The incorporation of Mg2+ and Sr2+ into calcite overgrowths: influences of growth rate and solution composition

The concentrations of Mg²⁺ and Sr²⁺ incorporated within calcite overgrowths precipitated from seawater and related solutions, determined at 25°C, were independent of the precipitation rate over approximately an order of magnitude. The saturation states used to produce this range of precipitation rates varied from 3 to 17 depending on the composition of the solution.The amount of Mg²⁺ incorporated in the overgrowths was not directly proportional to $\text{Mg}{}^{\mathrm{2+}}\text{Ca}{}^{\mathrm{2+}}$ in solution over the entire range (1–20) of ratios studied. Below a ratio of 7.5, the overgrowth was enriched in MgCO₃ relative to what is predicted by the constant distribution coefficient measured above a ratio of 7.5. This increased MgCO₃ correlates with the relative enrichment of adsorbed Mg²⁺. Above a ratio of 7.5 the concentration of MgCO₃ in the calcite overgrowths followed a classical thermodynamic behavior characterized by a constant distribution coefficient of 0.0123 (±0.008 std dev).The concentration of SrCO₃ incorporated in the overgrowths was linearly related to the MgCO₃ content of the overgrowths, and is attributed to increased solubility of SrCO₃ in calcite due to the incorporation of the smaller Mg²⁺ ions.The kinetic data indicate that the growth mechanism involves the adsorption of the cations on the surface of the calcite prior to dehydration and final incorporation. It is suggested that dehydration of cations at the surface is the rate controlling step.     

Mineralogy,nucleation and growth of dolomite in the laboratory and sedimentary environment: A review

Dolomite [CaMg(CO₃)₂] forms in numerous geological settings, usually as a diagenetic replacement of limestone, and is an important component of petroleum reservoir rocks, rocks hosting base metal deposits and fresh water aquifers. Dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of Ca²⁺ and Mg²⁺ cations interspersed with anion layers normal to the c‐axis. Dolomite has symmetry, lower than the (CaCO₃) symmetry of calcite primarily due to Ca–Mg ordering. High‐magnesium calcite also has symmetry and differs from dolomite in that Ca²⁺ and Mg²⁺ ions are not ordered. High‐magnesium calcite with near‐dolomite stoichiometry (≈50 mol% MgCO₃) has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high‐magnesium calcite. Many dolomites display some degree of cation disorder (Ca²⁺ on Mg²⁺ sites and vice versa), which is detectable using transmission electron microscopy and X‐ray diffractometry. Laboratory syntheses at high temperature and pressure, as well as studies of natural dolomites show that factors affecting dolomite ordering, stoichiometry, nucleation and growth include temperature, alkalinity, pH, concentration of Mg and Ca, Mg to Ca ratio, fluid to rock ratio, mineralogy of the carbonate being replaced, and surface area available for nucleation. In spite of numerous attempts, dolomite has not been synthesized in the laboratory under near‐surface conditions. Examination of published X‐ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near‐ambient conditions by microbial mediation are unsubstantiated. These laboratory products show no evidence of cation ordering and appear to be very high‐magnesium calcite. Elevated‐temperature and elevated‐pressure experiments demonstrate that dolomite nucleation and growth always are preceded by very high‐magnesium calcite formation. It remains to be demonstrated whether microbial‐mediated growth of very high‐magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high‐temperature experiments.     

Subsolidus and melting relations for the join CaCO3-MgCO3 at 10 kbar

Mixtures of pure dry CaCO₃ and MgCO₃ were reacted at 10 kbar in a piston-cylinder apparatus. Solidus and liquidus boundaries were delineated by interpretation of quenched textures. X-ray determined compositions of quenched carbonates are not a reliable guide to the phase relations. The binary melting loop for CaCO₃-MgCO₃ extends from CaCO₃ at 1460°C through a liquidus minimum near 30 wt% MgCO₃ and 1075°C, and it is terminated at the incongruent melting reaction for dolomite solid solution at 1125° C (liquid with 32 wt% MgCO₃) Magnesite solid solution dissociates at 1090°C to produce dolomite + periclase + CO₂, truncating the dolomite-magnesite solvus. The 10 kb liquidus minimum at 1075°C and 30 wt% MgCO₃ occurs at lower temperature and higher $\text{Ca}\text{Mg}$ ratio than the 27 kbar liquidus minimum at 1290°C and 38 wt% MgCO₃. This relationship suggests that the first liquid produced by melting of a carbonate-bearing peridotite has increasing $\text{Mg}\text{Ca}$ ratio with increasing pressure. These phase relations provide part of the framework required to trace paths of crystallization of kimberlite and carbonatite magmas.     

