PETROLOGICAL EVIDENCE FOR SHOCK MELTING OF CARBONATES IN THE MARTIAN METEORITE ALH84001

The meteorite ALH84001-a shacked igneous rock of probable martian orig in-contains chemically and isotopically heterogeneous carbonate globul es(1-8), associated with which are organic and inorganic structures th at have been interpreted? as possible fossil remains of ancient martia n biota, A critical assumption underlying this suggestion is that the carbonates formed from low-temperature fluids penetrating the cracks a nd voids of the host rock(3). Here we report petrological studies of A LH84001 which investigate the effects of shock on the various mineralo gical components of the rock We find that carbonate, plagiodase and si lica were melted and partly redistributed by the same shock event resp onsible for the intense local crushing(1,2) of pyroxene in the meteori te, Texture and compositional data show that, during the period of sho ck decompression, monomineralic melts were injected into pyroxene frac tures that were subsequently cooled and resealed within seconds, Our r esults therefore suggest that the carbonates in ALH84001 could not hav e formed at low temperatures, but instead crystallized from shock-melt ed material; this conclusion weakens significantly the arguments that these carbonates could host the fossilized remnants of biogenic activi ty.