Aqueous alteration

Meteorites on Mars. Meridiani Planum is the first Iron meteorite discovered on the surface of another planet, at the landing site of the Mars Exploration rover Opportunity [359]. Its maximum dimension is 30 cm (Fig. 8.38). Meteorites on the surface of solar system bodies can provide natural experiments for monitoring weathering processes. On Mars, aqueous alteration processes and physical alteration by Aeolian abrasion, for example, may have shaped the surface of the meteorite, which therefore has been investigated intensively by the MER instruments. Observations at mid-infrared wavelengths with the Mini-TES... 

Morris, R.V. et al. 2008. Iron mineralogy and aqueous alteration from Husband Hill through Home Plate at Gusev Crater, Mars Results from the Mossbauer instrument on the Spirit Mars Exploration Rover. Journal of Geophysical Research, 113, E12S42,... 

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.

The whole-rock samples are not immune from other open-system exchange, however. For example, we do not yet know how aqueous alteration manifests itself on these diagrams because detailed studies of the Mg isotopic effects of water-rock reactions have not been carried out at the time of this writing. 

Alteration in Allende chondmle C6 is concentrated at the margins where 5 Mg values are low (Fig. 17). Evaluation of the likelihood that low 8 Mg values could be the result of aqueous alteration will require studies of Mg isotope fractionation during low-T alteration of terrestrial mafic rocks. An alternative explanation is that low 8 Mg and alteration resulted from condensation (collision frequency of gaseous Mg is greater for the lighter isotopes). 

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). 

When water, the universal solvent, is present on a planet, an asteroid, or in a meteorite, a wide variety of chemical reactions take place that can completely alter the mineralogy and chemistry of an object. Some meteorites show extensive evidence of aqueous alteration. To understand the conditions under which the alteration occurred, one must be able to infer the amount, composition, and temperature of the fluids from the minerals that they produced. 

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). 

Over the next few decades, much effort was devoted to using the I- Xe system as an early solar system chronometer. However, because many of the results did not agree with the relative chronology indicated by the petrography of the samples and with ages determined by long-lived chronometers, and because people did not understand how the iodine was sited in the meteorites and the extent to which metamorphic heating and aqueous alteration... 

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

Chronology of secondary processes in the early solar system. Plot format and anchor points are the same as in Fig. 9.9. Dates related to thermal metamorphism are shown as open symbols, and dates related to aqueous alteration are shown as filled symbols. Both thermal metamorphism and aqueous alteration continued for tens to as much as 100 Myr after CAI formation. Data from Flohenberg and Pravdivtseva (2008), Flutcheon et al. (1998), Flua etal. (2005), Trinquier etal. (2008), Endress et al. (1996), Hoppe et al. (2004), and Zinner and Gopel (2002). 

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. 

Borg and Drake (2005) have determined the timing of aqueous alteration events in Martian meteorites from the ages of secondary minerals. Carbonates in ALH 84001 formed at 3.9 Ga, iddingsite in nakhlites formed -630 Myr ago, and salts in shergottites formed sometime after the crystallization of these rocks, 170 Myr ago. 

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. 

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