David N. Lumsden

Discrepancy between thin-section and X-ray estimates of dolomite in limestone

ABSTRACT Three independent errors affect X-ray diffraction estimates of dolomite in pre-Cenozoic carbonates. When using calcite/dolomite [104] peak ratios, each 1% of excess calcium in the dolomite lattice causes approximately a 2% overestimate of the amount of dolomite. Use of the [113] peak ratios avoids the stoichiometry problem; however. if quartz is present, the interference between the [102] quartz peak (d = 2.282 A) and the [113] calcite peak (d = 2.285 A) can cause a serious underestimate of the dolomite proportion. In addition, error will occur if the crystallite size in the powdered standards used to prepare X-ray calibration curves differs significantly from the crystallite size in the sample unknowns. All these errors can be avoided or corrected; however, point count r visual estimation of stained thin sections is a simple, reliable technique and is preferable for routine analysis of visibly crystalline, lithified, sediment.

Relationship between chlorite coatings on quartz grains and porosity, Spiro Sand, Oklahoma

ABSTRACT Authigenic chlorite coatings on detrital quartz grains influence the formation of overgrowths and preservation of porosity in the Spiro Sand, Arkoma Basin, Oklahoma. Thick continuous coatings preserve porosity because pressure solution is retarded and quartz overgrowths are nonexistent. If the coatings are sparse, thin, and discontinuous, the sandstone is nonporous because of pressure solution and quartz overgrowths. Intermediate stages between these two extremes also are present.

Mn(II) partitioning between calcium and magnesium sites in studies of dolomite origin

Abstract In an Electron Spin Resonance (ESR) study of over 30 dolomites, representing a variety of probable sedimentary origins, we observed a Mg-site to Ca-site ratio in the Mn distribution that suggests two groups of dolomite. In one group the dolomites have a Mn distribution ratio near 5.0 and an excess of Ca; in the other group the dolomites are near-stoichiometric but have a Mn distribution ratio that varies from near 5 to near 70. We suggest that these two groups reflect different origins for the dolomites. The wide variation in the manganese distribution ratio for the near-stoichiometric dolomites supplies a parameter against which other variables can be plotted in studies of dolomite origin.

Characteristics of Deep-Marine Dolomite

ABSTRACT Deep-marine sediment has not been subjected to the influences of meteoric waters, deep burial, or tectonic stress that have profoundly altered most rocks available to us in continental exposures; hence, it preserves the record of formation and early development of its dolomite. Evaluation of the DSDP X-ray and smear slide data files indicates that dolomite is a common component in deep-marine sediments; it averages 1% and is found in all sampled ocean basins throughout post-Jurassic time. Dolomite abundance is not related to the abundance of any other detrital, authigenic, or alteration mineral, except aragonite. Petrographic examination of 48 samples suggests that deep-marine dolomite crystal size and appearance are similar to the size and appearance in modern supratidal deposits. Do omite does not increase in abundance or in crystal size downhole, nor does crystal size increase with increasing age. The dolomite is commonly nonstoichiometric (avg. 56% CaCO3) and poorly ordered. Globally, it does not change in stoichiometry or order with depth or age. Dolomite in deep-marine sediments does not originate from unusual or regionally important conditions. It entered the sediments early, most probably as a chemical precipitate from pore waters. An uncertain amount, estimated to be 10%, is detrital.

Uranium series dating of stalagmites from Blanchard Springs Caverns, U.S.A.

Abstract Because stalagmites are enriched in 234U beyond the equilibrium value at the time of deposition, they are potentially datable by measuring their progress toward equilibrium. Procedures for uranium-thorium and uranium disequilibrium dating of speleothems were developed, making use of alpha spectrometry for thorium determinations and mass spectrometry for uranium determinations. The stalagmites tested were generally beyond the age range of the U-Th age dating method and, as a result, 234U/238U ratios provided the most useful dating technique. The estimated age range for a stalagmite from Blanchard Springs is 135,000 to 785,000 years BP with an interval of slow or suspended growth from about 320,000 to 570,000 years BP.

Mineralogy and Mn geochemistry of laboratory-synthesized dolomite

Abstract Dolomite was synthesized in hydrothermal bombs to assess influences on Mn partitioning into the Mg and Ca sites. Calcite was added to magnesium chloride solutions doped with 0, 10, 20, or 50 ppm Mn. Runs were made for 8 to 241 hr at 192 and 224°C. The calcium content of dolomite increased to 60–64% after 75 hr at 192°C and reverted to stoichiometry thereafter. Increases in the Mn content of the starter solution progressively retarded formation of dolomite. For dolomites formed without added Mn, or formed in less than 40 hr with 10 ppm added Mn, the ESR signals were too broad to permit determination of Mn site distribution ratios. Measurable ratios of Mn in the Mg site to Mn in the Ca site vary as a linear function of time, from 0.3 to 7 for samples prepared at 192°C and from 3 to 16 for those prepared at 224°C. Stoichiometric dolomites have higher Mn partitioning ratios than nonstoichiometric dolomites, as observed in natural systems. Dolomites formed in supratidal/sabkha, early groundwater, and Dorag conditions may differ in their Mn signature from those formed during burial diagenesis.

Secular variations in dolomite abundance in deep marine sediments

Change with time in the amount of dolomite in post-Jurassic deep marine sediments was tabulated using 844 Deep Sea Drilling Project samples that contain 5% or more dolomite and could be suitably dated. Superimposed on an overall decrease since Early Cretaceous time are two maxima in the Cretaceous, a maximum at the Paleocene/Eocene transition, and a maximum in the Neogene. Frequency trends in the Atlantic and Indo-Pacific are similar overall but differ in detail. Mediterranean and Red Sea data are Neogene and peak in the mid-Miocene and late Miocene to early Pliocene, respectively. Data from the Gulf-Caribbean are largely Neogene and suggest deepening since early Cenozoic time. The general similarity of the patterns in the Atlantic and Indo-Pacific, supported by data from the smaller basins, suggests that the frequency of occurrence of dolomite in deep marine sediments may be related to a common global cause, possibly the flooding and draining of shallow shelves and small ocean basins consequent upon eustatic sea-level changes.

Relationship between paleotemperatures of metamorphic dolomites and ESR determined Mn(II) partitioning ratios

Abstract An Electron Spin Resonance (ESR) study of the distribution of Mn(II) between Mg sites and Ca sites in 28 contact metamorphic dolomite samples suggests that the distribution is temperature sensitive. The data can be modelled by alternative interpretations. In the first a polynomial equation approximates the trend of the data suggesting an exponential relation with Mn(II) ratios decreasing to a stable value of 10 with increasing temperature (to near 900°C). The second suggests two groups of dolomites. One group, close to the intrusion (

The Role of Iron and Mg/Ca Ratio in Dolomite Synthesis at 192°C

The reaction path that dolomite follows in its formation, approach to stoichiometry, and approach to order differs in the presence or absence of iron. Dolomite was synthesized in hydrothermal bombs using eight starter combinations heated at 192°C for eight time periods. When iron was present dolomite became the dominant mineral within two days, whereas it took five days to produce similar amounts of dolomite without iron. Near-stoichiometric composition (51%-52% Ca) was reached in three days when iron was present, versus six days in iron-free experiments. Further, a better cation-ordered dolomite formed earlier in iron-containing experiments. The absence of a significant difference in the results of experiments using