Chondrite aqueous alteration

Figure 14. Comparison of A Mg (DSM3) and (SMOW) values for various chondrules and whole-rock chondrite samples. The sources of the oxygen isotope data are given in Table 3. Aqueous alteration on this diagram will move points approximately horizontally on this diagram.

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). 

We should also address the question of whether Cl chondrites represent the complete, low-temperature condensate from the solar nebula. Their bulk composition is consistent with such a model. However, Cl chondrites contain among the highest abundances of presolar grains that are not destroyed by aqueous alteration (the mineralogy of Cl chondrites is almost entirely due to such alteration). This suggests that it is more likely that Cl chondrites formed from representative samples of the dust inherited from the Sun s parent... 

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). 

What radiochronometers are best suited for dating aqueous alteration in chondritic... 

Once formed, the chondrite parent bodies experience a variety of processes, including thermal metamorphism, aqueous alteration, shock metamorphism due to impacts, and even disruption from large impacts. Several radiochronometers can provide information on the timing of metamorphism and aqueous alteration. The chronology of this processing is summarized in Figure 9.11. 

S-complex asteroids, which include the older E, S, and M groups, dominate the inner and middle belt out to 2.95 AU, C-complex asteroids are most common in the outer belt, and X-complex bodies, which include the P and D classes, are most common at about 3 AU (Fig. 11.7b). Note that this distribution represents only a part of the main asteroid belt shown in Figure 11.7a. Because of uncertainties in the interpretation of S-complex objects as either ordinary chondrites or achondrites, we can no longer say that the innermost asteroids are differentiated but we can infer that S-complex bodies were at least heated (recall from Chapter 6 that ordinary chondrites are mostly metamorphosed). Cl and 2 chondrites have suffered extensive aqueous alteration, suggesting they formed beyond a snowline marking the condensation of ice that later melted that snowhne likely marks the transition to C-complex objects at about 3 AU. 

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. 

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. 

A perplexing observation is that aqueous alteration appears to have been largely isochemical. The Cl chondrites, which provide the closest match to solar abundances, show the most extensive alteration. Likewise, the chemical compositions of CM chondrites are nearly uniform, despite significant differences in their degrees of aqueous alteration. Aqueous fluids can dissolve significant amounts of soluble materials during reactions, but there is little evidence in bulk Cl and CM chondrites that the dissolved material was transported anywhere by the fluids. Why solidified mud should have retained its cosmic composition is a mystery. 

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. 

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. 

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. 

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