Crystalline Crusts Research Articles

Depositional and diagenetic processes in travertines: A comprehensive examination of Tocomar basin lithotypes, Northwest Argentina

The use of travertines in active fault studies is known as travitonics. Their geobody shape, veins, faults and fractures, are commonly used as neotectonic proxies. The surficial travertines in the pull apart Tocomar Basin (Puna Plateau, Argentina) occur in the center of the Calama – Olacapato – El Toro (COT) transcurrent fault zone and have never been substantially buried. The Tocomar area exhibits complex tectonic with N–S thrust faults and NW-SE transfer faults, crucial in shaping its travertine deposits and geothermal activity. The travertine bodies are characterized in terms of geometry as a fissure ridge type, coexisting with ash tuff, siliciclastic sandstone, and conglomerate deposits. The travertines mainly consist of crystalline crusts, with occasional occurrences of granular, arborescent, and porous textures. Mineralogical analyses by X-ray diffraction (XRD) reveal the predominance of magnesian calcite, along with trace minerals identified as dolomite, anhydrite, gypsum, halite, fluorite, bassanite, and barite. Petrographic analyses, including cathodoluminerscence (CL), indicate intense diagenesis in the travertine samples. Isotopic signatures of C and O in these samples exhibit normally a negative trend, ranging from −6.38 to +2.45 ‰ VPDB for δ13C and from −20.48 to −11.21 ‰ VPDB for δ18O. Laboratory analysis and field observations allow to classify three distinct lithotypes in the Tocomar travertine: (1) crystalline crust, further divided into three sub-lithotypes: (1.1) laminated crystalline crust (LCC), (1.2) filamentous-like crystalline crust (FlCC) and (1.3) fiber-like crystalline crust (FbCC); (2) granular travertine (GT) and (3) porous travertine (PT). These different lithotypes likely reflect variations in fluid velocity during deposition, although some of them may result from diagenetic processes. The three lithotypes give rise to four facies association (steep slope, marshy/pools, drainage, and fracture-associated facies), forming two distinct depositional systems (slope and channel), which comprises the Tocomar travertine system.

Multitechnique characterization of secondary minerals near HI-SEAS, Hawaii, as Martian subsurface analogues

Secondary minerals in lava tubes on Earth provide valuable insight into subsurface processes and the preservation of biosignatures on Mars. Inside lava tubes near the Hawaii-Space Exploration and Analog Simulation (HI-SEAS) habitat on the northeast flank of Mauna Loa, Hawaii, a variety of secondary deposits with distinct morphologies were observed consisting of mainly sodium sulphate powders, gypsum crystalline crusts, and small coralloid speleothems that comprise opal and calcite layers. These secondary deposits formed as a result of hydrological processes shortly after the formation and cooling of the lava tubes and are preserved over long periods of time in relatively dry conditions. The coralloid speleothem layers are likely related to wet and dry periods in which opal and calcite precipitates in cycles. Potential biosignatures seem to have been preserved in the form of porous stromatolite-like layers within the coralloid speleothems. Similar secondary deposits and lava tubes have been observed abundantly on the Martian surface suggesting similar formation mechanisms compared to this study. The origin of secondary minerals from tholeiitic basalts together with potential evidence for microbial processes make the lava tubes near HI-SEAS a relevant analog for Martian surface and subsurface environments.

Katian (Late Ordovician) sphinctozoan-bearing reefs: Hybrid carbonates before the glacial maximum

With a significant rise of metazoan reef-builders in shallow marine environments, the Late Ordovician saw a substantial increase in precipitation of crystalline crusts in reef systems, which may reflect a dramatic environmental change in the ocean. Isopachous crystalline crusts occur abundantly in boundstone of Katian sphinctozoan-bearing microbial reefs on the Zhe-Gan Platform, South China. These deposits are characterized by in-place carbonate textures that are interpreted as associations of abiotic, bioinduced and biocontrolled fabrics, considered here to be hybrid carbonates. Little previous work has been done to investigate how these hybrid carbonates are related to the transition in biotic composition during the Ordovician. These fabrics occur in well-preserved sphinctozoan-bearing reefs in the lower member of the Sanqushan Formation at the Jitoushan Section in Yushan County, Jiangxi Province. The meter-scale reefs are dominated by Corymbospongia, Amsassia and microbial components, and are cemented by fibrous sparry crusts. The intertwined Corymbospongia and Amsassia build the primary skeletal framework. Microbes and subsequent cementation enhance the stability of the framework by covering most of the macrofossil skeletons. The studied skeletal-microbial-cement reefs share a lot of similarities with contemporaneous reefs of North China and Central Nepal, but differ from Early Ordovician reefs, which lack sparry crusts. Our results provide evidence for extensive synsedimentary cementation on the seafloor, which could have played a crucial role in the consolidation of Late Ordovician reefs during the icehouse conditions before the Hirnantian glacial maximum.

