MARINE DIAGENESIS OF FELDSPATHIC SAND - A FLOW-THROUGH EXPERIMENTAL-STUDY AT 200-DEGREES-C, 1-KBAR

Seawater was reacted with feldspathic sand at 200-degrees-C, 1 kbar to investigate the chemical and mineralogical changes that take place under conditions similar to those of burial diagenesis. A non-reactive flow-through system was used that allows pore-fluid chemistry to be monitored as reactions take place at low water/rock mass ratios while controlling temperature, confining pressure, pore-fluid pressure and average volumetric flow rate. Large and rapid changes in fluid chemistry occurred; Mg and Na were removed from solution while Ca, K, SiO2, Fe and Mn were leached from the solids. Smectite was the dominant alteration product although kaolinite, anhydrite, pyrite(?) and secondary albitic plagioclase were also produced. Mg removal from the fluid and smectite formation dominated the reactions. Mg removal generated acidity which attacked the rock and released K, Ca, SiO2, Fe and Mn. Na was also removed from the fluid by the formation of albitic plagioclase and smectite. The extent of chemical exchange was greatest early in the experiments when the ratio of surface area (A) to fluid mass (M) was very high. Preferential dissolution of "fines" lowered A/M which decreased the apparent rate of chemical exchange. Although apparent reaction kinetics decreased through time, the chemical exchange trends were the same throughout the experiments and the fluid composition was ultimately related to A/M and the flow rate. Pore-fluid evolutionary trends observed early in the experiments may reflect some of those that occur in the subsurface under rock-dominated conditions where A/M is high, reaction kinetics are fast, and/or flow rates are slow. Later trends may reflect more water-dominated conditions where apparent reaction kinetics are slowed due to decreased A/M and/or rapid fluid flow.