Abstract Electron Spin Resonance (ESR) spectroscopy can determine the absolute amounts of Mn(II) in the Ca and Mg sites in dolomite and in associated calcite. The ESR spectra of Mn(II) in dolomite can be qualitatively divided into three types that have little overlap. Type 1 spectra have sharp peaks, and the partitioning of Mn into cation sites can be determined. Such spectra are common in stoichiometric and nonstoichiometric dolomites from both lithified and unconsolidated deposits (14 of 20 modern dolomites; 13 of 27 deep-marine dolomites). There is no apparent relation between Mn partitioning ratios and the absolute amount of Mn, the presence of a free radical center peak, or the total amount of dolomite in the sample. Modern dolomite, deep-marine dolomite, and nonstoichiometric Phanerozoic dolomites have average Mn partitioning ratios of 2, 5, and 6, respectively, suggesting that the ratios are not age dependent. Stoichiometric dolomites have an average partitioning ratio of approximately 30; thus, ratios and stoichiometry may be related. Type 2 spectra were observed in six of 20 modern dolomites and in 10 of 27 deep-marine dolomites. These spectra have broad peaks, and the Mg and Ca sites cannot be individually resolved. Because they are not found in older lithifield Phanerozoic dolomites, type 2 spectra may be related to lattice disorder. Type 3 spectra, observed in four deep-marine dolomites, do not have interpretable Mn peaks. A center peak assignable to radiation damage and/or free radicals may be present, independent of the Mn spectra. Age and thermal history data can be obtained from this peak.

Abstract The concentrations and distribution of uranium and boron have been measured in dolomites and limestones from a core taken on the island of San Salvador in the Bahamas. The analyses reveal a wide range of concentrations both within and between the two predominant types of dolomite. The crystalline dolomites show unexpectedly high concentrations of U in skeletal components (2 to 7 ppm), but low values in void-filling cements (0.5 to 1 ppm). In contrast, the fabric-destructive microsucrosic dolomites are uniformly low in U (0.5 to 1 ppm) with occasional red algal fragments exhibiting concentrations as high as 1.5 ppm. Data presented here suggest that the U concentrations of the dolomites are inherited from original sedimentary and diagenetically altered components. It is suggested that the rocks that have higher concentrations of U, and in which the original fabrics are largely preserved, were dolomitized directly from the aragonite and high-Mg calcite (HMC) precursors. The U concentration is retained during dolomitization because in carbonite-rich fluids the uranyl ion (UO 2 2+ ) is complexed principally with the carbonate ion (CO 3 2- ). As the activity of CO 3 2- is usually limiting in producing solutions supersaturated with respect to dolomite, CO 3 2 produced from the dissolution of metastable precursors is reincorporated into dolomite. In contrast, dolomites with lower U concentration formed from a low-Mg calcite (LMC) precursor which previously lost U during stabilization by meteoric waters. Concentrations of B in the dolomites were similar (1 to 3 ppm) to values determined for modern LMC organisms (this study) and therefore suggest dolomitization from predominantly marine fluids. Comparisons with ranges reported in the literature show B concentrations in this investigation to be much lower. This is attributable to the ability of the nuclear-track technique to recognize contamination within the sample and consequently to allow it to be eliminated from the analysis.

Abstract Time series experiments relating to the dolomitization of calcite at 215° to 225°C in saline solutions of near-seawater salinity were conducted to ascertain the influence of sulfate and carbonate in solution on the rate of calcite dolomitization. A concentration of about 0.004M sulfate in solution prevented the dolomitization of calcite. At concentrations of less than 0.004M, dolomitization proceeded at a slower rate than in experiments where no sulfate was present. The final concentration of sulfate was controlled by the precipitation of anhydrite. The presence of sulfate in solution did not prevent the direct precipitation of dolomite in experiments in which the solid reactants were carbonate minerals other than calcite (BaCO 3 and 2PbCO 3 .PbOH). Also, the presence of sulfate in the calcite dolomitization experiments slowed the rate of calcite dissolution from 3 days in sulfate-free solutions to 6 or 7 days in sulfate-bearing solutions. These observations indicate that sulfate in solution may inhibit dolomitization primarily by retarding the rate of calcite dissolution, rather than by inhibiting the direct precipitation of dolomite from solution. The rate of calcite dolomitization was greater in solutions with higher carbonate/bicarbonate concentrations. This provides some confirmation for hypotheses regarding the importance of carbonate in solution as a kinetic factor that expedites dolomitization.

