They contain the interstellar dust grains and high carbon content that scientists prize for the clues they hold regarding the origin of the sun and planets and of life itself. Tagish Lake, in fact, contains 5.4 percent carbon by weight, more than any other carbonaceous chondrite, reported Monica Grady of the Natural History Museum in London, in the Science paper. But the oxygen isotopic characteristics of the Tagish Lake meteorite do not fit the existing categories of carbonaceous chondrites. Each classification represents a different parent body from which the meteorites sprang, usually an asteroid. Tagish Lake comes closest in isotopic composition to the CM and CI classifications for carbonaceous chondrites.
C stands for carbonaceous, meaning high in carbon content.
M stands for the Mighei meteorite from Ukraine, and I for the Ivuna meteorite from Tanzania, for which the classifications are named. We can make a pretty big case that the parent bodies of these carbonaceous chondrites had water on them. The oxygen isotopes serve nicely to give us some idea of both how wet the body was and also the temperature at which the alteration took place. Clayton's analysis showed that Tagish Lake is wetter than the CMs, but that it was chemically altered at much cooler temperatures than the CIs. But in keeping with scientific convention, that will have to await the discovery of four more meteorites similar to Tagish Lake.