Investigation of the hydrogeochemical processes in an alluvial channel aquifer located in a typical Karoo Basin of Southern Africa

An investigation was conducted to assess the hydrogeochemical processes of an alluvial channel aquifer located in a typical Karoo Basin of Southern Africa. The investigation was aimed at identifying and describing the groundwater chemistry evolution and its contribution to the overall groundwater quality. X-ray fluorescent spectrometry (XRF) and X-ray diffractometry (XRD) analyses were performed on geological samples to identify and quantify the major element oxides and minerals. The study utilises the conventional Piper diagram, bivariate plots and PHREEQC hydrogeochemical model to analyse groundwater chemistry data obtained during the wet (February and May) and dry seasons (August and December) of 2011. The XRF and XRD results show that the channel deposits are dominated by SiO₂ element oxides and quartz minerals, thus elevated concentrations of silicon (Si⁴⁺) were found in the groundwater. Dolomite and calcite minerals were also detected in the unconsolidated aquifer sediments. The detailed study of the alluvial aquifer system has shown that dissolution of dolomite and calcite minerals and ion exchange are the dominant hydrogeochemical processes influencing the groundwater quality. The groundwater evolves from Ca²⁺–Mg²⁺–HCO₃ ^? recharge water that goes through ion exchange with Na⁺ in the clay-silt sediment to give a Na⁺–HCO₃ ^? water type. The groundwater is supersaturated with respect to quartz, dolomite and calcite minerals. The study shows the potential usefulness of simple bivariate plots as a complimentary tool to the conventional methods for analyzing groundwater hydrogeochemical processes.     

Cathodoluminescent bimineralic ooids from the Pleistocene of the Florida continental shelf

A bored and encrusted late Pleistocene ooid grainstone was recovered from the seafloor at a depth of approximately 40 m on the outer continental shelf of eastern Florida. Ooid cortices are dominantly bimineralic, generally consisting of inner layers of radial magnesian calcite and outer layers of tangential aragonite. Ooid nuclei are dominantly rounded cryptocrystalline grains, although quartz grains and a variety of skeletal grains also occur as nuclei. Ooids are partially cemented by blocky calcite, and interparticle porosity is partially filled by micrite. Radial cortex layers are composed of brightly cathodoluminescent magnesian calcite having a composition of approximately 12 mol% MgCO₃ and 1000 ppm strontium. The iron and manganese concentrations in radial cortex layers are generally in the range of 500–1000 ppm and 100–250 ppm, respectively. Tangential cortex layers are composed of noncathodoluminescent aragonite containing approximately 11 500 ppm strontium and less than 0.5 mol% MgCO₃. Iron concentrations in tangential cortex layers are generally in the range of 150–400 ppm, and manganese concentrations are generally below the detection limit of 100 ppm. Echinoderm skeletal fragments, which are present as accessory grains, are composed of brightly cathodoluminescent magnesian calcite. Some ooid nuclei and the thin outer edges of some blocky calcite cement are cathodoluminescent; micrite matrix and the bulk of blocky calcite cement are noncathodoluminescent. Ooids do not exhibit textural evidence of recrystallization. The ooid grainstone underwent an episode of meteoric diagenesis. but ooid cortices were not affected by the event. We propose a previously unrecognized process by which the magnesian calcite cortex layers underwent diagenetic alteration in oxygen-depleted seawater. During this diagenesis, magnesium was lost and manganese was incorporated without apparent textural alteration and without mineralogical stabilization. Thus, we Suggest that cathodoluminescence may result from diagenetic alteration on the sea-floor.     

Dolomite Versus Calcite Weathering in Hydrogeochemically Diverse Watersheds Established on Bedded Carbonates (Sava and So?a Rivers, Slovenia)

The relative contributions of dolomite to calcite weathering related to riverine fluxes are investigated on a highly resolved spatial scale in the diverse watersheds of Slovenia, which previous work has shown have some of the highest carbonate-weathering intensities in the world and suggests that dolomite weathering is favored over limestone weathering in mixed carbonate watersheds. The forested Sava and So?a River watersheds of Slovenia with their headwaters in the Julian Alps drain alpine regions with thin soils (<30 cm) and dinaric karst regions with thicker soils (0 to greater than 70 cm) all developed over bedded Mesozoic carbonates (limestone and dolomite), and siliclastic sediments is the ideal location for examining temperate zone carbonate weathering. This study extends previous work, presenting geochemical data on source springs and documenting downstream geochemical fluctuations within tributaries of the Sava and So?a Rivers. More refined sampling strategies of springs and discrete drainages permit directly linking the stream Mg²⁺/Ca²⁺ ratios to the local bedrock lithology and the HCO₃ ^? concentrations to the relative soil depths of the tributary drainages. Due to differences in carbonate source lithologies of springs and tributary streams, calcite and dolomite weathering end members can be identified. The Mg²⁺/Ca²⁺ ratio of the main channel of the Sava River indicates that the HCO₃ ^? concentration can be attributed to nearly equal proportions by mass of dolomite relative to calcite mineral weathering (e.g., Mg²⁺/Ca²⁺ mole ratio of 0.33). The HCO₃ ^? concentration and pCO₂ values increase as soil thickness and alluvium increase for discrete spring samples, which are near equilibrium with respect to calcite. Typically, this results in approximately 1.5 meq/l increase in HCO₃ ^? from the alpine to the dinaric karst regions. Streams in general do not change in HCO₃ ^?, Mg²⁺/Ca²⁺, or Mg²⁺/HCO₃ ^? concentrations down course, but warming and degassing of CO₂ produce high degrees of supersaturation with respect to calcite. Carbonate-weathering intensity (mmol/km²-s) is highest within the alpine regions where stream discharge values range widely to extreme values during spring snowmelt. Overall, the elemental fluxes of HCO₃ ^?, Ca²⁺, and Mg²⁺ from the tributary watersheds are proportional to the total water flux because carbonates dissolve rapidly to near equilibrium. Importantly, dolomite weathers preferentially over calcite except for pure limestone catchments.     

