Cyclic development of large,complex, calcite dendrite crystals in the Clinton travertine,Interior British Columbia,Canada

Bands of large (up to 4 cm long) three-dimensional crystallographic dendrites form the terrace fronts in an old travertine mound exposed near Clinton, British Columbia. The dendrites, with their long axes perpendicular to the terrace front, are characterized by numerous levels of branching. Each branch is formed of multitudes of skeletal rhombs, four- and six(?)-sided bipyramidal crystals, or prismatic hexagonal crystals that are precisely aligned along crystallographic precepts. Although individual branches are formed of one type of subcrystal, neighbouring branches may be formed of different subcrystal types.Highly supersaturated waters that were generated by rapid CO₂ degassing of the spring water during its turbulent flow over the steep terrace fronts probably drove dendrite precipitation. The presence of growth lines indicates that growth was episodic. Type I growth lines probably formed annually in response to seasonal climate changes whereas Type II growth lines, which formed less frequently, may reflect changes in the flow velocity and/or flow patterns of the spring waters.Early diagenetic modification of the dendrites involved crystal face enlargement, cements formed of trigonal prisms or needle-fiber crystals, microbial infestation that mediated substrate dissolution, and/or deposition of detrital calcite crystals that formed in the water column. Much of the diagenetic modification may have taken place during the periods when the dendrites had temporarily stopped growing.The dendrites in the Clinton travertine are an excellent example of complex, episodic calcite crystal growth that was extensively modified by early diagenetic processes in a surface environment. The same spring waters from which the dendrites were precipitated mediated much of the early diagenesis.     

Impact of lake‐level changes on the formation of thermogene travertine in continental rifts: Evidence from Lake Bogoria,Kenya Rift Valley

Travertine is present at 20% of the ca 60 hot springs that discharge on Loburu delta plain on the western margin of saline, alkaline Lake Bogoria in the Kenya Rift. Much of the travertine, which forms mounds, low terraces and pool‐rim dams, is sub‐fossil (relict) and undergoing erosion, but calcite‐encrusted artefacts show that carbonate is actively precipitating at several springs. Most of the springs discharge alkaline (pH: 8·3 to 8·9), Na‐HCO₃ waters containing little Ca (<2 mg l^?1) at temperatures of 94 to 97·5°C. These travertines are unusual because most probably precipitated at temperatures of >80°C. The travertines are composed mainly of dendritic and platy calcite, with minor Mg‐silicates, aragonite, fluorite and opaline silica. Calcite precipitation is attributed mainly to rapid CO₂ degassing, which led to high‐disequilibrium crystal morphologies. Stratigraphic evidence shows that the travertine formed during several stages separated by intervals of non‐deposition. Radiometric ages imply that the main phase of travertine formation occurred during the late Pleistocene (ca 32 to 35 ka). Periods of precipitation were influenced strongly by fluctuations in lake level, mostly under climate control, and by related changes in the depth of boiling. During relatively arid phases, meteoric recharge of ground water declines, the lake is low and becomes hypersaline, and the reduced hydrostatic pressure lowers the level of boiling in the plumbing system of the hot springs. Any carbonate precipitation then occurs below the land surface. During humid phases, the dilute meteoric recharge increases, enhancing geothermal circulation, but the rising lake waters, which become relatively dilute, flood most spring vents. Much of the aqueous Ca²⁺ then precipitates as lacustrine stromatolites on shallow firm substrates, including submerged older travertines. Optimal conditions for subaerial travertine precipitation at Loburu occur when the lake is at intermediate levels, and may be favoured during transitions from humid to drier conditions.     