Slovenská terminológia travertínov, penovcov a príbuzných terestrických vápencov

The paper follows on a review articles on Slovak travertines and tufa published in English in 2021. Geomorphological terms such as ‘fissure ridge’, ‘coalesced mound’, ‘self-buiding channel’, ‘keeled waterfall’, ‘smooth slope’, ‘terrased slope’, ‘dams along stream’, ‘peached springline deposits’, ‘moss pillow’ were defined as new in Slovak terminology. Travertine forms are formed by defined facies: ‘crystalline crusts’ with different crystal types, ‘radiating dendrites’, ‘coated bubbles’, ‘rafts’, and ‘breccias’ of various origin. Biogenic facies such as ‘microphyte mats’, ‘microphyte crusts’, ‘microphyte shrubs’, and ‘macrophyte facies’ appear in various tufas and travertines. In lakes, fens and marshes, the ‘lime-mudstones’ and the terms ‘mottling (marmorisation)’ and ‘pseudomikcokarst’ were defined. ‘Travertinized tufa’ was established as a transitional form between travertine and tufa.

Physico-chemical and biological controls in a travertine system in the high Andes of northwestern Argentina

Unraveling the processes involved in travertines and tufa formation, and the textural and geochemical signatures they preserve, are relevant given their potential as world-class oil reservoirs and analogues for ancient microbial carbonates. This study yields insights into the processes controlling hydrochemistry, isotopic fractionation and carbonate precipitation within a travertine system located in the southern border of the Puna plateau (Argentina), at an active high-altitude volcano-tectonic setting. Geochemical and isotopic composition of hot spring waters, together with its physico-chemical parameters, are typical of deep-cycled meteoric waters with most of the CO2 provided by a magmatic source. Downstream CO2 degassing rises the pH and shifts carbonate chemical equilibrium providing the bulk conditions for CaCO3 precipitation. Precipitates occur mainly as aragonite near hot springs and calcite throughout the rest of the system (~1 km extent) in close association with cyanobacteria, diatoms and exopolymeric substances (EPS). Constructive morphologies and crystalline crusts develop where strong CO2 degassing occurs, driven by contrasting CO2 partial pressures between hot spring waters and the atmosphere, and also associated to incremental downstream turbulence (e.g., dams and waterfalls). Biological influence is preponderant within downstream calm pools by providing an EPS substrate suitable for calcite nucleation, contributing to clotted micrite textures. Downstream trend towards heavier isotopic compositions of waters and related carbonates is given by the preferential degassing and evaporation of lighter isotopes (12C and 16O) and enrichment of heavy isotopes (13C and 18O) in the carbonate phase. Nonequilibrium isotopic fractionation seems to relate with kinetically-controlled processes and pH changes along the system. This study highlights that, while at the macroscale physico-chemical processes control bulk water and carbonate chemistry, providing favorable conditions for carbonate precipitation, at the microscale microbial activity has some control, particularly on the development of carbonate microtextures.

MICROFACIES OF LATE PLEISTOCENE TRAVERTINE DEPOSITS IN JORDAN

The Late Pleistocene travertine outcrops from Deir Alla, Suwayma, and Az Zara were investigated, and their microfacies were identified. The microfacies of the Deir Alla travertines include micrite and spar groundmass, shrubs, crystalline crusts, a stromatolite-like structure, peloids, and cements. Shrub travertine includes spar calcite-coated stems with probably microbial micritic clumps. The crystalline crust travertine displays an alternation of micrite and sparite laminae. The micritic laminae are dark-coloured. Bundles of radial spar crystals are associated locally with micritic groundmass. The crystalline crust developed where biogenic activity is limited. Peloidal microfacies are less than 0.25 mm in diameter, cryptocrystalline, pale-dark green in colour, elliptical to spherical in shape, and usually associated with microorganisms. The microfacies of the Suwayma and Az Zara travertines include crystalline calcite rhombs and other composite scalenohedral crystals. They occur as small anhedral-subhedral crystals, monocrystalline to some polycrystalline, corroded, subrounded, and mainly coated with iron oxide and/or clay minerals. Peloids, ooids, and oncoids are common. They are dark-green coloured, cryptocrystalline to microcrystalline carbonates of spherical and ellipsoidal shape with less than 1 mm in diameter. Rich flora travertines include reed and paper-thin rafts with leaf impressions encrusted on moss cushions. The flora observed in the upper part of the Suwayma section was identified as charophyte oospores (gyrogonites). A few grains of quartz are present as small subhedral-euhedral crystals, monocrystalline, corroded, rounded, and mainly coated with iron oxide. The iron is irregularly distributed among the laminae and voids and is occasionally replaced by carbonates. The described macrophyte encrustation structures probably represent algae, cyanobacteria, or bryophytes. All samples of micrite and spar calcite appear as groundmass.

