Aqueous alteration of chondrites

Alteration minerals in carbonaceous chondrites, ( ) image of twisted phyllosilicate plates in the Ivuna Cl chondrite. (b) image of hollow serpentine tubes in the Murchison CM chondrite. 

Bulk hydrogen and 36Ar contents versus mineral alteration index for CM carbonaceous chondrites. These trends reflect progressive incorporation of water and destruction of the noble gas carrier as alteration advances. After Browning et al. (1996). 

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. 

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