Iron-rich aureoles in the CM carbonaceous chondrites Murray, Murchison, and Allan Hills 81002: Evidence for in situ aqueous alteration

Iron-rich aureoles in CM carbonaceous chondrites are previously unidentifie d domains of aqueously altered matrix material, whose FeO content may excee d that of the surrounding matrix by up to more than 15 wt%. We describe the petrography and mineralogy of these objects in the CM chondrites Murray, M urchison, and Allan Hills (ALH) 81002. The size of Fe-rich aureoles ranges from a few hundred microns to several millimeters in diameter and appears t o be a function of the degree of alteration of the host chondrite. The orig in of Fe-rich aureoles is related to the alteration of large metal grains t hat has resulted in the formation of characteristic PCP-rich reaction produ cts that are frequently observed at the centers of the aureoles. This sugge sts that Fe-rich aureoles in CM chondrites are the result of the mobilizati on of Fe from altering metal grains into the matrix. The fact that Fe-rich aureoles enclose numerous chondritic components such as chondrules, calcium -aluminum-rich inclusions (CAIs), and mineral fragments, as well as their r adial symmetric appearance, are strong evidence that they formed in situ an d that significant directional fluid flow was not involved in the alteratio n process. This and additional constraints, such as the distribution of S a nd other elements, as well as the inferred alteration conditions, are consi stent with in situ parent-body alteration. The observations are, however, e ntirely incompatible with preaccretionary alteration models in which the in dividual CM chondrite components have experienced diverse alteration histor ies. The presence of numerous intact aureoles in the brecciated CM chondrit es Murray and Murchison further suggests that the alteration occurred large ly after brecciation affected these meteorites. Therefore, the progressive aqueous alteration of CM chondrites may not be necessarily coupled to brecc iation as has been previously proposed.