Supergénna mineralizácia na II. horizonte Východnej šachty hnedouhoľnej bane Handlová (Slovenská republika)

At the II. horizon of the Eastern shaft of Handlová coal mine the occurrence of mineral assemblage of secondary minerals was identified. These accumulations consist of sulphate group minerals confirmed by PXRD and semiquantitative chemical analysis (EDS), namely gypsum, alunogen, pickeringite, melanterite, sideronatrite and epsomite. Sulphates form coatings, crystalline crusts and aggregates, which crystallized on the walls of mine corridors. The dimensions of these accumulations are reaching approximately 1.5 x 1.3 m with thickness up to 5 cm. The refined unit-cell parameters of alunogen (for the triclinic space group P-1) are: a 7.413(1), b 26.944(4), c 6.051(1) Å, α 90.05(1), β 97.65(1), γ 91.82(1)°, V 1197.1(3) Å3, pickeringite (for the monoclinic space group P21/c): a 6.1808(8), b 24.252(2), c 21.220(2) Å, β 100.36(1)°, V 3128.8(6) Å3 and melanterite (for the monoclinic space group P21/c): a 14.022(5), b 6.503(2), c 10.945(4) Å, β 105.83(3)°, V 960.2(7) Å3. Sideronatrite and epsomite were identified in mixture with the most significant diffraction maxima for sideronatrite: 10.252/100, 4.905/37, 3.424/10, 3.067/9 and epsomite: 5.347/18, 5.302/18, 4.489/26, 4.215/100, 4.201/59, 2.880/24, 2.747/23, 2.677/18. Accumulations of minerals were formed on the mine walls. Secondary sulphates are considered as products of the weathering of Fe-rich carbonates, pyrite and other sulphidic minerals which are part of associated pelosiderites embedded in the clays and claystones. Locally, gypsum and hexahydrite were also identified in the form of transparent crystals and white powdery aggregates on the calcite speleothems genetically linked to Mesozoic limestones and gypsum-rich Lower Triassic Werfen Formation. Unit-cell parameters refined from X-ray powder diffraction data are for hexahydrite (for the monoclinic space group C2/c): a 10.107(1), b 7.2114(8), c 24.424(3) Å, β 98.28(1)°, V 1761.7(3) Å3.

Dve nové lokality pyromorfitu v Slovenskej republike: Chvojnica a Hnilčík

Two new localities of pyromophite were recently discovered at the Chvojnica base metals occurrence in the Strážovské vrchy Mts. (Tatric Unit), and at the supergene zone of the Piata siderite-type hydrothermal vein near Hnilčík (Spišsko-gemerské rudohorie Mts., Gemeric Unit), Slovak Republic. At the Chvojnica occurrence pyromorphite occurs as bright green crystalline crusts and radial aggregates up to 3 mm developed in cavities and fractures in the porous quartz gangue with relicts of primary galena. It is associated with minor crystalline coatings of cerussite. The refined unit-cell parameters of pyromorphite from Chvojnica (for the hexagonal space group P63/m) are a 9.9750(2) Å, c 7.3569(2) Å and V 633.93(2) Å3. Except of dominant contents of Pb, P and Cl, only minor amounts of Ca (up to 0.10 apfu), Al (up to 0.05 apfu), Fe (up to 0.04 apfu), As (up to 0.05 apfu) and S (up to 0.02 apfu) were detected in studied pyromorphite. At the Hnilčík, pyromorphite occurs rarely in the cavities of the cavernous quartz-limonite gangue in the supergene zone of the Piata vein. It forms colourless to very light green prismatic crystals up to 2 mm in size, or crystalline aggregates associated with goethite, Mn oxides and malachite. The refined unit-cell parameters of pyromorphite from Hnilčík (for the hexagonal space group P63/m) are a 9.9484(15) Å, c 7.315(2) Å, V 627.0(2) Å3 (Ca-poor) and a 9.8983(6) Å, c 7.2589(8) Å, V 615.92(9) Å3 (Ca-rich). Pyromorphite from Hnilčík shows strong chemical zoning caused by variation of Pb and Ca contents, with outer parts of the crystals enriched in Ca. Contents of Ca are ranging between 0.09 to 0.96 apfu, with the atomic Pb/(Pb+Ca) ratio ranging from 0.98 to 0.81, which is close to the compositional boundary with phosphohedyphane. Interesting are also elevated concentrations of Cu reaching up to 0.07 apfu. Other minor elements detected in pyromorphite from Hnilčík are As (up to 0.12 apfu), Al, V and S (all up to 0.02 apfu).

Supergenní mineralizace hydrotermálního Ag-Pb-Zn ložiska Hříva u Louňovic pod Blaníkem (Česká republika)