Origin of anhedral Sr-rich calcite in deep-water mixed sediment, north-east Australia

At burial depths of 800-1000 m, within the epicontinental Queensland Trough of north-east Australia (ODP Site 823), microcrystalline inter- and intraskeletal mosaics of anhedral (loaf-shaped, rounded) calcite have Sr²⁺ values ranging from below microprobe detection limits (<150 ppm) to 8100 ppm. Host rocks are well lithified, fine-grained mixed sediment to clayey wackestone and packstone of Middle and Late Miocene age. Petrography demonstrates that calcite precipitation has spanned shallow to deep burial, overlapping formation of framboidal pyrite in the upper 50 m; shallow-burial dolomitization (<300 m); and dedolomitization during sediment consolidation and incipient chemical compaction at greater (>400–500 m) depths. Petrographic observations illustrate that the calcite microfabric formed through coalescing crystal growth resulting from one or a combination of displacive growth in clay, porphyroid neomorphism of aragonite/vaterite, and clay replacement by calcite. Sr²⁺ mean concentrations in calcite between depths of 800 and 1000 m are similar to an expected equilibrium pore-water concentration, using a D^sr of 0.06, and may indicate active calcite precipitation. However, Sr²⁺ variation (2000–5000 ppm) within and among crystals, and concentrations that range well above predicted equilibrium values for a given depth, illustrate either variable Sr²⁺ retention during recrystallization of shelf-derived aragonite (and authigenic local vaterite) or relative uptake of Sr²⁺ during calcite precipitation with burial. Within the context of calcite formation during burial to 1 km, diagenetic attributes that affect the latter process include increased concentrations of pore-water Sr²⁺ with depth associated with aragonite recrystallization/dissolution; upward migration of Sr-rich pore water; and increased D^Sr related to local variation in precipitation/recrystallization rates, differential crystal sector growth rates and/or microvariation in aragonite distribution.     

Salinity reflux and dolomitization of southern Australian slope sediments: the importance of low carbonate saturation levels

Anomalously saline waters in Ocean Drilling Program Holes 1127, 1129, 1130, 1131 and 1132, which penetrate southern Australian slope sediments, and isotopic analyses of large benthic foraminifera from southern Australian continental shelf sediments, indicate that Pleistocene–Holocene meso‐haline salinity reflux is occurring along the southern Australian margin. Ongoing dolomite formation is observed in slope sediments associated with marine waters commonly exceeding 50‰ salinity. A well‐flushed zone at the top of all holes contains pore waters with normal marine trace element contents, alkalinities and pH values. Dolomite precipitation occurs directly below the well‐flushed zone in two phases. Phase 1 is a nucleation stage associated with waters of relatively low pH (ca 7) caused by oxidation of H₂S diffusing upward from below. This dolomite precipitates in sediments < 80 m below the sea floor and has δ¹³C values consistent with having formed from normal sea water (? 1‰ to + 1‰ Vienna Pee Dee Belemnite). The Sr content of Phase 1 dolomite indicates that precipitation can occur prior to substantial metastable carbonate dissolution (< 300 ppm in Holes 1129 and 1127). Dolomite nucleation is interpreted to occur because the system is undersaturated with respect to the less stable minerals aragonite and Mg‐calcite, which form more readily in normal ocean water. Phase 2 is a growth stage associated with the dissolution of metastable carbonate in the acidified sea water. Analysis of large dolomite rhombs demonstrates that at depths > 80 m below the sea floor, Phase 2 dolomite grows on dolomite cores precipitated during Phase 1. Phase 2 dolomite has δ¹³C values similar to those of the surrounding bulk carbonate and high Sr values relative to Phase 1 dolomite, consistent with having formed in waters affected by aragonite and calcite dissolution. The nucleation stage in this model (Phase 1) challenges the more commonly accepted paradigm that inhibition of dolomitization by sea water is overcome by effectively increasing the saturation state of dolomite in sea water.     

