Diffusion and mass balance of Mg during early dolomite formation, Monterey Formation

Abstract

Diffusion from overlying seawater is capable of supplying enough Mg to account for all of the early diagenetic dolomite observed in the Monterey Formation (0.5–20 vol %). The amount formed depends on the Mg flux magnitude into the sediment, which is proportional to the rate of dolomite precipitation and inversely proportional to the sediment accumulation rate. The amount of dolomite formed is calculated for accumulation rates of 50 and 600 m/Ma, using reaction rate constants obtained from the Mg concentration profile of DSDP sites 479, 533 and 147, and proposed profiles that assume significant dolomite formation in the uppermost tens of meters of sediment. Advective pore water flow from compaction decreases the Mg flux and becomes increasingly significant as the accumulation rate increases and the reaction rate decreases. Advection alone during compaction supplies sufficient Mg to account for a realistic maximum of 2 vol % dolostone within a compacted section. An approximate evaluation of the Rayleigh number suggests that the low permeability of siliceous ooze prevents convective fluid flow at shallow burial depths. The Mg mass balance is calculated from dolomite abundance in Monterey Formation sections, Santa Maria basin area, California. Pore water and solid particle velocities are calculated as a function of sediment depth using a porosity-depth relation for average siliceous sediment based on a composite of DSDP data. The bulk sediment diffusion coefficient increases with sediment depth at a rate proportional to the geothermal gradient. At shallow burial depths seawater is probably the only significant source of Mg; dolomite, volcanic ash, and clay minerals are the only significant sinks. Clay mineral, organic matter, and silica diagenesis may provide additional Mg at deeper burial depths. The more or less regular spacing of dolomitic layers may be controlled by the residence time of the dolomite forming layer within the uppermost sediment, where the Mg flux is a maximum.