Identifying low-temperature hydrothermal karst and palaeowaters using stable isotopes: a case study from an alpine cave,Entrische Kirche,Austria

The area south of the prominent east–west trending Salzach Valley at the northern rim of the Central Alps of Austria has long been known to host anomalously warm springs emerging from a highly deformed calcite marble (Klammkalk). This unit also hosts cavities whose shapes suggest a hydrothermal karst origin and which are lined by calcite spar. We report here petrographic and isotopic evidence suggesting that dissolution by ascending low-temperature thermal waters also played an important role in the origin of a large cave in this region, Entrische Kirche. A paleo cave wall, preserved behind a thick flowstone in the interior of this cave, revealed a brownish bleaching zone which contrasts to the medium grey colour of the unaltered marble beneath. Across this zone the C and O isotope values gradually decrease by 3 and 11‰, respectively. These compositions are very different from those of the speleothem above but are similar to phreatic calcite spar from hydrothermal karst cavities in other outcrops in the area, where the absence of two-phase fluid inclusions suggests a low-temperature (less than ca. 50°C) hydrothermal origin. U/Th dating of the flowstone capping the alteration zone yielded a minimum age of the thermal water invasion in Entrische Kirche of ca. 240 kyr. There is no evidence in Entrische Kirche that these palaeowaters reached the point of calcite precipitation, but it is physically conceivable that higher and as yet unexplored parts of this deep (ca. 900 m) cave contain cavities lined by phreatic cave spar.     

Speleogenetic history of the Hungarian hydrothermal karst

The hydrothermal karst of Hungary displays at least two principal stages of development in two differing environments. Caves of an early stage were formed within a deep zone of low thermal gradient. These caves (vugs) are small (tens of centimeters) and lined with scalenohedral crystals of calcite that are often in association with barite. Calcite yields fluid inclusion temperatures of 55–95°C and is depleted in¹⁸O (–11.2 to –17.6 per mil PDB). The caves were formed by ascending thermal waters charged with CO₂. Solubility of CaCO₃ in such a system gradually increases with the ascent of the fluid (solutional zone) but drops sharply at a depth of –250 m to –500 m below the water surface (depositional zone). Caves formed in the solutional zone may be shifted into the depositional zone due to tectonic uplift, and calcite lines their walls. Large caves (tens to thousands of cubic meters) of a late stage were formed within a shallow zone of high thermal gradient immediately below and above the thermal water table. The calcite of the phreatic crusts has a rhombohedral habit, displays lower fluid inclusion temperatures (35–55°C and less), and a depletion in¹⁸O of –9.5 to –14.6 per mil PDB. Several powerful cave-forming processes may operate there including convection, mixing/cooling corrosion, and condensation corrosion. Due to differences in the rate of tectonic uplift, rate of hydrothermal system decay, and hydrogeologic pattern, these caves were either filled with water for a long period of time (phreatic calcite crusts are formed) or partly dewatered early in their history (waterline and subaerial speleothems are formed). The zones of thermal cave formation recognized in Hungary may have a universal character. Very similar features are found in other hydrothermal karst areas of the world (Kirghizia, Algeria, South Dakota).     

Evidence for a hypogene paleohydrogeological event at the prospective nuclear waste disposal site Yucca Mountain,Nevada, USA,revealed by the isotope composition of fluid-inclusion water

Secondary calcite residing in open cavities in the unsaturated zone of Yucca Mountain has long been interpreted as the result of downward infiltration of meteoric water through open fractures. In order to obtain information on the isotopic composition (δD and δ¹⁸O) of the mineral-forming water we studied fluid inclusions from this calcite. Water was extracted from inclusions by heated crushing and the δD values were measured using a continuous-flow isotope-ratio mass spectrometry method. The δ¹⁸O values were calculated from the δ¹⁸O values of the host calcite assuming isotopic equilibrium at the temperature of formation determined by fluid-inclusion microthermometry.The δD values measured in all samples range between ? 110 and ? 90‰, similar to Holocene meteoric water. Coupled δ¹⁸O–δD values plot significantly, 2 to 8‰, to the right of the meteoric water line. Among the various processes operating at the topographic surface and/or in the unsaturated zone only two processes, evaporation and water–rock exchange, could alter the isotope composition of percolating water. Our analysis indicates, however, that none of these processes could produce the observed large positive δ¹⁸O-shifts. The latter require isotopic interaction between mineral-forming fluid and host rock at elevated temperature (>100 °C), which is only possible in the deep-seated hydrothermal environment. The stable isotope data are difficult to reconcile with a meteoric origin of the water from which the secondary minerals at Yucca Mountain precipitated; instead they point to the deep-seated provenance of the mineral-forming waters and their introduction into the unsaturated zone from below, i.e. a hypogene origin.     