Geochemistry of meteoric calcite cements in some Pleistocene limestones

In this study, the stable isotope and trace element geochemistries of meteoric cements in Pleistocene limestones from Enewetak Atoll (western Pacific Ocean), Cat Island (Bahamas), and Yucatan were characterized to help interpret similar cements in ancient rocks. Meteoric calcite cements have a narrow range of δ¹⁸O values and a broad range of δ¹³C values in each geographical province. These Pleistocene cements were precipitated from water with stable oxygen isotopic compositions similar to modern rainwater in each location. Enewetak calcite cements have a mean δ¹⁸O composition of ?6.5%₀ (PDB) and δ¹³C values ranging from ?9.6 to +0.4%0 (PDB). Sparry calcite cements from Cat Island have a mean δ¹⁸O composition of ?4.1%₀ and δ¹³C values ranging from ?6.3 to + 1.1%₀. Sparry cements from Yucatan have a mean δ¹⁸O composition of ?5.7%₀ and δ¹³C values of ?8.0 to ?2.7%₀. The mean δ¹⁸O values of these Pleistocene meteoric calcite cements vary by 2.4%₀ due to climatic variations not related directly to latitude. The δ¹³C compositions of meteoric cements are distinctly lower than those of the depositional sediments. Variations in δ¹³C are not simply a function of distance below an exposure surface. Meteoric phreatic cements often have δ¹³C compositions of less than —4.0%₀, which suggests that soil-derived CO₂ and organic material were washed into the water table penecontemporaneous with precipitation of phreatic cements. Concentrations of strontium and magnesium are quite variable within and between the three geographical provinces. Mean strontium concentrations for sparry calcite cements are, for Enewetak Atoll, 620 ppm (σ= 510 ppm); for Cat Island, 1200 ppm (σ= 980 ppm); and for Yucatan, 700 ppm (σ= 390 ppm). Equant cements, intraskeletal cements, and Bahamian cements have higher mean strontium concentrations than other cements. Equant and intraskeletal cements probably precipitated in more closed or stagnant aqueous environments. Bahamian depositional sediments had higher strontium concentrations which probably caused high strontium concentrations in their cements. Magnesium concentrations in Pleistocene meteoric cements are similar in samples from Enewetak Atoll (mean =1.00 mol% MgCO₃; σ= 0.60 mol% MgCO₃) and Cat Island (mean = 0.84 mol% MgCO₃; σ= 0.52mol% MgCO₃) but Yucatan samples have higher magnesium concentrations (mean = 2.20 mol% MgCO₃: σ= 0.84mol% MgCO₃). Higher magnesium concentrations in some Yucatan cements probably reflect precipitation in environments where sea water mixed with fresh water.     

Silicate and carbonate mineral weathering in soil profiles developed on Pleistocene glacial drift (Michigan, USA): Mass balances based on soil water geochemistry

Geochemistry of soil, soil water, and soil gas was characterized in representative soil profiles of three Michigan watersheds. Because of differences in source regions, parent materials in the Upper Peninsula of Michigan (the Tahquamenon watershed) contain only silicates, while those in the Lower Peninsula (the Cheboygan and the Huron watersheds) have significant mixtures of silicate and carbonate minerals. These differences in soil mineralogy and climate conditions permit us to examine controls on carbonate and silicate mineral weathering rates and to better define the importance of silicate versus carbonate dissolution in the early stage of soil-water cation acquisition.Soil waters of the Tahquamenon watershed are the most dilute; solutes reflect amphibole and plagioclase dissolution along with significant contributions from atmospheric precipitation sources. Soil waters in the Cheboygan and the Huron watersheds begin their evolution as relatively dilute solutions dominated by silicate weathering in shallow carbonate-free soil horizons. Here, silicate dissolution is rapid and reaction rates dominantly are controlled by mineral abundances. In the deeper soil horizons, silicate dissolution slows down and soil-water chemistry is dominated by calcite and dolomite weathering, where solutions reach equilibrium with carbonate minerals within the soil profile. Thus, carbonate weathering intensities are dominantly controlled by annual precipitation, temperature and soil pCO₂. Results of a conceptual model support these field observations, implying that dolomite and calcite are dissolving at a similar rate, and further dissolution of more soluble dolomite after calcite equilibrium produces higher dissolved inorganic carbon concentrations and a Mg²⁺/Ca²⁺ ratio of 0.4.Mass balance calculations show that overall, silicate minerals and atmospheric inputs generally contribute <10% of Ca²⁺ and Mg²⁺ in natural waters. Dolomite dissolution appears to be a major process, rivaling calcite dissolution as a control on divalent cation and inorganic carbon contents of soil waters. Furthermore, the fraction of Mg²⁺ derived from silicate mineral weathering is much smaller than most of the values previously estimated from riverine chemistry.     