At the Ag-Pb-Zn deposit Hříva near Louňovice pod Blaníkem (Central Bohemian Region, Czech Republic), supergene mineralization with relatively abundant samples of coronadite, pyromorphite and hinsdalite was newly detected. Coronadite forms black to several mm thick crusts on quartz vein or hinsdalite crusts. Coronadite is monoclinic, space group I2/m, the unit-cell parameters refined from X-ray powder diffraction data are: a 9.946, b 2.866(2), c 9.839 Å, β 90.34(2)° and V 280.4(4) Å3. Chemical analyses of coronadite correspond to the empirical formula (Pb1.32Na0.05Co0.03Ba0.02K0.01Ca0.01Mg0.01)Σ1.45Mn4+5.25(Mn3+1.95Al0.38Cu0.30 Fe0.08Si0.02Zn0.01P0.01)Σ2.74 (O15.82F0.17mCl0.01)Σ16 calculated on the basis (Mn+Fe+Al+Cu+Zn+Si+P) = 8 apfu. Pyromorphite occurs there as white to creamy crusts up to 2 mm thick. Its empirical formula calculated on the basis P+As = 3 apfu is Ca0.04Pb5.04[(PO4)2.98(AsO4)0.02]Σ3.00Cl1.14. Most interesting is relatively rare hinsdalite, which forms light green to blue-green crystalline crusts up to 0.1 mm thick on quartz or pyromorphite. Hinsdalite is trigonal, space group R-3m, the unit-cell parameters refined from X-ray powder diffraction data are: a 7.002(6), c 16.807(7) Å and V 713.6(7) Å3. Chemical analyses of hinsdalite correspond to the empirical formula Pb1.06Al2.73Cu0.11 [(PO4)1.18(SO4)0.82]Σ2.00(OH)6.36 on the basis P+S+As = 2 apfu.

Carbonate tufas as archives of climate and sedimentary dynamic in volcanic settings, examples from Gran Canaria (Spain)

AbstractThree Holocene tufas from Gran Canaria volcanic island were studied with the aim of deciphering their sedimentary evolution through space and time. Las Temisas tufa (south‐eastern arid part of the island) is dominantly composed of oncoids, intraclasts, phytoclasts, coated stems, minor thin stromatolites, and a high amount of siliciclastics. It was deposited in a fluvial system with variable flow velocities and palustrine conditions areas, which alternated with high energy events. Azuaje tufa (northern humid part of the island) is composed of coated stems, stromatolites, oncoids and phytoclasts, with relatively low amounts of siliciclastics, suggesting slow‐flowing and palustrine conditions and a relatively low incidence of (high energy) floodings. Los Berrazales tufa (north‐west of Gran Ganaria, the most humid one), is mainly composed of coated stems and crystalline crusts, formed in a laminar flow regime. Dominant clastic sedimentation in Las Temisas and high calcite growth rates in Los Berrazales led to a poor development of stromatolites in comparison with Azuaje. Las Temisas and Azuaje deposits have similar upward evolution with decreasing trend in siliciclastics and increasing trend in carbonates. However, Las Temisas has higher siliciclastic and lower phytoclastic contents suggesting a less vegetated area and more arid climate than in the other deposits. Additionally, tufas record local events common in volcanic terrains. Azuaje presents three units bounded by erosive discontinuities, which reveal significant erosion by enhanced runoff that could be caused by loss of vegetation due to wildfires related to volcanic eruptions at headwaters. Las Temisas record a possible interruption in sedimentation represented by aligned boulders due to rockfalls from the hillsides. These deposits formed from waters with similar chemistry providing to the carbonates their similar signals in δ13C–δ18O stable isotopes and 87Sr/86Sr ratios like that of the volcanic rocks. This work shows how, in volcanic areas, tufas are unique archives of the climate, vegetation and volcanic‐related processes, because all imprint the sedimentary regime of tufa deposition.

Mid–Late Ordovician tetradiid–calcimicrobial–cement reef: A new, peculiar reef-building consortium recording global cooling

Skeletal–microbial–cement reefs are a triple hybrid carbonate that mainly formed during the Pennsylvanian to Mid-Triassic, when a marked increase in microbial carbonate formation coincided with extensive precipitation of crystalline crusts on the seafloor. We report a new type of reef-building association from Middle–Upper Ordovician strata of western North China, in which erect thin tubes of tetradiids (coralomorph) are encrusted by the calcimicrobes Renalcis and Angusticellularia and then by a large amount of early marine cement that is presumably high-Mg calcite or aragonite in composition. The resulting meter-scale mound is embedded within intraclastic–bioclastic grainstone, implying high-energy shallow-marine conditions. The thin tetradiid tubes, which would have been unable to physically withstand strong waves and currents, are interpreted to have been consolidated by encrusting calcimicrobes and then by extensive early marine cementation. Tetradiid-bearing reefs have generally been reported from muddy successions; the results of the present study suggest that consolidators were important in reef-building in high-energy environments during the later Ordovician. Considering also the coeval bivalve–sponge–microbial–cement reef reported from the same area and a sponge–microbial–cement reef from Arctic Canada, early marine cementation appears to have been at least locally important in the late Ordovician, similar to the Pennsylvanian through the Mid-Triassic. These triple hybrid carbonates may have formed by a combination of: (1) emergence of newly evolved skeletal reef-builders during the Great Ordovician Biodiversification Event; (2) development of CO2-concentrating mechanisms in calcimicrobes induced by a decrease in atmospheric CO2; and (3) an increase in the calcium saturation state in seawater resulting in extensive abiotic cementation as well as calcification of microbes. All of these factors might have been induced by global cooling throughout the Mid–Late Ordovician.