Abstract The Drakes Bay Formation is an upper Miocene sequence of siliceous mudstones containing many small dolomite nodules. The nodules probably formed without a precursor biogenic calcite supplying Ca or HCO 3 for dolomitization. Dolomite formation preferentially took place in sediment layers slightly richer in organic C than the surrounding sediments. More extensive sulfate reduction in these layers raised the porewater HCO, concentration and caused carbonate precipitation. The initial carbonate may have been dolomite or calcite, which was later converted to dolomite. Carbon and oxygen stable isotope ratios vary systematically and clearly illustrate changes in the isotopic composition of dissolved CO 2 that occurred with depth. The isotopic analyses show that dolomite formation did not begin until the pore waters were free of dissolved sulfate. The Ca contents of the dolomites decrease, and both the Mg and Fe contents increase, with depth of formation. Manganese contents correlate with Fe contents. Sodium contents of the dolomites are relatively high, probably reflecting their poor ordering and nonstoichi-ometry. Strontium contents of the dolomites are typical of those from hemipelagic sediments with moderate sedimentation rates.

Abstract A 1.2-km-thick section of the Miocene Monterey and overlying Pliocene Sisquoc formations in the Santa Maria basin area of California contains highly variable amounts of biogenic silica, detrital clay and silt, organic matter, carbonate, pyrite, and francolite. Organic-matter diagenesis resulted in the early precipitation of dolomite, pyrite, and francolite, and the concentration of trace metals. Dolostone horizons occur 1 to 10 m apart and consist of 50 to 95 weight percent pore-filling dolomite. The dolomite is low in Fe and Mn and contains an average of 0.8 to 5.3 mole percent excess Ca. Dolomite composition is related to texture in some samples and suggests several different episodes of dolomitization. There is a positive correlation between organic matter, pyrite, and the trace metals V, Cr, Ni, Cu, and Zn. Pyrite formation probably occurred below the sediment/seawater interface (noneuxinic basin) in the microbial-sulfate reduction zone, and was limited by Fe in sediment having a high organic matter-to-clay ratio and by reduced sulfur in sediment having a low organic matter-to-clay ratio.

Abstract The Bonneterre Formation (Cambrian), southeast Missouri, is characterized by dolomitized algal bioherms and associated shelf carbonates that were deposited around the Precambrian St. Francois Mountains, which were islands during Late Cambrian time. West of the dolomitized shelf carbonates, the offshore facies of the Bonneterre consists of a deeper water limestone and shale sequence composed of oolitic and skeletal wackestones and packstones interbedded with silty lime mudstones and green illitic shales. Individual limestone and shale beds range in thickness from less than 1 cm to several meters. At the base of the offshore facies, immediately overlying the Lamotte Sandstone, is a regionally extensive basal dolomite that averages about 6 m thick. The basal dolomite contains coarse crystalline, nonplanar dolomite. This dolomite is relatively low in iron and nearly stoichiometric with regard to CaCO 3 . Stable carbon and oxygen isotope values for the basal dolomite are similar to values obtained for epigenetic dolomite associated with nearby sulfide ore bodies. The interbedded limestones and shales of the offshore facies contain abundant ferroan dolomite occurring as individual crystals and patches of crystals replacing limestones and floating in shale beds. This dolomite is commonly concentrated near solution seams, in argillaceous seams in limestones, in shale beds, and also may selectively replace allochems. The ferroan dolomite is commonly enriched in CaCO 3 and zoned with respect to iron. Stable carbon and oxygen isotope values are low, indicating elevated temperature and an organic source of carbon. The basal dolomite was formed by warm basinal brines circulating through the Lamotte Sandstone aquifer. This water may have been genetically related to the fluids that produced nearby Mississippi Valley-type ore deposits. The presence of impermeable overlying shale beds had restricted these fluids (and the resultant dolomite) to the lower few meters of the offshore facies. The ferroan dolomite was formed after burial by Mg +2- and Fe +2 -rich water evolved during the illitization of smectite in the interbedded shale.

Abstract A major concern with Viburnum Trend lead-zinc deposits is the nature of the mineralizing fluids. The chemical compositions of secondary recrystallized and sparry dolomites in the Bonneterre and Davis formations were determined to help define the nature of the coexisting mineralizing fluids in the Viburnum Trend and surrounding areas. Chemical compositions of the recrystallized host basal dolomite in the lowermost part of the Bonneterre Formation show a general south to north decrease of iron and manganese and an associated increase of strontium. This is consistent with a southern source for the mineralizing fluids. A similar trend of decreasing iron and manganese and increasing strontium upsection in the Viburnum Trend and the back reef indicates that the fluids moved from the underlying Lamotte Sandstone into the Bonneterre Formation. Low-iron and manganese concentrations in the backreef sparry dolomite, as well as relatively constant strontium values in the entire area, suggest the mixing of basinal brines with meteoric waters. A proposed sequence of dolomitization and ore-forming events can be summarized as: (1) dolomitization of the backreef unit by depositional marine or diagenetic waters, (2) updip movement of mineralizing brines from a southern source, possibly the Ouachita-Arkoma Basin, causing epigenetic dolomitization and ore deposition in the Viburnum Trend, and (3) changing of mineralizing conditions due to the mixing in the back reef of principal basinal brines with dilute meteoric waters from an eastern source.