'Forbidden zone' subduction of sediments to 150 km depth— the reaction of dolomite to magnesite + aragonite in the UHPM metapelites from western Tianshan, China

The solid‐state reaction magnesite (MgCO₃) + calcite (aragonite) (CaCO₃) = dolomite (CaMg(CO₃)₂) has been identified in metapelites from western Tianshan, China. Petrological studies show that two metamorphic stages are recorded in the metapelites: (1) the peak mineral assemblage of magnesite and calcite pseudomorphs after aragonite which is only preserved as inclusions within dolomite; and (2) the retrograde glaucophane‐chloritoid facies mineral assemblage of glaucophane, chloritoid, dolomite, garnet, paragonite, chlorite and quartz. The peak metamorphic temperatures and pressures are calculated to be 560–600 °C, 4.95–5.07 GPa based on the calcite–dolomite geothermometer and the equilibrium calculation of the reaction dolomite = magnesite + aragonite, respectively. These give direct evidence in UHP metamorphic rocks from Tianshan, China, that carbonate sediments were subducted to greater than 150 km depth. This UHP metamorphism represents a geotherm lower than any previously estimated for subduction metamorphism (< 3.7 °C km^?1) and is within what was previously considered a ‘forbidden’ condition within Earth. In terms of the carbon cycle, this demonstrates that carbonate sediments can be subducted to at least 150 km depth without releasing significant CO₂ to the overlying mantle wedge.     

Determining fluid source and possible pathways during burial dolomitization of Maryville Limestone (Cambrian), Southern Appalachians, USA

Detailed petrographic analyses along a depositional transect from a carbonate platform to shale basin reveals that dolomite is the principal burial diagenctic mineral in the Maryville Limestone. This study examines the role of burial dolomitization of subtidal carbonates. Dolomite occurs as a replacement of precursor carbonate and as inter- and intraparticle cements. Four different types of dolomite are identified based on detailed petrographic and gcochemical analyses. Type I dolomite occurs as small, irregular disseminations typically within mud-rich facies.Type II dolomite typically occurs as inclusions of planar euhedral rhombs (ferroan), 5–300 μm in size, in blocky clear ferroan calcite (meteoric) spar. Type II dolomite is non-luminescent. Type I and II dolomite formed during shallow to intermediate burial diagenesis. Type III dolomite consists of subhedral to anhedral crystals 10–150 μm in size occurring as thin seams along stylolites and as thick bands a few millimetres in width. This dolomite consists of dominantly non-luminescent rhombs and, less commonly, orange luminescent and zoned rhombs. Type IV dolomite consists of baroque or saddle-shaped, 100–1500 μm crystals, and is non-luminescent. Type IV dolomite formed during the period of maximum burial. Types III and IV dolomite increase in abundance downslope. Type III dolomite contains 1.2–2.6 wt% Fe and a maximum of 1000 ppm Mn. The distribution of these elements displays no distinct vertical or lateral trends. In contrast, Fe and Mn distributions in Type IV dolomite exhibit distinct spatial trends, decreasing from 3.5–4.5 wl% Fe and 0.1–0.3 wt% Mn in the west (slope/basin) to 1.5–2.5 wt% Fe and less than 600 ppm Mn in the east (shelf margin), a distance of approximately 60 km. Spatial trends in Fe and Mn distributions in Type IV saddle dolomite, suggest a west-east fluid flow during late burial diagenesis. Types III and IV dolomite have a mean δ¹⁸O value of - 7.8%₀₀ and a mean δ¹³C value of + 1.1%₀₀ (relative to the PDB standard). Based on a range of assumed basinal water composition of 2.8%₀₀ SMOW, temperatures calculated from δ¹⁸O values of Types III and IV dolomite range between 75 and 160°C. ⁸⁷Sr/⁸⁶Sr data for Types III and IV dolomite range from 0.7111 to 0.7139. These values are radiogenic when compared to Cambrian marine values and are consistent with the presence of a diagenetic fluid that interacted with siliciclastic sediments. The distribution of Palaeozoic facies in the southern Appalachians indicates a Cambrian shale source for the fluids, whilst burial curves suggest a Middle Ordovician age for burial fluid movement.     