A geochemical evaluation of perennial spring activity and associated mineral precipitates at Expedition Fjord,Axel Heiberg Island,Canadian High Arctic

Two groups of perennial springs are observed in the Canadian High Arctic at Expedition Fjord on Axel Heiberg Island at Colour Peak and Gypsum Hill. Saline discharge (∼1.3–2.5 molal NaCl) produces a variety of calcite (travertine) and gypsum-rich precipitates. Saturation index calculations of the spring waters at Colour Peak suggest CO₂ degassing from the waters causes calcite precipitation. Gypsum precipitation dominates at Gypsum Hill, where spring waters have lower alkalinity and higher SO₄ concentrations. Mineral accumulations form both channel and rimstone pool morphologies as a result of varying slope conditions. At Colour Peak, confined flow in steep slope areas develop massive structures in contrast to more friable, porous accumulations in areas where waters fan out on shallower slopes; these morphological variations lead to corresponding varying apparent rates of mineral precipitation. Mineral precipitation at Gypsum Hill is far less notable as a result of lower discharge rates and annual degradation by icing formation. Microscopic observations and geochemical analyses of the channel precipitates at Colour Peak reveal alternating light (calcite spar) and dark (anhedral microcrystalline calcite combined with organic matter and non-carbonate minerals) laminae. Rimstone pools forming in lower sections of spring discharge are composed of accumulations of large euhedral calcite crystals interbedded with allochthonous inputs. High concentration of dissolved solids is responsible for slow travertine precipitation rates, which occurs during winter. This precipitation is further retarded during summer months by the introduction of crystal growth inhibitors such as Fe³⁺ and deposition of organic matter and soil sediments.     

Microenvironmental controls on mineralogy and habit of CaCO3 precipitates: an example from an active travertine system

Analysis of water and associated carbonate precipitates from a small, warm-spring travertine system in SW Colorado, USA, provide an example of the: (i) great variability of the geochemical parameters within these dynamic systems, and (ii) significance of the microenvironment in controlling mineralogy and morphology of carbonate precipitates. Waters emerged from the springs highly charged in CO₂, with an initial p_CO2 of 1.2 × 10⁵ Pa. Degassing of the CO₂ from the waters decreased the pH from 6.1 to 8.0, resulting in an increase of 8%‰ in δ¹³C values downflow in the total CO₂ in solution and an increase in the I_SAT from 2.1 to as high as 63 times supersaturation with respect to calcite. Due to changes in the stable isotopic composition of the waters downflow as well as changes in the degree of supersaturation, stable isotopic analyses range greatly from locale to locale within this small system. Near the spring vents, at relatively low I_SAT levels, well-developed rhombohedra of calcite formed as biotically induced precipitates around diatom stalks and other algae as well as abiotic crusts. In contrast, near the distal end of the system, very high I_SAT levels were reached and resulted in the precipitation of skeletal-dendritic crystals of calcite on copper substrates, floating rafts of laterally linked hemispheres of aragonite crystals, and bimineralic carbonate-encrusted bubbles. Microenvironmental parameters control the mineralogy and habit of these precipitates.     

A preliminary analysis of the formation of travertine and travertine cones in the Jifei hot spring,Yunnan, China

The Jifei hot spring emerges in the form of a spring group in the Tibet–Yunnan geothermal zone, southwest of Yunnan Province, China. The temperatures of spring waters range from 35 to 81°C and are mainly of HCO₃–Na·Ca type. The total discharge of the hot spring is about 10 L/s. The spring is characterized by its huge travertine terrace with an area of about 4,000 m² and as many as 18 travertine cones of different sizes. The tallest travertine cone is as high as 7.1 m. The travertine formation and evolution can be divided into three periods: travertine terrace deposition period, travertine cone formation period and death period. The hydrochemical characteristics of the Jifei hot spring was analyzed and compared with a local non-travertine hot spring and six other famous travertine springs. The results indicate that the necessary hydrochemical conditions of travertine and travertine cones deposition in the Jifei area are (1) high concentration of HCO₃ ⁻ and CO₂; (2) about 52.9% deep source CO₂ with significantly high value; (3) very high milliequivalent percentage of HCO₃ ⁻ (97.4%) with not very high milliequivalent percentage of Ca²⁺ (24.4%); and (4) a large saturation index of calcite and aragonite of the hot water.     

