On the origin of rim textures surrounding anhydrous silicate grains in CM carbonaceous chondrites

Fine-grained, optically opaque rims coat individual olivine and pyroxene gr ains in CM matrices and chondrules. Bulk chemical analyses and observations of these rims indicate the presence of phyllosilicates and disseminated op aques. Because phyllosilicates could not have survived the chondrule format ion process, chondrule silicate rims must have formed entirely by late-stat e aqueous reactions. As such, these textures provide a useful benchmark for isolating alteration features from more complex CM matrix materials. Both chondrule silicate and matrix silicate rims exhibit morphological features commonly associated with advancing stages of replacement reactions in terre strial serpentinites. Contacts between many matrix silicate rims and the ad jacent matrix materials suggest that these rims formed entirely by aqueous reactions in a parent-body setting. This contrasts with previous assertions that rim textures can only form by the accretion of nebular dust but does not imply an origin for the rims surrounding other types of CM core compone nts, such as chondrules.