Geochemistry Of Dolomites Research Articles

Geochemical Evolution of Dolomites in Thanagazi Formation of Mesoproterozoic Alwar Basin, Northwest India

The geochemistry of dolomite in part of the Thanagazi Formation in the Mesoproterozoic Alwar Basin has been studied employing major and trace elements, including rare earth elements (REEs) to understand their provenance and paleoweathering conditions. In the current study, two distinct types of dolomites were identified viz. amorphous and crystalline. Various geochemical discriminants such as SiO2 vs Al2O3 and CaO vs MgO indicate that the dolomites were originated from sedimentary sources with substantial metasomatic and biogenic contributions. Geochemical constraints from plots of Fe2O3/Al2O3 vs. Al2O3/(Al2O3+Fe2O3) and rare earth element compositions for provenance signatures suggest likely deposition of dolomites in shallow marine sedimentary environment. Based on geothermometric estimates and base metal concentrations in dolomites, it is inferred that the mixing of sediments derived from two different sources (metasomatic and biogenic) were likely responsible for the dolomitization in the Thanagazi Formation of the Mesoproterozoic Alwar Basin.

Petrography, geochemistry and genesis of dolomites in the upper Cambrian Sanshanzi Formation of the western Ordos Basin, northern China

The Cambrian Sanshanzi Formation is an important dolomite horizon in the Ordos Basin of northern China, and is well exposed in the western part of that basin. This paper focuses on the petrography and geochemistry of dolomites in this formation with an aim to provide a better understanding of their genesis. Reflux seepage, burial, and hydrothermal dolomites are identified in the Sa nshanzi Formation. Reflux seepage dolomites include dolomitic grainstone and powder crystal dolomite, and have high Sr and low Mn contents, a high CaO/MgO ratio and low δ13 C values, and the reflux seepage dolomitization was formed by high-salinity and Mg2+-enriched fluid penetrates downward into the limestone. Burial dolomites are the main type of dolomite distributed through the Sanshanzi Formation. Burial dolomites are composed of fine- to coarse-crystal dolomite and have low Sr and high Mn contents, a low CaO/MgO ratio, and high δ13C values. The features of high salinity, strong reducibility and negative δ18O values indicate the burial dolomites formed under high-temperature and high-pressure conditions, the burial dolomitization was due to the limestone deeply buried during the later diagenetic stage. Hydrothermal dolomites are mainly characterized by saddle dolomite features, a positive δEu anomaly, and low δ18O values, and the hydrothermal dolomitization occurred during Mg2+-rich hydrothermal fluid invaded. The reflux seepage and hydrothermal dolomite developed in local areas, with the hydrothermal dolomite affected by the distribution of faults.

Origins and Geochemistry of Dolomites and Their Dissolution in the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

Triassic dolomites occur pervasively in the Western Sichuan Basin. Although these strata have been deeply buried and affected by multiple phases of dolomitization and dissolution, some intervals in the upper part of the Leikoupo Formation have developed high porosity. Based on their petrographic and geochemical features, three major types of dolomite fabrics are recognized, namely, dolomicrite, fabric-retentive dolomite, and fabric-destructive dolomite. Geochemical evidence indicates that the dolomicrite formed following the Sabkha model in a low-temperature hypersaline environment, as these rocks exhibit abnormally high Sr and Na contents, lower Fe and Mn contents, δ18O values generally ranging from −1.70‰ to −1.67‰ (with an average value of −1.69‰), and higher Mg/Ca ratios. The fabric-retentive dolomite formed following the seepage-reflux model in a shallow burial environment, and these rocks exhibit the highest 87Sr/86Sr ratios, δ18O values generally ranging from −6.10‰ to −2.50‰ (with an average value of −3.98‰), and a wide range of Fe and Mn contents, indicating that they may have been altered by meteoric water. The fabric-destructive dolomite formed following the burial model at elevated temperatures; these rocks exhibit the lowest Sr and Na contents, δ18O values generally ranging from −7.01‰ to −6.62‰ (with an average value of −6.79‰), relatively higher Mg/Ca values, and lower 87Sr/86Sr ratios. The early Sabkha, seepage-reflux dolomitization and penecontemporaneous periodic meteoric freshwater selective dissolution processes formed multi-period, overlapping moldic pores, algal framework pores, and intragranular dissolution pores. The superposition of organic acid dissolution during the burial period is the main controlling factor of the formation of deeply buried, high-quality dolomite reservoirs in the Leikoupo Formation.

