ABSTRACT: Calcite Cements in Expanded Micas Suggest Bacterial Activity

P. A. O'Day, D. R. Pevear, J. J. Zullig, R. E. Klimentidis

Highly expanded detrital biotite grains with intercleavage spaces filled by calcite cements in sandstones from DSDP sites 178 (Alaskan abyssal plain, 660 m depth) and 474A (Gulf of California, 520 m depth)

provide evidence for biologically mediated carbonate precipitation during early diagenesis. Microprobe analyses of biotites and intercleavage cements show no preferential dissolution or alteration of biotite at grain edges or splays. Likewise, transmission electron micrographs of ion-milled sections exhibit disrupted 10-A biotite crystallites (10-500 nm wide) in a calcite matrix which are not potassium depleted (determined by semiquantitative EDAX analysis) relative to undisrupted biotite. Cathodoluminescent microscopy of expanded biotites reveals bright, luminescent calcite rims at biotite edges and in intercleavage spaces which generally correspond to lower trace concentrations of Mg, Mn, and Fe (0.1-0.5 XCO₃, X = Mg + Mn + Fe) than in nonluminescent pore-filling calcite. Thus, trace elements in early formed intercleavage calcite are not linked to biotite dissolution. Calcite microsamples from biotite intercleavages have ^dgr¹⁸O = -1.36 to -8.92^pmil (PDB) and ^dgr¹³C = -12.73 to -22.19^pmil (PDB), indicating a significant fraction of organically derived carbon. Calculated activities of aqueous species and gases from published pore fluid analyses (using computer program EQ3) at Site 474A show that fluids are saturated with respect to calcite, have a high activity of HCO_3^-, and have a high fugacity of CH₄ under anoxic conditions. Trace element distributions in intercleavage calcite may be controlled by local changes in CO₂, HCO_3^-, and/or pH caused by bacterial activity. Sulfate-reducing or methanogenic bacteria (documented at Site 474A) living in biotite cleavages would produce CH₄, CO₂, and/or HCO_3^-, leading to enhanced calcite precipitation in intercleavage spaces and expansion of biotite by net positive volume changes.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990