Supergénne minerály z U-Cu rudného výskytu Východná-Nižný Chmelienec v Nízkych Tatrách (hronikum, Slovensko)

An association of supergene U-Cu and Y/REE minerals was found in a relic of old ore dump at the abandoned U deposit occurrence Východná-Nižný Chmelienec, the northern slopes of the Nízke Tatry Mts., Slovakia. They have partially recent origin, since exploration of the locality took place between 1965 and 1966. The studied mineral assem- blage is represented by goethite, malachite, uranophane and (meta)zeunerite, in a lesser extent baryte and rare zálesíite. Uranophane appears separately (globular aggregates, thin coatings) and it also forms the main part of the yellow to yellow-green crystalline crusts on the rock cracks. The chemical composition of the uranophane was determined by electron microprobe analyses and it is close to its ideal chemical formula Ca(UO2)2(SiO3OH)2·5H2O. The average chemical composition of the studied uranophane can be expressed by an empirical formula (Ca1.0Mg0.02K0.01Fe0.01Ba0.01)Σ1.05 (UO2)2.08(SiO3OH)1.84·5H2O. The infrared vibrational spectra of the studied uranophane show 3 (UO2)2+ at 850-760 cm-1; the 3 (SiO4)4- antisymmetric stretching vibration at 1000-900 cm-1; the 1 (SiO4)4- symmetric stretching vibration at 1150-1199 cm-1; the  H2O bending vibration at 1800-1600 cm-1 and OH stretching vibrations at 3407; 3408 and 3409 cm-1. The weak bands 2648; 2646 and 2651 cm-1 may be assigned to organic impurities. The calculated U-O bond length 1.83 Å corresponds to short U-O bonds in uranophane. The accessory admixtures of uranophane coatings are (meta)zeunerite and zálesíite. (Meta)zeunerite occasionally forms thin coatings of light green to emerald green tabular crystals (up tu 0.5 mm) on the surface of the rocks. Chemical analyses of (meta)zeunerite correspond to the empirical formula (Cu0.66K0.03Fe0.01Ca0.01)Σ0.71(UO2)2.11[(AsO4)1.96(PO4)0.01]Σ1.97·12H2O. Zálesíite occurs as crystalline aggregates, nests, formed by tiny acicular crystals, up to 100 µm in length. This is the second finding (occurrence) of this mineral in Slovakia. An average zálesíite chemical composition is (Ca0.83REE0.18U0.05Al0.03Ti0.01)Σ1.10(Cu5.81Fe0.06Zn0.02)Σ5.90[(AsO4)2.75 (SiO4)0.21(PO4)0.02(SO4)0.03]Σ3.01(OH)5.10·3H2O. Malachite, which has been also found in the association, is only a minor mineral in the studied locality. The formation of uranyl silicates (uranophane) and minerals of the mixite group (zálesíite), present at the studied locality, points to neutralization of acidic supergene fluids in the mine dumps. Possibly, this environment later (precipitation of baryte) passed to neutral or slightly basic conditions (precipitation of carbonates - malachite). The identified uranyl phosphates/arsenates (zeunerite/metazeunerite), typical of an acidic environment, are therefore rare.

Fluorapophyllite-(NH4), NH4Ca4(Si8O20)F⋅8H2O, a new member of the apophyllite group from the Vechec quarry, eastern Slovakia

AbstractThe new mineral fluorapophyllite-(NH4), ideally NH4Ca4(Si8O20)F⋅8H2O, was found at the Vechec andesite quarry located near Vechec village, Vranov nad Topľou Co., Prešov Region, Slovak Republic. It occurs in cavities of quartz–illite–saponite–tobelite xenolith embedded in pyroxene andesite. Fluorapophyllite-(NH4) is associated with calcite, tridymite, pyrite, chabazite-Ca and heulandite-Ca. It forms clusters, aggregates or crystalline crusts consisting of individual, well-developed crystals up to 4 mm in size, exhibiting the forms {110}, {101} and {001}. The mineral is colourless to light pink and translucent with white streak and vitreous to pearly lustre; it is non-fluorescent under ultraviolet radiation. The Mohs hardness is ~4½ to 5, tenacity is brittle, fracture is irregular, and there is perfect cleavage on {001}. The calculated density is 2.325 g cm–3. Fluorapophyllite-(NH4) is optically uniaxial (+) with ω = 1.5414(5) and ɛ = 1.5393(8) (λ = 589 nm). It is non-pleochroic. The empirical formula (based on 29 O + F apfu) is [(NH4)0.55K0.32Na0.07Ca0.06]Σ1.00(Ca4.01Mg0.02)Σ4.03Si7.97O20[F0.84(OH)0.16]Σ1.00⋅8H2O. Fluorapophyllite-(NH4) is tetragonal, space groupP4/mnc,a= 8.99336(9) Å,c= 15.7910(3) Å,V= 1277.18(3) Å3andZ= 2. The seven strongest X-ray powder diffraction lines are [dobsin Å,(I,%)(hkl)]: 7.897(32)(002), 7.812(13)(101), 4.547(14)(103), 3.946(100)(004), 2.985(39)(105), 2.4841(11)(215) and 1.5788(12)(00.10). The crystal structure of fluorapophyllite-(NH4), refined toR1= 0.0299 for 743 unique (I> 3σI) observed reflections, confirmed that the atomic arrangement is very similar to that of the other members of the apophyllite group. The new mineral is named according to the current nomenclature scheme for apophyllite-group minerals and is an NH4dominant analogue of fluorapophyllite-(K), fluorapophyllite-(Na) and fluorapophyllite-(Cs).