Sedimentology, mineralogy and isotopic analysis of Pellet Lake, Coorong region, South Australia

ABSTRACT Gravity cores of Holocene sediments from a shallow ephemeral lake in the Coorong region (Pellet Lake, southeastern coastal Australia) show a mineral assemblage and sequence particular to its hydrology. The mineralogical sequence above an initial dolomitic siliciclastic sand reflects conditions of increasing salinity in the lower portions of the core (i.e. organic-rich aragonite to magnesite + hydromagnesite + aragonite) followed by a relative decrease in salinity (i.e. magnesite + aragonite + hydromagnesite to aragonite + hydromagnesite) in the upper portions of the core. This sequence is capped by ? 0.4 m of micritic dolomite and minor amounts of hydromagnesite, with the relative abundance of dolomite increasing upwards. Three stratigraphically and spatially distinct dolomite units (upper, lower and margin) are recognized using stable carbon and oxygen isotope data, unit cell calculations and MgCO₃ mole per cent data of the dolomite. Detailed X-ray diffraction (XRD) analyses of samples with more than 80% dolomite shows that the dolomite is ordered. Average unit cell parameters, calculated from the XRD patterns, indicate that the upper dolomite unit has crystal lattices expanded in the c_o direction (c_o= 16.09 Å) relative to ideal dolomite (c_o= 16.02 Å) and contracted in the a_o direction (a_o= 4.796 Å) relative to ideal dolomite (a_o= 4.812 Å). The mol fraction of MgCO₃ in the upper dolomite shows up to 4.0 ±M 2.0 mole per cent excess Mg in the dolomite crystal lattice (calculated from XRD). This unusual dolomite crystal chemistry is probably generated by rapid precipitation from solutions which have greatly elevated Mg/Ca ratios. Transmission electron microscopy reveals that the upper dolomite has a heterogeneous microstructure which also suggests rapid precipitation from solution. The modulated microstructure found in calcium-rich dolomite is completely lacking. Dolomite ordering reflections are present in electron diffraction patterns, but are weak. Stable oxygen and carbon isotope values of the upper dolomite are tightly grouped (ave. δ¹⁸O ～+ 7.55%_o, δ¹³C ～+ 4.10%_o), yet show three upward-lightening oxygen cycles. The oxygen cycles correlate with three upward decreases in the calculated Mg content of the dolomite zone. These cycles may indicate the increased importance of rain-water dilution of the brine at times when the water in the lake was at its shallowest levels. Analyses of the lower dolomite and the margin dolomite suggest that these units precipitated more slowly from less evaporitic brines than the upper dolomite unit. The lower dolomite is close to stoichiometric, has less evaporitic stable isotope values than the upper dolomite, and has only a slightly expanded c_o-axis. The margin dolomite is Ca-rich, has a more homogeneous microstructure, and has expanded a_o and c_o axes. The abundance of relatively soluble Mg-bearing phases, such as hydromagnesite and magnesite, may supply additional magnesium for the dolomitization of aragonite and calcite during subsequent diagenesis and burial of the sediment. This process may leave a finely laminated dolomicrite deposit which retains little, if any, evidence of evaporite minerals.     

Geochemical controls on a calcite precipitating spring

A small spring fed stream was found to precipitate calcite by mainly inorganic processes and in a nonuniform manner. The spring water originated by rainwater falling in a 0.8 km² basin, infiltrating, and dissolving calcite and dolomite followed by dissolution of gypsum or anhydrite. The Ca²⁺/Mg²⁺ indicates that calcite is probably precipitated in the subsurface from a supersaturated solution. This water emerges from the spring still about 5 times supersaturated with respect to calcite and continues calcite precipitation. When 10 times supersaturation is reached, due to CO₂ degassing the precipitation is more rapid. The calcite accumulation from the stream with a flow of 5 l/s is calculated to be 12600 kg/yr with the highest rates in areas where CO₂ degassing is the greatest. The non-equilibrium, as shown by the high calcite supersaturation, is also reflected in a variable partitioning pattern for Sr²⁺ between the water and calcite.     

Reaction kinetics of dolomite rim growth

Reaction rims of dolomite (CaMg[CO₃]₂) were produced by solid-state reactions at the contacts of oriented calcite (CaCO₃) and magnesite (MgCO₃) single crystals at 400 MPa pressure, 750–850 °C temperature, and 3–146 h annealing time to determine the reaction kinetics. The dolomite reaction rims show two different microstructural domains. Elongated palisades of dolomite grew perpendicular into the MgCO₃ interface with length ranging from about 6 to 41 µm. At the same time, a 5–71 µm wide rim of equiaxed granular dolomite grew at the contact with CaCO₃. Platinum markers showed that the original interface is located at the boundary between the granular and palisade-forming dolomite. In addition to dolomite, a 12–80 µm thick magnesio-calcite layer formed between the dolomite reaction rims and the calcite single crystals. All reaction products show at least an axiotactic crystallographic relationship with respect to calcite reactant, while full topotaxy to calcite prevails within the granular dolomite and magnesio-calcite. Dolomite grains frequently exhibit growth twins characterized by a rotation of 180° around one of the $[11\bar{2}0]$ equivalent axis. From mass balance considerations, it is inferred that the reaction rim of dolomite grew by counter diffusion of MgO and CaO. Assuming an Arrhenius-type temperature dependence, activation energies for diffusion of CaO and MgO are E _a (CaO) = 192 ± 54 kJ/mol and E _a (MgO) = 198 ± 44 kJ/mol, respectively.     

