CORRELATED ALTERATION EFFECTS IN CM CARBONACEOUS CHONDRITES

Three parameters are proposed to determine the relative extent of alte ration in CM chondrites. The mineralogic alteration index monitors the relative progress of coupled substitutions in the progressive alterat ion of cronstedtite to Mg-serpentine and increases with increasing alt eration. To calculate values of this index, an algorithm has been deve loped to estimate the average matrix phyllosilicate composition in ind ividual CM chondrites. The second parameter is the volume percent of i solated matrix silicates, which decreases with progressive alteration due to mineral hydration. Finally, the volume percent of chondrule alt eration monitors the extent of chondrule phyllosilicate production and increases as alteration proceeds. These parameters define the first C M alteration scale that relies on multiple indicators of progressive a lteration. The following relative order of increasing alteration is es tablished by this model: Murchison less than or equal to Bells < Polle n less than or equal to Murray < Mighei < Nogoya < Cold Bokkeveld. The relative degree of aqueous processing Cochabamba and Boriskino experi enced is less precisely constrained, although both fall near the middl e of this sequence. A comparison between the mineralogic alteration in dex and literature values for the whole-rock chemistry of CM chondrite s reveals several correlations. A positive, nearly linear correlation between bulk H content and progressive CM alteration suggests an appro ximately constant production rate of new phyllosilicates relative to t he mineralogical transition from cronstedtite to Mg-serpentine. The ab undance of trapped planetary Ar-36 decreases systematically in progres sively altered CM chondrites, suggesting the wholesale destruction of primary noble gas carrier phase(s) by aqueous reactions. Because low t emperature fluid-rock reactions are generally associated with large is otopic mass fractionation factors, we also compared our model predicti ons with delta(18)O values for bulk CM samples. Although some of these data are poorly resolved, the order of increasing delta(18)O values a pproximates the order of increasing alteration predicted by our model parameters. Multiple correlations between diverse alteration parameter s strongly suggest that (a) different CM chondrites experienced simila r kinds of processes and conditions, and (b) CM materials experienced in situ alteration on the CM parent body or bodies.