FINE-GRAINED SERPENTINE IN CM2 CARBONACEOUS CHONDRITES AND ITS IMPLICATIONS FOR THE EXTENT OF AQUEOUS ALTERATION ON THE PARENT BODY: A REVIEW

Outer main-belt asteroids are predominantly of the C-type (carbonaceous), suggesting that they are likely parent bodies of carbonaceous chondrites. Abundant phyllosilicates in some classes of carbonaceous chondrites have chemical compositions, mineral associations, and textures that preserve direct evidence of the processes by which carbonaceous chondrites and their parent asteroids originated and evolved to their present state. Serpentine is the dominant hydroxyl-bearing mineral in the most abundant (CM) groupof carbonaceous chondrites. Serpentine may have formed as a direct nebular condensate during cooling of the solar nebula, or by aqueous alteration of anhydrous Mg, Fe-silicate precursors. Such alteration of anhydrous precursors may have occurred in the solar nebula prior to assembly of the meteorites' parent bodies or on the parent bodies. The relative proportions of Fe and Mg in fine-grained CM2 serpentines have been used to compare the degree of aqueous alteration of different CM2 chondrites with one another. The Mg content of serpentine increases with increasing overall degree of aqueous alteration, so CM2 chondrites with Mg-rich serpentines experienced a more advanced degree of aqueous alteration than CM2 chondrites with Fe-rich serpentines. Attempts to quantify aqueous alteration of CM chondrites by interpreting electron microprobe analyses in terms of charge-balance and site-occupancy constraints from serpentine stoichiometry have met with mixed success. Despite its imperfections, one widely used alteration index based on serpentine stoichiometry is strongly correlated with the elapsed time since the fall and recovery of witnessed CM chondrite falls. Additionally, volatile organic contaminants introduced during sample processing in the laboratory are associated with serpentine and other matrix phyllosilicates. Together, these post-recovery changes in scientifically important sample attributes imply that oxidation-reduction and other types of weathering and contamination affect these meteorites even during curatorial storage and laboratory processing. The same phyllosilicates that make their carbonaceous-chondritic host rocks scientifically important research targets also render those same rocks extraordinarily vulnerable to terrestrial contamination of some of their most scientifically important attributes. This has possible implications for reconstructing pre-terrestrial (parent body) aqueous alteration phenomena from carbonaceous chondritic meteorites and eventually from samples returned by future missions to asteroids with spectral reflectance properties similar to carbonaceous chondrites.