A Geochemical Study of Dolomite in the Monterey Formation, California

Abstract

ABSTRACT Isotopic and trace-element analyses of dolomites from the Miocene Monterey Formation of California show that there is a strong correlation between the 13C values and the iron, manganese, and strontium contents of dolomites from different localities. Dolomites with negative 13C values have low trace-element contents, averaging less than 1 mole % iron, 300 to 400 ppm manganese, and 200 to 250 ppm strontium. Those with positive 13C values have much higher trace-element contents, averaging greater than 2 mole % iron, with values as high as 10 mole %, 1,200-1,400 ppm manganese, and 600 to 800 pm strontium. Monterey sections with dolomites with low trace-element contents contain higher percentages of dolomite and have lower sedimentation rates and lower detrital mineral contents than sections with dolomites with high trace-element contents. Differences in iron and manganese contents of dolomites from different sections are probably attributable to variation in the amount of readily available iron and possibly manganese oxide coatings on detrital minerals. Whether a dolomite forms in or below the zone of organic-matter oxidation by microbial sulfate reduction also may affect the availability of iron and manganese. In the zone of sulfate reduction, reduced iron, and possibly manganese, may be precipitated as sulfide minerals rather than be incorporated into dolomite. Differences in the strontium contents of Monterey dolomites are probably the result of different reaction stoichiometries of the dolomitization process. Calculations of the maximum depth of dolomite formation based on a diffusion-limited seawater source of Mg2+ for dolomitization show that for sections with high percentages of dolomite (10-20%), dolomitization must take place within the uppermost few meters of the sediment column.