The dolomitization of CaCO3: an experimental study at 252–295°C

The results of experiments on the hydrothermal dolomitization of calcite (between 252 and 295°C) and aragonite (at 252°C) by a 2 M CaCl₂-MgCl₂ aqueous solution are reported and discussed. Dolomitization of calcite proceeds via an intermediate high (ca. 35 mole %) magnesian calcite, whereas that of aragonite is carried out through the conversion of the reactant into a low (5.6 mole %) magnesian calcite which in turn transforms into a high (39.6 mole %) magnesian calcite. Both the intermediate phases and dolomite crystallize through a dissolution-precipitation reaction. The intermediate phases form under local equilibrium within a reaction zone surrounding the dissolving reactant grains. The volume of the reaction zone solution can be estimated from Sr²⁺ and Mg²⁺ partitioning equations. In the case of low magnesian calcite growing at the expense of aragonite at 252°C, the total volume of these zones is in the range of 2 × 10^?5 to 2 × 10^?4 1., out of 5 × 10^?3 1., the volume of the bulk solution.The apparent activation energies for the initial crystallization of high magnesian calcite and dolomite are 48 and 49 kcal/mole, respectively.Calcite transforms completely into dolomite within 100 hr at 252°C. The overall reaction time is reduced to approximately 4 hr at 295°C. The transformation of aragonite to dolomite at 252°C occurs within 24 hr. The nature of the reactant dictates the relative rates of crystallization of the intermediate phases and dolomite. With calcite as reactant, dolomite growth is faster than that of magnesian calcite; this situation is reversed when aragonite is dolomitized.Coprecipitation of Sr²⁺ with dolomite is independent of temperature (within analytical error) between 252 and 295°C. Its partitioning, with respect to calcium, between dolomite and solution results in distribution coefficients in the range of 2.31 × 10^?2 to 2.78 × 10^?2.     

Oxygen isotope fractionation during the dolomitization of calcium carbonate

The oxygen isotope fractionation accompanying the hydrothermal dolomitization of CaCO₃ between 252 and 295°C has been investigated. Dolomitization (which occurs via the crystallization of one or more intermediate magnesian calcite phases) is characterised by a progressive lowering in δ⁸O, which smoothly correlates with the change in the Mg/(Mg + Ca) and the $\text{Sr}\text{(Mg + Ca)}$ ratios and with the sequential phase formation. The data support the proposals of Katz and Matthews (1977) that (a) all reaction occurs by solution and reprecipitation, (b) intermediate phases and dolomite form sequentially and (c) the intermediate phases form within limited solution zones surrounding the dissolving precursor. Calculated volumes of the solution zone for the aragonite → low magnesian calcite transformation are within the range 3.7–6.7 × 10^?5 liters (out of 5 × 10^?3 liters, the volume of the bulk solution used in the present study), and agree well with those calculated from strontium and magnesium partitioning data. Dolomite precipitates in apparent isotopic equilibrium with the bulk solution. The temperature dependence of the fractionation is defined by the equation 1000 Inα_D-H2O = 3.06 × 10⁶T^?2 ? 3.24 Dolomite-water fractionations from this equation are significantly lower than those obtained by extrapolation of the Northrop And Clayton (1966) calibration. The reaction zone model can be applied to explain near zero dolomite-calcite oxygen isotope fractionations reported by Epsteinet al. (1964).     

Low-Mg calcite marine cement in Cretaceous turbidites: origin, spatial distribution and relationship to seawater chemistry

Lower Cretaceous (Hauterivian) bioclastic sandstone turbidites in the Scapa Member (North Sea Basin) were extensively cemented by low-Mg calcite spars, initially as rim cements and subsequently as concretions. Five petrographically distinct cement stages form a consistent paragenetic sequence across the Scapa Field. The dominant and pervasive second cement stage accounts for the majority of concretions, and is the focus of this study. Stable-isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions. Nevertheless, trends from whole-rock data, augmented by cement separates from synlithification fractures, indicate an early calcite δ¹⁸O value of+0·5 to -1·5‰ PDB. As such, the calcite probably precipitated from marine pore fluids shortly after turbidite deposition. Carbon isotopes (δ¹³C=0 to -2‰ PDB) and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution. Textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation, as expected for a mineral-controlled transformation. Electron probe microanalyses demonstrate that early calcite cement contains <2 mol% MgCO₃, despite its marine parentage. Production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine Mg²⁺/Ca²⁺ ratio in early Cretaceous ‘calcite seas’, relative to modern oceans. Scapa cement compositions concur with published models in suggesting that Hauterivian ocean water had a Mg²⁺/Ca²⁺ ratio of ≤1. This is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics. In contrast to the dominant early cement, late-stage ferroan, ¹⁸O-depleted calcites were sourced outwith the Scapa Member and precipitated after 1–2 km of burial. Our results emphasize that bioclast dissolution and low-Mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis.     