Origin of pedogenic needle-fiber calcite revealed by micromorphology and stable isotope composition—a case study of a Quaternary paleosol from Hungary

Pedogenic needle-fiber calcite was studied regarding its morphology, texture and stable isotope composition from the paleosol of the Quaternary Várhegy travertine (Budapest, Hungary). The needle-fiber calcite is composed of 40–200 μm long monocrystals. Smooth rods as well as serrated-edged crystals with calcite overgrowths were identified by SEM. Needles have several textural varieties: randomly distributed crystals in vugs and pores with calcite hypocoatings, bundles of subparallel crystals forming coatings around grains and alveolar structure with bridging needles in vugs.The morphological study of needle-fiber calcite suggests that needles are calcified fungal sheaths and produced by fungal biomineralization, a common process in recent and fossil soils and calcretes. The stable isotope composition of needle-fiber calcite (average: δ¹⁸O=-7.1‰ and δ¹³C=-7.3‰ vs. V-PDB) indicates significant incorporation of organically derived CO₂ and probably biological influence on needle genesis. Dissolved host rock travertine and/or atmospheric CO₂ could also contribute some carbon to the acicular calcite.     

Mineralogy and origin of rhizoliths on the margins of saline,alkaline Lake Bogoria,Kenya Rift Valley

A wide range of rhizoliths occurs around the margins of Lake Bogoria, Kenya. These include root casts, moulds, tubules, rhizocretions, and permineralised root systems. These rhizoliths are variably composed of opaline silica, calcite, zeolites (mainly analcime), fluorite, and possibly fluorapatite, either alone or in combinations. Some rhizoliths are infilled moulds with detrital silicate grains. Most rhizoliths are in situ, showing both vertical and horizontal orientations. Reworked rhizoliths have been concentrated locally to form dense rhizolites.Hot-spring fluids, concentrated by evapotranspiration and capillary evaporation, have provided most of the silica for the permineralisation of the plant tissues. Precipitation involved the growth of silica nanospheres and microspheres that coalesced into homogeneous masses. Calcite rhizoliths formed following evaporative concentration, evapotranspiration, and (or) CO₂ degassing of Ca-bearing runoff water that infiltrated the sediment, or by mixing of runoff with saline, alkaline groundwater. Fluorite precipitated in areas where mixing of hot-spring and meteoric waters occurred, or possibly where hot-spring fluids came into contact with pre-existing calcite. Zeolitic rhizoliths formed during a prolonged period of aridity, when capillary rise and evaporative pumping brought saline, alkaline waters into contact with detrital silicate minerals around roots.     

Hydrochemical and isotope characteristics of spring water and travertine in the Baishuitai area (SW China) and their meaning for paleoenvironmental reconstruction

A method of combining hydrochemical data logging and in situ titrating with measurement of stable carbon and oxygen isotopes was used to reveal the hydrochemical and isotopic characteristics in the Baishuitai travertine scenic area of SW China. It was found that the travertine-forming springs have a very high concentration of calcium and bicarbonate, and accordingly very high CO₂ partial pressures, which are not likely to be produced by biological activity in soil alone. Further analysis of the stable carbon isotopes of the springs shows that the high pressure of CO₂ is mainly related to an endogenic CO₂ source. That means the Baishuitai travertine is endogenic in origin. This is contrast to the commonly accepted saying that the travertine deposition in this study simply is a product of warm and humid conditions in a karst ecological environment. Rapid CO₂ degassing from the water is triggered by the much higher partial pressures in water than that of the surrounding air. Consequently, as the waters flow downstream of the spring the pH increases, the waters become supersaturated with respect to calcite, and travertine is deposited. The preferential release of ¹²CO₂ to the atmosphere results in a progressive increase of travertine ¹³C downstream. This is concluded with a preliminary discussion of variation in travertine-forming water temperatures, according to differences in stable oxygen isotopic compositions of the travertine formed in different epochs at Baishuitai. It was found that the change in water temperature is as high as 13 °C, i.e., from 23 °C at about 2500 years b.p., to 10 °C at present. This may mainly reflect that the effect of geothermal source on water temperature is decreasing. The problems involved in paleoenvironmental reconstruction with endogene travertine are also discussed. They are the impacts of "dead carbon" in radiocarbon dating and the enrichment in ¹³C of travertine by endogenic CO₂ and degassing of CO₂ from water, which has to be considered in paleovegetation reconstruction when using ¹³C data of the endogene carbonate deposits.     