Speleothem record of the last 180 ka in Villars cave (SW France): Investigation of a large δ18O shift between MIS6 and MIS5

The Vil-car-1 flowstone core from Villars cave (SW France) provides one of the first European speleothem records extending back to 180 ka, based on U–Th TIMS and MC-ICP-MS measurements. The core offers a continuous record of Termination II and the Last Interglacial. The penultimate deglaciation is characterized by a prominent 5‰ depletion in calcite δ¹⁸O. Determining which specific environmental factors controlled such a large oxygen isotopic shift offers the opportunity to assess the impact of various factors influencing δ¹⁸O variations in speleothem calcite.Oxygen isotope analyses of fluid inclusions indicate that drip water δ¹⁸O remained within a very narrow range of ±1‰ from Late MIS6 to the MIS5 δ¹⁸O optimum. The possibility of such a stable behaviour is supported by simple calculations of various effects influencing seepage water δ¹⁸O.Although this could suggest that the isotopic shift in calcite is mainly driven by temperature increase, attempts to quantify the temperature shift from Late MIS6 to the MIS5 δ¹⁸O optimum by assuming an equilibrium relationship between calcite and fluid inclusion δ¹⁸O yield unreasonably high estimates of ～20 °C warming and Late MIS6 cave temperatures below 0 °C; this suggests that the flowstone calcite precipitated out of thermodynamic equilibrium at this site.Using a method proposed by Guo et al. (submitted for publication) combining clumped isotope measurements, fluid inclusion and modern calcite δ¹⁸O analyses, it is possible to quantitatively correct for isotopic disequilibrium and estimate absolute paleotemperatures. Although the precision of these absolute temperature reconstructions is limited by analytical uncertainties, the temperature rise between Late MIS6 and the MIS5 optimum can be robustly constrained between 13.2 ± 2.6 and 14.6 ± 2.6 °C (1σ), consistent with existing estimates from Western Europe pollen and sea-surface temperature records.     

Isotope compositions of C and O of magmatic calcites from the Udachnaya–East pipe kimberlite,Yakutia

It has been demonstrated for the first time that the isotopic compositions of carbon (δ¹³C) in magmatic calcites from the Udachnaya–East pipe kimberlite groundmass varies from–2.5 to–1.0‰ (V-PDB), while those of oxygen (δ¹⁸O) range from 15.0 to 18.2‰ (V-SMOW). The obtained results imply that during the terminal late magmatic and postmagmatic stages of the kimberlite pipe formation, the carbonates in the kimberlite groundmass became successively heavier isotopically, which indicates the hybrid nature of the carbonate component of the kimberlite: it was formed with contributions from mantle and sedimentary marine sources.

     

Comment on: “Origin, timing, and temperature of secondary calcite-silica mineral formation at Yucca Mountain, Nevada” by N. S. F. Wilson, J. S. Cline, and Y. V. Amelin

Yucca Mountain in southern Nevada is being evaluated as a potential site for the geological disposal of high-level nuclear waste. A reliable assessment of the future performance of the repository will require detailed paleohydrogeological information. Hydrogenic secondary minerals from the vadose zone of Yucca Mountain are being studied as paleohydrogeological indicators. A phenomenological model envisaging the deposition of secondary minerals by meteoric fluids infiltrating downward though the vadose zone was proposed in the reviewed paper. Our evaluation reveals that the model is not supported by empiric evidence reported in the paper.     

Overview of calcite/opal deposits at or near the proposed high-level nuclear waste site,Yucca Mountain,Nevada, USA: Pedogenic,hypogene, or both?

Calcite/opal deposits (COD) at Yucca Mountain were studied with respect to their regional and field geology, petrology and petrography, chemistry and isotopic geochemistry, and fluid inclusions. They were also compared with true pedogenic deposits (TPD), groundwater spring deposits (GSD), and calcite vein deposits (CVD) in the subsurface. Some of the data are equivocal and can support either a hypogene or pedogenic origin for these deposits. However, Sr-, C-, and O-isotope, fluid inclusion, and other data favor a hypogene interpretation. A hypothesis that may account for all currently available data is that the COD precipitated from warm, CO₂-rich water that episodically upwelled along faults during the Pleistocene, and which, upon reaching the surface, flowed downslope within existing alluvial, colluvial, eluvial, or soil deposits. Being formed near, or on, the topographic surface, the COD acquired characteristics of pedogenic deposits. This subject relates to the suitability of Yucca Mountain as a high-level nuclear waste site.     

Isotope-Hydrochemical Study of Natural Waters on the Western Border of the Ufa Plateau

Water Resources - Based on studies conducted in 2017, the features of formation and changes in the isotopic (δ18O and δ2H) and chemical composition of natural waters on the western border...     

Design of two crushing devices for release of the fluid inclusion volatiles

Two crushing cells have been described for the release of volatiles from fluid inclusions in minerals in vacuum, static gas, and gas-flow applications. To minimize the adsorption of released volatiles on the freshly created mineral surfaces, both devices employed heated crushing. In the MTSN (Museo Tridentine di Scienze Naturali) crusher, samples were disintegrated by a piston driven by an induction coil. For efficient crushing, the electromagnet operated in dynamic impulse mode. In the LFU (Leopold-Franzens-Universit?t) crusher, the sample was disintegrated through the combined action of compression (manually operated hydraulic ram) and attrition. Crushers are able to be used in off-line and on-line modes, in gas chromatographic and mass spectrometric analyses.