On the controls of mineral assemblages and textures in alkaline springs, Samail Ophiolite, Oman

Interactions between meteoric water and ultramafic rocks in the Oman Ophiolite generate waters of variable physicochemical characteristics. The discharge of these waters forms complex alkaline pool networks, in which mineral precipitation is triggered by mixing, evaporation, and uptake of atmospheric CO2. A systematic and co-localized sampling of waters and solids in two individual spring sites allowed us to determine the saturation state of a range of minerals and correlate them to the different water and precipitate types. We subdivided the waters of the spring sites into three distinctive types: i) Mg-type; moderately alkaline (7.9<pH<9.5), Mg2+–HCO3−-rich waters, ii) Ca-type; hyperalkaline (pH>11.6), Ca2+–OH−-rich waters, and iii) Mix-type; alkaline to hyperalkaline (9.6 < pH < 11.5) waters with intermediate chemical composition. We first report the occurrence of hydrated magnesium (hydroxy-) carbonate phases in Mg-type waters. Nesquehonite forms in these waters via evaporation and transforms into dypingite and hydromagnesite under CO2-rich conditions. In Ca-type waters, the coupling of atmospheric CO2 uptake with evaporation leads to the formation of a calcitic crystalline crust on the air-water interface. The crusts are aragonite- and brucite-bearing, where Mg-type and Ca-type waters discharge and vigorously mix at the same pool. Unlike the Mg-type and Ca-type waters, the pools of Mix-type waters host massive aragonite-dominated deposits due to high Mg/Ca ratio that favors the growth of aragonite over calcite. The hydrodynamics during mixing spatially control brucite precipitation and restrict its formation and accumulation around specific mixing zones, where a continuous supply of Mg of inflowing Mg-type waters takes place. Crystal morphologies record the effect on the values of supersaturation and supersaturation rates in the pools due to mixing processes, evaporation and CO2 uptake. In Ca-type waters, CO2 uptake and evaporation dictate the textural characteristics of calcite both in crystalline crusts and rock coatings. Textural evolution of aragonite from crystalline sheaves to spheroidal shapes underlines the different supersaturation rates of calcium carbonate crystallization in flocculent material of Mix-type waters. Geochemical models of mixing between Mg-type and Ca-type waters revealed the evolution of mineral saturation indices under various mixing proportions, and their relation to the observed mineralogy and geochemistry of the pool waters. The thorough documentation of mineral assemblages and crystal morphologies enabled us to provide a more detailed account of how water composition, mixing, and mineral precipitation co-evolve in the alkaline spring systems, where CO2 is sequestered.

Geochemistry and mineralogy of serpentinization-driven hyperalkaline springs in the Ronda peridotites

We present a detailed study of the water geochemistry, mineralogy and textures in serpentinization-related hyperalkaline springs in the Ronda peridotites. Ronda waters can be classified into hyperalkaline fluids and river waters that are broadly similar to Ca2+–OH- and Mg2+–HCO3- water types described in serpentinite-hosted alkaline springs elsewhere. At the discharge sites of the fluids (fractures or human made outlets) and ponds along the fluid flow paths, the fluids are hyperalkaline (10.9 < pH < 12) and characterized by low Mg and high Na, K, Ca, and Cl- concentrations. River waters, occurring near the spring sites, are mildly alkaline (8.5 < pH < 8.9) and enriched in Mg and DIC compared to Na, K, Ca and Cl-. The chemistry of Ronda Mg–HCO3 river waters is likely due to the hydrolysis of ferromagnesian peridotite minerals in equilibrium with the atmosphere by infiltrated meteoric water and shallow groundwater in the serpentinized peridotite. The Ronda Ca–OH hyperalkaline fluids are generated by the combination of low temperature serpentinization reactions from infiltrated surface Mg–HCO3 river waters —or Ca–HCO3 waters from near karst aquifers— and deep carbonate precipitation isolated from atmospheric CO2. Mass balance calculations indicate that the weathering of Ca-bearing peridotite silicates, such as diopside, is a feasible source of Ca in Ronda Ca–OH hyperalkaline fluids; however, it requires steady-state dissolution rates substantially greater than those determined experimentally.Travertine, crystalline crusts and sediment deposits comprise the types of solid precipitates observed in Ronda hyperalkaline spring sites. Calcite, aragonite, dolomite and Mg-Al-rich clays are the main mineral phases identified in the spring sites. As illustrated in the Baños del Puerto spring site, (i) calcite-dominated precipitation is due to uptake of atmospheric CO2 by Ca–OH hyperalkaline fluids during discharge, and (ii) aragonite-dominated precipitation is due to mixing of Ca–OH hyperalkaline fluids with Mg–HCO3 river waters. Aragonite and dolomite contents increase away from the springs and toward the river waters that uniquely reflects the effect of Mg ions on the precipitation of aragonite versus calcite. Other potential factors controlling the precipitation of these CaCO3 polymorphs are the Mg/Ca ratio, the CO2 content, and the temperature of the fluids. Dolomite forms during lithification of travertine due to periodic flooding of river water combined with subsequent evaporation.

