Chondritic meteorites 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 degree of equilibrium isotopic fractionation among phases depends on temperature, so the isotopic compositions of co-existing phases can be used for thermometry. Oxygen is widely used in this way. For example, Clayton and Mayeda (1984) found that the oxygen isotopic compositions of calcite and phyllosilicates from Murchison lie on a mass-dependent fractionation line and differ in 6180 by 22%o. This difference requires a temperature of around 0 °C, which is interpreted to be the temperature of aqueous alteration on the Murchison parent asteroid. Similar measurements for Cl chondrites indicate that aqueous alteration for these meteorites occurred at higher temperature, 50-150 °C (Clayton and Mayeda, 1999). 

The CV, CO, and CR chondrites are mostly anhydrous and were considered in Chapter 11. However, the CV3oxB chondrites experienced significant aqueous alteration. The matrices of these meteorites are heavily altered and contain phyllosilicates, fayalite, Fe,Ni sulfides and carbides, Ca,Fe pyroxene, and andradite garnet. The CVoxA chondrites were apparently also aqueously altered, but were subsequently dehydrated by thermal metamorphism. The matrices of CR2 chondrites contain alteration minerals that resemble those in Cl chondrites, including phyllosilicates, magnetite (Fig. 12.15d), carbonates and sulfides, although the alteration is not as extensive. Chondrule mesostasis was affected in some CR chondrites. Minor phyllosilicates occur in the matrix of chondrites, but these meteorites contain no carbonates or sulfates. 

The least metamorphosed ordinary chondrites (petrologic types 3.0-3.1) show evidence of minor aqueous alteration that primarily affected the matrix, but in some cases also affected chondrule mesostasis. In these meteorites, the amount of aqueous fluid was very small, limiting the degree of alteration. 

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. 

Brearley, A. J. (2006) The action of water. In Meteorites and the Early Solar System II, eds. Lauretta, D. S. and McSween, H. Y., Jr. Tucson University of Arizona Press, pp. 587-624. The best available review of aqueous alteration processes and materials in chondritic meteorites. 

Guo W, Eiler JM (2007) Evidence for methane generation during the aqueous alteration of CM chondrites. Meteoritics and Planetary Sciences (in press)... 

Meteorites are divided into two broad categories chondrites, which retain some record of processes in the solar nebula and achondrites, which experienced melting and planetary differentiation. The nebular record of all chondritic meteorites is obscured to varying degrees by alteration processes on their parent asteroids. Some meteorites, such as the Cl, CM, and CR chondrites, experienced aqueous alteration when ice particles that co-accreted with the silicate and metallic material melted and altered the primary nebular phases. Other samples, such as the ordinary and enstatite chondrites, experienced dry thermal metamorphism, reaching temperatures ranging from about 570 to 1200 K. In order to understand the processes that occurred in the protoplanetary disk, we seek out the least-altered samples that best preserve the record of processes in the solar nebula. The CV, CO,... 

Several types of the early Solar System materials are available for laboratory analysis (see Chapter 1 and Table 1.1 and Fig. 1.1). Each material has unique characteristics and provides specific constraints on the chemistry of the solar nebula. Major components of this sample are meteorites, fragments of asteroids, that serve as an excellent archive of the early Solar System conditions. Primitive chondritic meteorites contain glassy spherical inclusions termed chondrules, some of the oldest solids in the Solar System. Most chondrites were modified by aqueous alteration or metamorphic processes in parent bodies but there are some chondrites that are minimally altered (un-equilibrated chondrites, UCs). They have yielded a wealth of information on the chemistry, physics, and evolution of the young Solar System. 

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. 

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. 

Endress M. and Bischoff A. (1993) Mineralogy, degree of brecciation, and aqueous alteration of Cl chondrites Orgueil, Ivtma, and Alais. Meteoritics 28, 345—346. 

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. 

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

Chizmadia L. J., Rubin A. E., and Wasson J. T. (2002) Mineralogy and petrology of amoeboid olivine inclusions in C03 chondrites relationship to parent-body aqueous alteration. Meteorit. Planet. Sci. 37, 1781-1796. 

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. 

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). 

The Cl chondrites have long been cited as the classic example of asteroidal aqueous alteration, because of the presence of ubiquitous sulfate veins (DuFresne and Anders, 1962 Richardson, 1978 Fredriksson and Kerridge, 1988). These veins crosscut the dark, fine-grained matrix and can extend across the entire meteorite sample or stone. These veins have commonly been attributed to the widespread movement of water within the Cl parent body. However, Gounelle and Zolensky (2001) have reappraised the origin of these veins and concluded that they are terrestrial, not asteroidal, in origin. Their preferred interpretation is that the veins formed as a result of the dissolution, local transport, and precipitation of extraterrestrial sulfates by absorbed terrestrial water. Thus, one of the widely accepted lines of evidence to support parent-body alteration should now be treated with caution. 

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