Microscopic calcite dendrites in cold-water tufa: implications for nucleation of micrite and cement

Dendritic calcite forms in an active cold-water tufa system in association with extracellular polymeric substances (EPS) that discontinuously coat bryophytes and cyanobacteria. Dendrites consist of 100–200 nm thick calcite fibres that form 3D lattice-like domains. In each dendrite domain, fibres have three structurally equal orientations, which correspond in disposition to radii from the centre of a calcite unit cell to the convex triple face junctions on its surface. Fibres do not form in the orientation of the c-axis. The external form of each dendrite has the shape of half of a shortened octahedron, with an upper triangular surface parallel to the substrate. Dendrite nucleation takes place on or in microbial EPS, whether microbial cells are present or not, and is probably effected by attraction of Ca²⁺ cations to negatively charged EPS, together with CO₂-degassing and concomitant pH increase of supersaturated spring water in stream splash zones. Ensuing dendrite growth is abiogenic and controlled by diffusion. Dendrite c-axes are perpendicular to the substrate, probably because the negative charge of EPS forces the orientation of Ca²⁺ and CO planes within the developing dendrite crystal to be parallel to the EPS film surface. Dendrites are eventually filled and overgrown by solid, syntaxial calcite, which gradually and completely obliterates the dendrites as more familiar calcite crystal forms develop. No trace of the dendritic nucleus remains in the rock record. Calcite crystal nucleation may take place by this mechanism in many marine and meteoric settings, given that microbial EPS is now assumed to be virtually ubiquitous in these environments. This phenomenon could contribute to the development of familiar fabrics such as marine micrite cement and fibrous calcite cement, radial ooids, peloids, ‘abiogenic’ stromatolites, sea floor precipitates, microbialites, tufa, travertine, speleothems, and some meteoric cements. It may also contribute to the substrate-normal orientation of c-axes of common cement fabrics.     

Abiotic versus biotic controls on the development of the Fairmont Hot Springs carbonate deposit, British Columbia, Canada

The relict Fairmont Hot Springs deposit, formed largely of carbonates, covers an area of 0·5 km², and is up to 16 m thick. The triangle‐shaped discharge apron, which broadens down‐valley, is divided into a proximal part with beds dipping at <10° and a distal part with beds dipping at 10° to 15°. The deposit is formed of the: (1) Basal Macrophyte; (2) Lower Carbonate; (3) Middle Clastic; (4) Upper Carbonate; and (5) Upper Clastic Sequences. Two charcoal samples embedded in the Lower Carbonate Sequence yielded dates of 8690 ± 90 and 8270 ± 70 cal yr bp , indicating that much of the deposit formed post‐glacially during the Early to Mid‐Holocene. Deposit aggradation ceased in the Mid to Late Holocene when the Fairmont Creek valley was incised. The Lower and Upper Carbonate Sequences, which are the thickest sequences, are composed of nearly equal parts of travertine (abiotic) and tufa (biotic), with feather dendrite travertine, radiating dendrite travertine and stromatolite tufa dominating. Competition between calcite precipitation rates and biotic growth rates controlled the distribution of tufa and travertine across the discharge apron. Calcite and biotic growth rates were controlled largely by flow velocity across the apron which, in turn, was controlled by topography and regular fluctuations in spring water discharge volume. During times of high spring discharge, slow sheet flow over the proximal part of the apron promoted stromatolite growth, whereas fast, turbulent flow on the distal part of the apron induced rapid feather dendrite formation. During times of low spring discharge, quiescent, shallow evaporative pools, conducive to radiating dendrite formation, formed on the proximal part of the apron, whereas slow flow on the distal part promoted stromatolite growth. Facies with high calcite supersaturation experienced rapid abiotic dendrite growth that precluded most biotic growth.     

