Phyllosilicate veins in a CI meteorite: evidence for aqueous alteration on the parent body

Abstract

CARBONACEOUS chondrites of type CI have elemental abundances close to those of the Sun1, and are therefore regarded as the most primitive Solar System material available for study. These meteorites consist in large part of fine-grained phyllosilicates, which account for most of the water in the meteorites (∼20 wt.% H2O). Previous studies (see, for example, ref. 2) have suggested that the phyllosilicates were formed by aqueous alteration on the meteorite parent body, but formation by direct condensation from the solar nebula has not been ruled out. Here I report the observation of fracture-filling veins of phyllosilicates in the Yamato-82162 CI chondrite. Such veins were previously known to contain only sulphates and carbonates3–6. Transmission electron microscope observations show that the phyllosilicates are coherent intergrowths of iron-bearing, sodium-rich saponite and minor chlorite-like phyllosilicate; thus, they are similar to the phyllosilicates in the Orgueil CI chondrite7. These observations provide new evidence that the phyllosilicates in CI chondrites were probably formed by the activity of aqueous solutions on the meteorite parent body, and that phyllosilicate formation overlapped with the period of impact brecciation.