DISSOLUTION AND TIME-DEPENDENT COMPACTION OF ALBITE SAND - EXPERIMENTS AT 100-DEGREES-C AND 160-DEGREES-C IN PH-BUFFERED ORGANIC-ACIDS AND DISTILLED WATER

Aqueous fluids are important in the diagenesis and deformation of crus tal rocks. Both chemical and physical interactions are involved and of ten they are strongly coupled. For example, pore waters not only disso lve, transport, and precipitate chemical species, but they also substa ntially affect the mechanical behavior of the rocks that contain them. Stresses magnified at grain contacts by differences in pore-fluid pre ssure (P-p) and confining pressure (P-c) can, in turn, influence the r ate and extent of chemical exchange. To begin investigation of these c oupled systems, compaction experiments were conducted using albite san d (250-500 mu m) and distilled water (pH 5.8), 0.07 M acetate (pH 4.7) , and 0.07 M acetate + 0.005 M citrate (pH 4.4) solutions in a hydroth ermal flow-through system at conditions that simulate diagenesis. Pore -fluid chemistry and pore-volume loss were monitored to quantify the e ffects of organic acids on time-dependent compaction rates. The effect s of stress and fluid chemistry on the dissolution kinetics were also examined. Albite dissolution rates, monitored by steady-state fluid ch emistry, increased when an effective pressure (P-e = P-c - P-p) was ap plied, probably due to increases in total surface area caused by grain breakage at contacts. These effects were transient in distilled water , however, Si and Al concentrations remained elevated in the acetate p ore fluid. The average Si-based release rates indicate approximate to 35% increase in reactive surface area by application of P-e = 34.5 MPa . At 100 degrees C with P-e = 34.5 MPa, steady-state Si concentrations were approximate to 2.3 times higher in 0.07 M acetate and 5.8 times higher in 0.07 M acetate + 0.005 M citrate than in distilled water. Al increased by even larger factors (3x in the acetate buffer and 10x in the citrate solution). These changes in fluid chemistry are attribute d to both pH and ligand-enhanced reactions. Albite dissolution appears to be controlled by surface complexation reactions at Al sites. Rapid dissolution of albite in the organic acid solutions is probably due t o the ability of organic acid ligands to selectively complex with alum inum. Time-dependent compaction was observed at 100 and 160 degrees C with P-e = 34.5 MPa. Strain rates increased with temperature from appr oximate to 10(-9) s(-1) at 100 degrees C to approximate to 10(-8) s(-1 ) at 160 degrees C and decreased with strain in all pore fluids, espec ially at 100 degrees C. Compaction rates in distilled water and in the acetate solution had similar magnitudes and strain dependencies; howe ver, small amounts of citrate species apparently enhance compaction co mpared to the other fluids at similar strains. Textural data indicate that time-dependent compaction of the albite sand occurred primarily b y brittle mechanisms at these temperatures. However, the deformation i s clearly thermally activated and may be chemically assisted by the aq ueous pore fluid. (C) 1998 Elsevier Science B.V. All rights reserved.