Paleohydrogeologic reconstruction of Triassic carbonate paleolakes from stable isotopes: Encompassing two lacustrine models

Reconciling interpretations of geochemical, sedimentologic, and stratigraphic observations is a common challenge with many datasets, especially those from paleolake systems. The Triassic Cerro de las Cabras and Cerro Puntudo formations, part of the sedimentary fill of the Cuyana rift Basin in central-west Argentina, represent carbonate-rich playa-lake systems with abundant microbialites, but different sedimentologic and stratigraphic attributes such as presence or absence of efflorescent crystalline crusts and aggrading versus prograding-aggrading stacking patterns. A detailed study of the stable-carbon and oxygen-isotope composition of the limestones deposited in these paleolakes, together with sedimentological and mineralogical evidence as petrography, microprobe, XRD and cathodoluminescence data, enabled interpretation of their complex hydrology.Lack of correlation between carbon and oxygen stable-isotope values for both Cerro de las Cabras (δ13C χ–6.03‰; σ 0.67‰/δ18O χ −12.64‰; σ 0.55‰) and Cerro Puntudo (δ13C χ −3.10‰; σ 1.15‰/δ18O χ −12.58‰; σ 1.8‰) paleolakes strongly indicates open hydrology (underground flow), in apparent contradiction with sedimentologic and stratigraphic evidence for intermittent to persistently closed hydrography (overland flow). Stable-isotopic values for the Cerro de las Cabras paleolake together with sedimentology data indicate a recharge (hydrologically open) lake, and for the Cerro Puntudo a through-flow (hydrologically open) playa-lake. To resolve the apparent contradictions, we propose an integration of playa-lake hydrology (groundwater) with lake-basin type sedimentology, stratigraphy, and hydrography. This allowed identifying and proposing a balanced-fill–through-flow lake subtype for the Cerro Puntudo system and an underfilled–recharge-lake subtype for the Cerro de las Cabras paleolake; both with hydrology open to groundwater flow. This work suggests a starting point for expanding the lake-basin-type model to explicitly incorporate different groundwater and overland flow pathways.

Coexisting active travertines and tufas in the southeastern border of the Puna plateau

The Terma Los Hornos is an active hydrothermal system located in the southeastern border of the Andean Puna plateau in Catamarca province, northwest Argentina. It exhibits a complete range from high- to low-temperature Quaternary carbonate products still actively forming. Travertines and tufas can be considered lithological end members within the depositional system, but the lack of clear boundaries has generated confusion when trying to distinguish them in the fossil record. In the Terma Los Hornos a continuum between both extremes is recorded, through a transition of different morphologies and dominant but non-diagnostic fabrics, among which we recognize a variety of crystalline crusts, shrubs, spheroidal particles, encrusted bubbles and reeds, lithoclastic, grumulous micritic and silty-sandy carbonates. These were grouped in the following depositional subsystems: 1) vents and pinnacles, 2) mound-apron, 3) terraced-slope, 4) steep slopes and 5) flooding areas and palustrine. Hydrochemistry results show substantial variabilities ranging from 65 °C, pH = 6.41, 1029 ppm alkalinity and 37 ppm calcium concentrations at the vents site to 21 °C, pH = 8.13 and 800 ppm alkalinity and 14 ppm calcium concentrations at ~1 km downstream, were carbonate precipitation is no longer visible. The set of linked subenvironments tend to merge into a larger scale geobody that defines the travertine system, with a proximal area dominated by physicochemical processes and a distal extent with pervasive microbial and common macrophyte features. The travertine system should not be understood as a fixed model, and its limits as well as the existing subenvironments, their extents and degree of overlapping, will depend mainly on three factors: 1) hydrothermal input, 2) climate, and 3) landscape. Perched past travertine deposits together with internal architectures and extent of the presently active travertine system allows thinking about constructive and destructive stages, respectively thought as dominantly expanding and erosional periods. Alternation of these two evolutionary stages and their balance will dictate the volume of primary facies and potential reworking products. A better understanding of these stages and their litho-stratigraphic products, will allow developing predictive stratigraphic and correlation models based on a sequence stratigraphic approach.