Travertine formation in Plitvice National Park, Yugoslavia: chemical versus biological control

Hydrochemical studies of the Plitvice Lakes and their tributaries (Croatia/Yugoslavia) were coupled with micromorphological investigations on carbonate lake sediments and recent travertines. Karst springs discharge water from aquifers in Triassic and Jurassic dolomites and limestones and collect in lakes, which are ponded behind accreting travertine dams. Waters at springs have a high CO₂ partial-pressure (greater than 7000 ppm) and are slightly undersaturated with respect to calcite (saturation index less than —0·03). CO₂ partial pressure is quickly reduced in swift running streams, leading to very high supersaturation with carbonate minerals (saturation indices between 0·74 and 0·53). Calcite deposition, however, is restricted to the lake bottoms (formation of lake marl) and to the tufa dams. The annual carbonate precipitating capacity of the system based on water balance and downstream loss of dissolved ions is estimated to be on the order of 10 000 t CaCO₃ as cascade deposits (tufa dams) or as micrite in lakes behind the travertine dams. The initial stages of travertine formation as a result of morphological, biological, and chemical factors are (i) moss settling on small ridges in the creek courses, (ii) epiphytes (diatoms and cyanobacteria) settling on the moss surface, (iii) micrite particles resuspending from lake bottoms and being trapped on mucous excretions from bacteria and diatoms, and (iv) inorganic calcite precipitating as sparite at nucleation sites provided by these crystal seeds. Geochemical studies of the lake marl and tufa dams show that amino acids are dominated by aspartic acid. Carbohydrates come from structural polysaccharides of diatoms. The sticky excretions, rich in aspartic acid, are necessary for the initiation of calcite precipitation. They may be a response of algal and bacterial metabolism to environmental stress by either nutrient depletion or high calcium concentrations in ambient waters. The formation of tufa and micrite (lake marl) appears to be initiated by localized biological factors and is not governed by mere calcite supersaturation of the water. Oligotrophy may be an essential precondition for the formation of fresh water carbonate deposits.     

Diurnal Variations of Hydrochemistry in a Travertine-depositing Stream at Baishuitai, Yunnan, SW China

Diurnal variations of hydrochemistry were monitored at a spring and two pools in a travertine-depositing stream at Baishuitai, Yunnan, SW China. Water temperature, pH and specific conductivity were measured in intervals of 5 and 30 min for periods of 1 to 2 days. From these data the concentrations of Ca²⁺, HCO₃⁻, calcite saturation index, and CO₂ partial pressure were derived. The measurements in the spring of the stream did not show any diurnal variations in the chemical composition of the water. Diurnal variations, however, were observed in the water of the two travertine pools downstream. In one of them, a rise in temperature (thus more CO₂ degassing) during day time and consumption of CO₂ due to photosynthesis of submerged aquatic plants accelerated deposition of calcite, whereas in the other pool, where aquatic plants flourished and grew out of the water (so photosynthesis was taking place in the atmosphere), the authors suggest that temperature-dependent root respiration underwater took place, which dominated until noon. Consequently, due to the release of CO₂ by the root respiration into water, which dominated CO₂ production by degassing induced by temperature increase, the increased dissolution of calcite was observed. This is the first time anywhere at least in China that the effect of root respiration on diurnal hydrochemical variations has been observed. The finding has implications for sampling strategy within travertine-depositing streams and other similar environments with stagnant water bodies such as estuaries, lakes, reservoirs, pools and wetlands, where aquatic plants may flourish and grow out of water.     

Holocene pisoliths and encrustations associated with spring-fed surface pools, Pastos Grandes, Bolivia

Calcite pisoliths, with diameters ranging from 1 to 200 mm, are forming now on the surface of a playa (salar) in the Andean Altiplano (4500 m above MSL) of Bolivia. They are associated with active or recently active hot springs (20-75°C) which flow onto the playa surface. Encrustations of pieces of an older caliche-type crust, of pisoliths, of indurated mud and of older concretions are also found as well as series of small (1-3 cm high) sinter terraces (rimstone dams). Arborescent concretions and overgrowths are common and they are reminiscent of drip-stone textures. Water analyses demonstrate that calcite supersaturation (about twenty times) occurs mainly through CO₂ loss, with photosynthesis by algae and degassing the main removal mechanisms. The two available analyses indicate slight evaporation and a calcium loss between spring and pool of 2.3 mmol per litre of water. It is thought that the hot springs pick up much of their solute load from the playa sediments. The closest analogues to these deposits have been reported from caves (cave pearls and concretions). Although the depositional processes may be similar, the environment on an evaporitic playa surface is quite different. The geological implications for this newly observed pisolith environment may be considerable.     