Marine-derived dolomite in the shallowly buried temperate Port Campbell Limestone (Miocene), Otway Basin, Australia

The Miocene Port Campbell Limestone in the Otway Basin (Port Campbell Embayment), south-eastern Australia, is a shallowly buried (<350 m), temperate carbonate grainstone which consists primarily of benthonic foraminifera, bryozoans, brachiopods, echinoids and planktonic foraminifera. Volumetrically insignificant calcite cements include scalenohedral, blocky and syntaxial overgrowths. Dolomite is present in variable amounts (1–25%), scattered throughout the unit as euhedral rhombs, usually comprising <2% of the whole rock volume. The dolomite post-dates the calcite cements and is mainly an interparticle cement with crystal size varying between 10 and 150 μm (mean 50 μm). Under cathodoluminescence the dolomite rhombs have a dull core and a nonluminescent outer rim. The dolomite is nonstoichiometric, Ca-rich (Ca_54–62Mg_38–46), with high trace element concentrations. The Mn concentrations range from 0 to 310 p.p.m. in the crystal cores (mean 140 p.p.m.) and 80–240 p.p.m. in the crystal rims (mean 140 p.p.m.). The Fe concentrations increase from the crystal cores (range 640–5690 p.p.m.; mean 2030 p.p.m.) to the crystal rims (range 2840–9440 p.p.m.; mean 6040 p.p.m.), whereas the Sr concentrations decrease from the crystal cores (range 690–1510 p.p.m.; mean 1280 p.p.m.) to the crystal rims (620–1240 p.p.m.; mean 930 p.p.m.). The δ₁₃C_PDB values of the dolomite range between +2.5 and +2.6%, whereas the δ₁₈O_PDB values range from +0.3% to+0.6%. This dolomite occurrence supports the idea that marine or near-marine dolomite can form not only syndepositionally, but also in the shallow subsurface of temperate units, soon after sediment deposition, under reducing conditions. The fine-grained, low-permeability nature of the Port Campbell Limestone contributed to the reducing conditions at shallow depth, the high trace element concentrations of the dolomite (especially in Fe) and the near marine composition of the dolomitizing fluids, as large volumes of meteoric water were inhibited.     

Calcium and magnesium‐limited dolomite precipitation at Deep Springs Lake,California

Dolomite [Ca,Mg(CO₃)₂] precipitation from supersaturated ionic solutions at Earth surface temperatures is considered kinetically inhibited because of the difficulties experienced in experimentally reproducing such a process. Nevertheless, recent dolomite is observed to form in hypersaline and alkaline environments. Such recent dolomite precipitation is commonly attributed to microbial mediation because dolomite has been demonstrated to form in vitro in microbial cultures. The mechanism of microbially mediated dolomite precipitation is, however, poorly understood and it remains unclear what role microbial mediation plays in natural environments. In the study presented here, simple geochemical methods were used to assess the limitations and controls of dolomite formation in Deep Springs Lake, a highly alkaline playa lake in eastern California showing ongoing dolomite authigenesis. The sediments of Deep Springs Lake consist of unlithified, clay‐fraction dolomite ooze. Based on δ¹⁸O equilibria and textural observations, dolomite precipitates from oxygenated and agitated surface brine. The Na‐SO₄‐dominated brine contains up to 500 mm dissolved inorganic carbon whereas Mg²⁺ and Ca²⁺ concentrations are ca 1 and 0·3 mm , respectively. Precipitation in the subsurface probably is not significant because of the lack of Ca²⁺ (below 0·01 mm ). Under such highly alkaline conditions, the effect of microbial metabolism on supersaturation by pH and alkalinity increase is negligible. A putative microbial effect could, however, support dolomite nucleation or support crystal growth by overcoming a kinetic barrier. An essential limitation on crystal growth rates imposed by the low Ca²⁺ and Mg²⁺ concentrations could favour the thermodynamically more stable carbonate phase (which is dolomite) to precipitate. This mode of unlithified dolomite ooze formation showing δ¹³C values near to equilibrium with atmospheric CO₂ (ca 3‰) contrasts the formation of isotopically light (organically derived), hard‐lithified dolomite layers in the subsurface of some less alkaline environments. Inferred physicochemical controls on dolomite formation under highly alkaline conditions observed in Deep Springs Lake may shed light on conditions that favoured extensive dolomite formation in alkaline Precambrian oceans, as opposed to modern oceans where dolomites only form diagenetically in organic C‐rich sediments.