Doctoral Dissertations

Date of Award

12-1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Harry Y. McSween Jr.

Committee Members

Claudia Mora, Theodore Labotka, Alexander Van Hook

Abstract

The first multi-parameter scaling procedure for determining the relative degree of aqueous alteration that individual CM carbonaceous chondritic meteorites experienced is introduced. Modal proportions and average phyllosilicate compositions from 9 CM falls define 3 progressive alteration indices that are then used to identify 3 additional alteration trends based on whole-rock chemical data obtained from the literature. The proposed parameters produce a self-consistent order for the relative extent of alteration that the analyzed samples experienced, while the identified correlations themselves suggest constraints on asteroidal conditions during the alteration process. Sulfide rinds are ~10 μm thick coatings of fine-grained sulfides and phyllosilicates around many individual silicate grains within CM chondrules. Two mechanisms of sulfide rind formation were considered to constrain the environmental conditions and processes that favor their production on the CM parent body: direct precipitation from a supersaturated fluid, and in situ replacement. The direct precipitation of sulfide rinds is kinetically inhibited under the most plausible of asteroidal conditions, except where S-bearing fluids are confined to olivine grain boundaries during the incipient stages of alteration. Petrological evidence supports the additional possibility that some sulfide rind textures formed by in situ replacement processes. Regardless of which mechanism prevailed, S was probably supplied to the initial CM alteration fluids in response to the equilibration of primary chondrule sulfides with grossly undersaturated alteration fluids. Sulfide rind production probably occurred at low activities of total dissolved S, pH values ranging from ~7 to 9, and Ehs between ~-0.3 to -0.6. Similar sulfide-bearing rind textures observed in CM matrices could have formed by analogous processes during parent body aqueous alteration. The origin of altered rim materials that surround chondrules, inclusions, and mineral fragments in CM chondrites provide further insight into the location of CM alteration. An extensive petrologic investigation of the innermost and outermost contacts that separate CM rim assemblages from adjacent materials strongly supports an interactive exchange of dissolved components on the final CM parent body.

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