'Sediment'–cement relationships in a Pleistocene speleothem from Italy: a possible analogue for 'replacement' cements and Archaeolithoporella in ancient reefs

Ancient carbonate buildups may contain extraordinarily large amounts of early diagenetic precipitates. In some, host rock lamination may be traced into inclusion bands within the 'cement' crystals, suggesting that the crystals are replacive. By analogy with a Pleistocene speleothem from the Sorrento Peninsula, however, these relationships can be explained differently. In the speleothem, large, repeatedly split and dendritic calcite crystals occur within a laminated carbonate. Lamination consists of sub-mm alternations of micrite and microspar. Micritic laminae pass laterally into inclusion-rich growth bands in the dendritic calcite crystals, and have replaced an aragonitic cement, whereas the microspar laminae were primary calcite cements. Three types of inclusion-rich bands occur in the dendrite crystals: (1) with aragonite relicts, (2) 'ribbon calcite' and (3) with oriented micropores. When aragonite precipitated, the calcite dendrite branches were unable to keep growing as single crystals and split into crystallites (separated by micropores, some forming ribbon calcite), whereas during episodes of calcite lamina precipitation, the larger crystals were regenerated by crystallite coalescence. Calcite crystals are primary: they did not replace a micritic precursor. By analogy with the Italian speleothem, some ancient reefal sparry carbonates may not be replacements of earlier laminated sediments, but may have grown concurrently with them. It is also probable that some ancient laminated sediments were instead sea-floor precipitates, and that stromatolites containing cross-cutting crystal fabrics, and the alternating micrite-microspar laminae typical of Archaeolithoporella , could be largely abiotic crystal growths.     

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.     

Geochemical characterization of surface water and spring water in SE Kashmir Valley,western Himalaya: Implications to water–rock interaction

Water samples from precipitation, glacier melt, snow melt, glacial lake, streams and karst springs were collected across SE of Kashmir Valley, to understand the hydrogeochemical processes governing the evolution of the water in a natural and non-industrial area of western Himalayas. The time series data on solute chemistry suggest that the hydrochemical processes controlling the chemistry of spring waters is more complex than the surface water. This is attributed to more time available for infiltrating water to interact with the diverse host lithology. Total dissolved solids (TDS), in general, increases with decrease in altitude. However, high TDS of some streams at higher altitudes and low TDS of some springs at lower altitudes indicated contribution of high TDS waters from glacial lakes and low TDS waters from streams, respectively. The results show that some karst springs are recharged by surface water; Achabalnag by the Bringi stream and Andernag and Martandnag by the Liddar stream. Calcite dissolution, dedolomitization and silicate weathering were found to be the main processes controlling the chemistry of the spring waters and calcite dissolution as the dominant process in controlling the chemistry of the surface waters. The spring waters were undersaturated with respect to calcite and dolomite in most of the seasons except in November, which is attributed to the replenishment of the CO₂ by recharging waters during most of the seasons.     

Mixed-water and hydrothermal dolomitization of the Pliocene Shirahama Limestone, Izu Peninsula, central Japan

The early Pliocene Shirahama Limestone is a grainstone-packstone principally composed of fragments of algae, bryozoa, and echinoderm and subordinate volcanic rocks. The limestone was variously dolomitized and the regional distribution of dolomite is patchy. Dolomite occurs as isolated crystals filling pores, moulds, and solution vugs, and mosaic aggregates replacing bioclasts. Calcite occurs as rim and pore-filling sparry cements, and as calcareous skeletons. Isotopically, the dolomites are classified into a heavy oxygen group (?2 to ? 3.5%₀ PDB) and a light oxygen group (?5.5 to ? 7.5%₀ PDB). Calcite associated with heavy oxygen dolomite has δ¹⁸O of ? 6.5 to ?8.5%₀ PDB, whereas those associated with light oxygen dolomite have a wide range from ?7.5 to ?14%₀ PDB. Calcite in dolomite-free limestone has an oxygen isotopic composition of ?2 to ?8.5%₀ PDB. Textures, chemistry, and isotopic evidence indicate that heavy oxygen calcite formed in freshwater, and heavy oxygen dolomite in a meteoric-marine mixture of 10–30% seawater. Light oxygen calcite and dolomite precipitated from modified hydrothermal fluids at approximately 30–65°C. Petrographic features, and both isotopic and chemical evidence suggest that the Shirahama Limestone was exposed to freshwater soon after deposition. Subsequently blocky calcite precipitated (Stage I). The limestone was locally submerged in the meteoric-marine mixture due to gradual subsidence or eustatic movement. This led to the precipitation of heavy oxygen, zoned dolomite and dolospar (Stage II). Hydrothermal alterations occurred in the area a few Myr ago, and related hydrothermal fluids and mixed meteoric-hydrothermal waters caused dedolomitization of some zoned dolomite, partial dissolution of vuggy dolomite, precipitation of limpid dolomite and recrystallization of some earlier dolomites (Stage III). Zeolites were also precipitated from these fluids. Finally, the Shirahama Limestone was exposed again to freshwater and sparry calcite precipitated to plug some of the remaining pores (Stage IV).     