ASPECTS OF THE DOLOMITIZATION OF THE MÕHKÜLA BEDS (SILURIAN, ESTONIA)

The dolomites of the Mohkula Beds represent the uppermost middle Llandovery in the northeastern, marginal part of the Baltic Palaeobasin. Secondary dolomitization has changed the primary composition of rocks, but according to the geological data available they originate from the normal-marine sediments of the shelf facies. X-ray diffraction, X-ray fluorescence, and titration analyses were applied to study the chemical composition of rocks and differences in the dolomitizing environment. The CaCO 3/MgCO3 molar ratio and lattice parameters of dolomites, irrespective of the content of insoluble residue or the texture of rocks, are close to those of ideal dolomites. Compared to limestones dolomites are depleted in Sr and enriched in Mn. The concentrations and correlations of these elements vary by lithological varieties along the section, which supports the suggestion that there was no unique dolomitizing fluid and the dolomitization began soon after the deposition. The geochemistry of dolomites is almost consistent with dolomitization by normal or modified (dilute) sea water. In general, dolomitization correlates with regression. The area favourable for dolomitization is related to the inner shelf, migrating in accordance with the fluctuation of sea level and oscillation of the shoreline.

ABSTRACT: Petrography and Geochemistry of Dolomites in the Lower Cretaceous Edwards Formation in Taylor County, Texas

ABSTRACT: Petrography and Geochemistry of Dolomites in the Lower Cretaceous Edwards Formation in Taylor County, Texas

ABSTRACT: Petrography and Geochemistry of Dolomites in the Lower Cretaceous Edwards Formation in Taylor County, Texas

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Hydraulic Seals and Their Origin: Evidence from the Stable Isotope Geochemistry of Dolomites in the Middle Ordovician St. Peter Sandstone, Michigan Basin

Regionally extensive, overpressured natural gas pools within the St. Peter Sandstone (Middle Ordovician) of the Michigan basin are bounded by rock types that include 3-7-m-thick zones of low-permeability, low-porosity carbonates or diagenetically banded quartz sandstones. Replacive dolomite from an approximately 5-m-thick carbonate interval in the east-central portion of the Michigan basin has very low ^dgr13C values that systematically decrease from approximately -5^pmil at the top to -10^pmil (PDB) at the base. ^dgr18O values for the replacive dolomite also decrease systematically with depth from approximately 27 to 23^pmil (SMOW). These data suggest an upward decrease in isotope exchange between the replacive dolomitization fluid and the precursor rock (i.e., the system was rock dominated at the top of the carbonate interval), which implies upward, cross-formational movement of the dolomitizing fluid. Fluid-rock interaction modeling suggests that the dolomitizing fluid had a total dissolved carbon (TDC) content of approximately 4000 ppm and a ^dgr13C value of -27^pmil, which indicates that the carbon was primarily derived from organic diagenesis. Sr isotope and major element data suggest that this dolomitizing fluid had a modified seawater origin. Carbonate intervals in the St. Peter Sandstone contain dolomite cement in quartz sandstone laminations and fractures that have ^dgr13C values that closely mimic the large ^dgr13C variations of the adjacent replacive dolomite with depth. This correlation indicates that the replacive dolomite controlled the ^dgr13C value of the carbon dissolved in the parent fluids of the later dolomite cement. Moreover, these data suggest that the parent fluids of the dolomite cement were not confined to a common fracture network, but flowed pervasively through the carbonate intervals. The ^dgr13C values of dolomite cement in diagenetically banded quartz sandstone intervals from three locations in the central portion of the Michigan basin range from -9 to -4^ mil and are relatively invariant at a particular locality; therefore, the TDC of the dolomitizing fluid in the central Michigan basin is interpreted to have contained only about 20-30% organic carbon. The fact that all dolomites analyzed in the St. Peter Sandstone have much lower ^dgr13C values than carbonates in adjacent formations indicates that dolomitization and the formation of hydraulic seals were related to organic matter diagenesis.

Geochemistry of Dolomites from Late Miocene Reefs in Southern Spain

Geochemistry of Dolomites from Late Miocene Reefs in Southern Spain

Stratigraphic Patterns, Sedimentology and Diagenesis of Capitan Backreef Strata, Permian, Guadalupe Mnts, New Mexico

Capitan backreef strata, Yates Formation, are characterized by six cycles, each with a lower siliciclastic and an upper carbonate unit. The factors controlling the deposition of these packages remain controversial. Traditional sedimentary and stratigraphic approaches have been integrated with diagenetic events to understand the main controls on the genesis of these cycles. Syndepositional and syn-unconformity diagenetic features are: (1) marine calcite or aragonite cementation, (2) dissolution of metastable mineral phases, and (3) dolomitization (both replacive and primary precipitate). Geochemistry of dolomites is consistent with evaporation-concentrated and slightly reducing marine waters. Meteoric calcite cements were not found associated with the subaerial exposure surfaces at cycle tops in shelf strata. Arid climate probably prevented the establishment of stable freshwater lenses. Postdepositional diagenesis includes meteoric and shallow burial calcite cements, dissolution vugs, kalinite, and vadose calcite cements. Combination of sedimentologic and diagenetic studies of Guadalupe Mountains outcrops suggests that sea level fluctuations probably were responsible for the deposition of cyclic strata and syndepositional diagenetic features. Relative sea level falls exposed parts of the shelf driving dolomitizing fluids through shelf strata. Relative sea level rises flooded the shelf and deposited subtidal to intertidal siliciclastic and carbonate rocks.