Ultra High Resolution Transmission Electron Microscopy of Matrix Mineral Grains in CM Chondrites: Preaccretionary or Parent Body Aqueous Processing?

CM chondrites are highly hydrated \nmeteorites associated with a parent asteroid that has \nexperienced significant aqueous processing. The meteoritic \nevidence indicates that these non-differentiated \nasteroids are formed by fine-grained minerals embedded \nin a nanometric matrix that preserves chemical \nclues of the forming environment. So far there are two \nhypothesis to explain the presence of hydrated minerals \nin the content of CM chondrites: one is based on textural \nfeatures in chondrule-rim boundaries [1-3], and \nthe other ‘preaccretionary’ hypothesis proposes the \nincorporation of hydrated phases from the protoplanetary \ndisk [4-6]. The highly porous structure of these \nchondrites is inherited from the diverse materials present \nin the protoplanetary disk environment. These \nbodies were presumably formed by low relative velocity \nencounters that led to the accretion of silicate-rich \nchondrules, refractory Ca- and Al-rich inclusions \n(CAIs), metal grains, and the fine-grained materials \nforming the matrix. Owing to the presence of significant \nterrestrial water in meteorite finds [7], here we \nhave focused on two CM chondrite falls with minimal \nterrestrial processing: Murchison and Cold Bokkeveld. \nAnhydrous carbonaceous chondrite matrices are usually \nrepresented by highly chemically unequilibrated \nsamples that contain distinguishable stellar grains. \nOther chondrites have experienced hydration and \nchemical homogeneization that reveal parent body \nprocesses. We have studied CM chondrites because \nthese meteorites have experienced variable hydration \nlevels [8-10]. It is important to study the textural effects \nof aqueous alteration in the main minerals to \ndecipher which steps and environments promote bulk \nchemistry changes, and create the distinctive alteration \nproducts. It is thought that aqueous alteration has particularly \nplayed a key role in modifying primordial \nbulk chemistry, and homogenizing the isotopic content \nof fine-grained matrix materials [7, 11, 12]. Fortunately, \nthe mineralogy produced by parent-body and terrestrial \naqueous alteration processes is distinctive [5, 11].