clays, as well as a variety of carbonates, sulfates, oxides, sulfides, halides, and oxy-hydroxides, some of which are pictured in Figure 12.15. The alteration minerals in carbonaceous chondrites have been discussed extensively in the literature (Zolensky and McSween, 1988 Buseck and Hua, 1993 Brearley, 2004) and were most recently reviewed by Brearley (2006). In the case of Cl chondrites, the alteration is pervasive and almost no unaltered minerals remain. CM chondrites contain mixtures of heavily altered and partially altered materials. In CR2 and CV3oxb chondrites, matrix minerals have been moderately altered and chondrules show some effects of aqueous alteration. For other chondrite groups such as CO and LL3.0-3.1, the alteration is subtle and secondary minerals are uncommon. In some CV chondrites, a later thermal metamorphic overprint has dehydrated serpentine to form olivine. 

Asteroids in the outer asteroid belt show considerable spectral variability, due in part to differences in the degree of aqueous alteration. However, alteration alone is not sufficient to explain all the compositional variability observed in meteorites derived from these objects. Laboratory studies of carbonaceous chondrites show significant differences in the compositions and proportions of the various primary components, demonstrating that accreted materials in the asteroid belt were not uniform. 

Rubin, A. E., Trigo-Rodriguez, J. M., Huber, H. and Wasson, J. T. (2007) Progressive aqueous alteration of CM carbonaceous chondrites. Geochimica et Cosmochimica Acta, 71, 2361-2382. 

Brearley A. J. (1997) Phyllosilicates in the matrix of the unique carbonaceous chondrite, LEW 85332 and possible imph-cations for the aqueous alteration of Cl chondrites. Meteorit. Planet. Sci. 32, 377-388. 

Keller L. P. and Buseck P. R. (1990b) Aqueous alteration in the Kaba CV3 carbonaceous chondrite. Geochim. Cosmochim. Acta 54, 2113-2120. 

Keller L. P. and McKay D. S. (1993) Aqueous alteration of the Grosnaja CV3 carbonaceous chondrite. Meteoritics 28, 378. 

Tomeoka K., KojimaH., and YanaiK. (1989) Yamato-86720 a CM carbonaceous chondrite having experienced extensive aqueous alteration and thermal metamorphism. Proc. NIPR Symp. Antarct. Meteorit. 2, 55—74. 

Some carbonaceous chondrites are rich in carbon (Cl and CM chondrites have 1.5-6% carbon), but others are not. Carbonaceous chondrites are now defined on the basis of their refractory elemental abundances, which equal or exceed those in Cl chondrites. Carbonaceous chondrites are derived from very diverse asteroids, which probably formed in very different locations. The parent bodies of Cl and CM chondrites are highly altered, yet the parent bodies of CH and CB chondrites are less altered than all other chondrite bodies. Young et al (1999) infer from oxygen isotopic compositional data that Cl, CM, and CV chondrites could have been derived from different zones in a single, aqueously altered body. However, bulk chemical differences between these groups indicate fractionation during nebular processes, not aqueous alteration (see below), and the components in CM and CV chondrites are quite different. 

Bischoff A. (1998) Aqueous alteration of carbonaceous chondrites—arevitw. Meteorit. Planet. Sci. 33, 1113-1122. 

Chizmadia L. J. and Brearley A. J. (2003) Mineralogy and textural characteristics of fine-grained rims in Yamato 791198 CM2 carbonaceous chondrite constraints on the location of aqueous alteration. In Lunar Planet. Sci. XXXIV, 1419. The Lunar and Planetary Institute, Houston (CD-ROM). 

Hanowski N. P. and Brearley A. J. (2000) Iron-rich aureoles in the CM carbonaceous chondrites, Murray, Murchison, and Allan Hills 81002 evidence for in situ aqueous alteration. Meteorit. Planet. Sci. 35, 1291-1308. 

McSween H. Y., Jr. (1987) Aqueous alteration in carbonaceous chondrites mass balance constraints on matrix mineralogy. Geochim. Cosmochim. Acta 51, 2469-2477. 

PAHs are believed to be a major class of carbon-bearing molecules in the interstellar medium 1138], They are found in carbonaceous chondrites tliat have fallen to Earth (see section 4.2.1) and in interplanetary dust particles [28]. Shock and Schulte [139] suggested that amino acids could be syntliesized by aqueous alteration of precursor PAHs in carbonaceous chondrites. We directed attention to shock reaction of PAHs [135,140,141], and conducted shock reactions using benzene, tire simplest aromatic hydrocarbon, as a starting material to simulate possible reactions occurring in interstellar space. Furtliermore, we examined the mechanism of shock reaction on the basis of quantum chemistry and discussed the implication for cosmocheniistiy. 

Non-volatile materials in asteroids included 1. presolar (interstellar) dust, and 2. dust condensed from a chondritic vapor in the inner solar system where presolar silicate and carbon dusts had evaporated at 2000 K during a thermal flare-up phase of the young Sun [71]. Evidence for both dust sources can be found in carbonaceous meteorites albeit they were modified by aqueous alteration below 400 K, or thermal alteration up to 800 K in their parent bodies [69,72,73]. When metastable carbynes were not obliterated by parent body alteration, these particular meteorites would be candidates for searches of carbynes formed (probably by condensation) around C-rich YSOs (interstellar dust), in the inner solar system, or both. 

The Cl chondrites experienced severe aqueous alteration on their parent asteroid, and if they ever contained chondrules, metal, and sulfides as the other carbonaceous chondrites do, these phases were largely erased. The Cl chondrites mainly consist of fine grained, hydrous silicates, magnetite (Fe304) probably produced by oxidation of FeNi metal, Fe-Ni bearing sulfides other than troilite, and various salts. Despite the absence of chondrules, the Cl chondrites are still chondrites because their overall elemental composition for many elements is much closer to chondritic meteorites than to iron meteorites or so-called achondrites, that mainly consist of silicates and experienced severe melting. 

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