Sloping fan travertine, Belen, New Mexico, USA

Pliocene to Quaternary age travertines are very well-exposed in quarries near Belen, New Mexico, U.S.A., on the western edge of the Rio Grande Rift system. A series of hillside springs produced travertine tongues tens of meters thick and hundreds of meters long. The accumulations represent deposits from individual springs as well as the amalgamation of deposits. The overall architecture is predominantly composed of sloping fans with a smaller component of terrace mounds. The sloping fan deposits commonly have a dip of <10°, however, they range from horizontal to near vertical. Individual strata display significant changes in depositional dip, beds pinch and swell, and some are completely truncated. Centimeter to meter scale terrace mounds exhibit the common stair-step morphology. As a consequence of vertical accretion in the pools, terrace mounds morphed into sloping fans. The travertine is composed of a variety of commonly reported constituents, i.e., centimeter thick laminae of bacterial shrubs and oncoids, foam rock, sheets and rafts, and finely crystalline crusts that occur throughout the sloping fan and terrace mound accumulations. Sheets and rafts formed as precipitates in pools on the surfaces of the fans and terraces as well as spelean deposits on the water surfaces of pools within cavities in the overall accumulation. Thus, the spelean rafts provide valuable indicators of original horizontality in the sloping fan strata. In addition, intraformational breccias, composed of locally torn-up travertine intraclastic boulders and deposited in with other travertine, and extraformational breccias, composed of torn-up travertine intraclasts mixed with siliciclastic fines and sand and Paleozoic limestone clasts transported downslope from higher on the hillside, are a common constituent in the sloping fan accumulation. The Belen travertines provide a very well-exposed example of sloping fan travertines and may provide relevant data with regard to the subsurface Aptian Pre-Salt deposits, offshore Brazil.

Travertine: Distinctive depositional fabrics of carbonates from thermal spring systems

AbstractThe terrestrial limestones forming at the emergence of thermal springs show a variety of unusual depositional facies. The specific lithological and petrological features of these deposits have few counterparts in the marine, and continental, karst‐related carbonates, but they are typical of the epigean limestone that has been quarried since antiquity in the surroundings of Tivoli (Rome) under the name of travertine, where it is still forming in hydro‐geothermal fields linked to extensional tectonics. The physicochemical, hydrodynamic and geological conditions specific to the Tivoli thermal spring system imply hypersaturated alkaline–sulphate, warm to hot waters, upwelling from springs fed through open fractures/faults in extensional and/or volcanic regimes. These features, together with the hydrodynamic behaviour of the water flows running from the vents, control the petrogenetic features of the travertine, a well‐bedded, mostly finely laminated, porous but quite compact limestone. The results of a detailed comparative petrological analysis carried out on the lithofacies of travertine limestones, and of those observed during formation within numerous active thermal spring fields, provide the elements required for an exhaustive textural classification of the travertine lithofacies, which has not yet been described systematically. According to the genetic processes and fabrics, the thermal deposits that originate from such hypersaturated alkaline–sulphate, warm to hot waters, can be subdivided into: abiotic crystalline crusts, microbially mediated crusts (microbialites) and granular deposits mostly represented by small accumulations of lime‐mudstone. Some of the granular deposits and the microbialites are only partially comparable with analogous sediments forming on tidal flats/sabkhas or other continental sites; however, the facies association of crystalline crusts and laminar curled microbialites has no counterpart in the marine realm. The widespread presence of thermophile bacteria and sulphobacteria, and the general absence of autochthonous eukaryote organisms, unable to live in poisonous sulphate waters, are also undeniable evidence of their thermal origin.

A multidisciplinary approach to understanding carbonate deposition under tectonically controlled hydrothermal circulation: A case study from a recent travertine mound in the Euganean hydrothermal system, northern Italy

AbstractA multidisciplinary study was carried out on an Upper Pleistocene travertine mound (Montirone Hill; Abano Terme, south of Padova, Veneto, Italy) with the aim of developing a comprehensive understanding of the processes interacting in structurally controlled thermal springs and the consequent deposition of thermal limestones. The sedimentology, geochemistry, structural characteristics and hydrogeology of the mound, which is associated with the Euganean geothermal field and the related complex structural framework of the Schio‐Vicenza fault system, were investigated. A palaeo‐environmental model of the deposit has been reconstructed clarifying the connection between the travertine deposition and the regional structural setting. Calcium‐rich thermal waters rose from spring orifices (estimated temperature from 54°C to 61°C based on geochemical calculations) and produced a mound made of coalescent shield‐like bodies. The dominant lime‐mudstone facies resulted from the accumulation of lime mud at the bottom of shallow crater‐like basins located on top of stacked cones. Soft sediment deformations affecting the lime mud sediment were most probably produced by the recurrent uprising of gas bubbles. The overlying crystalline crusts were deposited on the flanks of the mound by waters overflowing from the vents with laminar flux. A deep hydrothermal circuit with a long period of water–rock interaction is supported by geochemical analyses. Furthermore, a local extensional regime, enhancing the migration of thermal fluids to the surface, is inferred based on the results of the structural analysis carried out on the fault/fracture mesh that affects the mound. These results corroborate the well‐established relation among travertine deposits, hydrothermal systems and fault systems, and substantiate the structurally controlled conceptual model of the Euganean hydrothermal system, suggesting that both the thermal and the fault systems were active at least since 34 ka.