基于水化学和同位素特征的四川黄龙沟泉群分类研究

本文采用水化学和同位素方法对四川黄龙沟沿途出露的7个泉点进行了分析.结果表明,泉水水化学和同位素的时空变化反映了CO2逸出、钙华沉积和蒸发效应等诸多因素的共同影响,是由这些泉水处在四个水循环转化段决定的.根据水化学和同位素特征可将这些泉水划分为三种不同的类型:深部泉、表生泉和转化泉.这些认识将为四川黄龙沟景观水资源的管...     

In-situ Growth of Calcite at Devils Hole, Nevada: Comparison of Field and Laboratory Rates to a 500,000 Year Record of Near-Equilibrium Calcite Growth

Calcite grew continuously for 500,000 years on the submerged walls of an open fault plane (Devils Hole) in southern Nevada, U.S.A. at rates of 0.3 to 1.3 mm/ka, but ceased growing approximately 60,000 years ago, even though the fault plane remained open and was continuously submerged. The maximum initial in-situ growth rate on pre-weighed crystals of Iceland spar placed in Devils Hole (calcite saturation index, SI, is 0.16 to 0.21 at 33.7 °C) for growth periods of 0.75 to 4.5 years was 0.22 mm/ka. Calcite growth on seed crystals slowed or ceased following initial contact with Devils Hole groundwater. Growth rates measured in synthetic Ca-HCO₃ solutions at 34 °C, CO₂ partial pressures of 0.101, 0.0156 (similar to Devils Hole groundwater) and 0.00102 atm, and SI values of 0.2 to 1.9 were nearly independent of P_{CO 2}, decreased with decreasing saturation state, and extrapolated through the historical Devils Hole rate. The results show that calcite growth rate is highly sensitive to saturation state near equilibrium. A calcite crystal retrieved from Devils Hole, and used without further treatment of its surface, grew in synthetic Devils Hole groundwater when the saturation index was raised nearly 10-fold that of Devils Hole water, but the rate was only 1/4 that of fresh laboratory crystals that had not contacted Devils Hole water. Apparently, inhibiting processes that halted calcite growth in Devils Hole 60,000 years ago continue today.     

Carbonate chemistry of groundwater from chalk,Givendale, East Yorkshire

Precipitation, soil and spring waters from an outlier of Chalk were analysed over a one year period for field pH, and contents of Ca⁺², Mg⁺², HCO₃^? and other dissolved solids. Measured soil log PCO₂ (atm) varied between a minimum of ?2.60 and maximum of ?1.46, and could be predicted from measurements of soil temperature. Soil waters evolved under open system conditions with respect to soil CO₂, and were undersaturated with calcite during the winter recharge period.The chemistry of the springs is related to their topographic position. Group 1 springs, located below a feather edge of chalk, had both their minimum and maximum PCO₂s predicted by the soil CO₂ data, suggesting open system CO₂ evolution. Group 2 springs, located along the scarp slope had minimum PCO₂s predicted by the soil data, but maximum PCO₂s which could only be explained by a closed system evolution from the maximum soil CO₂ observed. Group 1 springs were close to calcite saturation, whereas Group 2 springs were significantly undersaturated with calcite. The two groups could be identified by linear discriminant analysis of measured Ca²⁺, pH and HCO₃^? concentrations.