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Title:Experimental diagenesis of lime mud
Author(s):Papenguth, Hans William
Doctoral Committee Chair(s):Sandberg, Philip A.
Department / Program:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Textures similar to those found in natural microcrystalline limestones (micrites) were produced by closed-system (70-200$\sp\circ$C/1-15.6 bars/1-486 days) and open-system (90-95$\sp\circ$C/1 bar/63-311 days; 10-200 mL/day) hydrothermal experiments on a variety of lime muds ($<$25 $\mu$m size fraction), including natural lime muds, laboratory precipitated carbonates, and disaggregated skeletal materials.
Mean crystal size ranges of about 1-3 $\mu$m and 5-25 $\mu$m were determined for calcite-dominated precursor (CDP) micrites and for aragonite-dominated precursor (ADP) micrites, respectively. Mean crystal size ranges in natural samples interpreted as CDP and ADP micrites by Lasemi (1983) and Lasemi and Sandberg (1982, 1983, 1984) match those experimentally-determined results fairly well.
Precipitation of secondary calcite in aragonitic lime muds was controlled by pre-existing calcite or Mg-calcite seed crystals. The mean size of the seed crystals acts as the lower limit for mean crystal size in the final micrite, whereas the mean distance between the centers of the seed crystals provides an upper size limit. The calcitization of Mg-calcite-rich mud during the formation of CDP micrites differs from the calcitization of aragonitic mud because some Mg$\sp{2+}$ released during dissolution of Mg-calcite adsorbs onto secondary calcite grains and because all sediment is potentially available to serve as seeds for localized precipitation of secondary calcite. The result is smaller crystal sizes in CDP micrites.
Growth of cements in fractures and intraskeletal voids was the first noticeable change during experimental diagenesis. Micrite crystal sizes were generally fairly uniform throughout the change from mud to micrite. Partially-transformed samples appeared to be mixtures of the starting material and end-product. After mineralogical transformation, crystal size did not change during extended reaction times (no evidence of multiple dissolution-reprecipitation). The similarity of textures in experimental and ancient ADP micrites support conclusions by earlier authors that substantial compaction is not as important in the formation of micrites as was once thought.
Similarities in the sizes and distribution of aragonite relics and initial aragonite grains indicate that those aragonite relics were engulfed by secondary calcite during reactions. Absence of relics in reacted synthetic aragonite muds suggests that natural relic preservation is linked to the presence of resistive coatings (e.g., by organic compounds).
Issue Date:1991
Rights Information:Copyright 1991 Papenguth, Hans William
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9136691
OCLC Identifier:(UMI)AAI9136691

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