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 geochemistry of Swan Lake Basin in the Nebraska Sandhills,USA: Large 13C enrichment in sediment-calcite records

This study presents isotope geochemical analyses conducted on water column samples and core sediments collected from the Swan Lake Basin. Water analyses include the dissolved methane (CH₄) content and the ratio of carbon-13 to carbon-12 (δ¹³C) in dissolved inorganic carbon (DIC). The core sediments – sandy muds containing inorganic calcite, organic matter, and opal phases ± ostracods – were examined by X-ray diffraction, dated by radiocarbon (¹⁴C), analyzed for wt% organic carbon, wt% organic nitrogen, wt% organic matter, wt% calcite, δ¹³C of bulk-sediment insoluble organic matter (kerogen), ¹⁸O:¹⁶O ratio (δ¹⁸O) and δ¹³C of bulk and ostracod calcite. Of particular significance is the large enrichment in carbon-13 (δ¹³C = +4.5 to +20.4‰ V-PDB) in the calcite of these sediments. The ¹³C-enriched calcite is primarily formed from DIC in the water column of the lake as a result of the following combined processes: (i) the incorporation of ¹³C enriched residual carbon dioxide (CO₂) after partial reduction to CH₄ in the sediments and its migration into the water column-DIC pool; (ii) the preferential assimilation of ¹²C by phytoplankton during photosynthesis; (iii) the removal of ¹³C-depleted CH₄ by ebullition and of organic matter by sedimentation and burial. The ¹³C enrichment was low between 3624 and 2470 yr BP; high between 2470 and 1299 yr BP; and moderate since 1299 yr BP. Low ¹³C enrichment was formed under low water-column carbon levels while higher ones were formed under elevated rates of biomass and calcite deposition. These associations seem to imply that biological productivity is the main reason for carbon-13 enrichments.     

Origin of CO2 and carbonate veins in mantle-derived xenoliths in the Pannonian Basin

The origin and evolution of CO₂ inclusions and calcite veins in peridotite xenoliths of the Pannonian Basin, Hungary, were investigated by means of petrographic investigation and stable isotope analyses. The fluid inclusions recovered in paragenetic olivine and clinopyroxene belong to distinct populations: type A (texturally early) inclusions with regular shapes (often with negative crystal forms) forming intragranular trails; type B (texturally late) inclusions defining randomly oriented trails that reach grain boundaries. Type B inclusions are often associated with silicate melt (type C) inclusions. Stable carbon isotope compositions in inclusion-hosted CO₂ were obtained by vacuum crushing followed by conventional dual inlet as well as continuous flow mass spectrometry in order to eliminate possible lab artifacts. Olivines, clino- and orthopyroxenes of the host peridotite have oxygen isotope compositions from 5.3 to 6.0‰ (relative to V-SMOW), without any relationship with xenolith texture. Some of the xenoliths contained calcite in various forms: veins and infillings in silicate globules in veins, secondary carbonate veins filling cracks and metasomatic veins with diffuse margins. The former two carbonate types have δ¹³C values around –13‰ (relative to V-PDB) and low Sr contents (< 0.5 wt.%), whereas the third type,veins with high-temperature metasomatic features have a δ¹³C value of –5.0‰ and high Sr contents up to 3.4 wt.%. In spite of the mantle-like δ¹³C value and the unusually high Sr content typical for mantle-derived carbonate, trace element compositions have proven a crustal origin. This observation supports the conclusions of earlier studies that the carbonate melt droplets found on peridotite xenoliths in the alkaline basalts represent mobilized sedimentary carbonate. The large δ¹³C range and the ¹²C-enrichment in the carbonates can be attributed to devolatilization of the migrating carbonate or infiltration of surficial fluids containing ¹²C-rich dissolved carbon.Carbon isotope compositions of inclusion-hosted CO₂ range from –17.8 to –4.8‰ (relative to V-PDB) with no relation to the amount of CO₂ released by vacuum crushing. Low-δ¹³C values measured by stepwise heating under vacuum suggest that the carbon component is pristine and not related to surficial contamination, and that primary mantle fluids with δ¹³C values around –5‰ were at least partly preserved in the xenoliths. Tectonic reworking and heating by the basaltic magma resulted in partial CO₂ release and local ¹³C-depletion.     

Hydro-geochemical and isotopic fluid evolution of the Los Azufres geothermal field,Central Mexico

Hydrothermal alteration at Los Azufres geothermal field is mostly propylitic with a progressive dehydration with depth and temperature increase. Argillic and advanced argillic zones overlie the propylitic zone owing to the activity of gases in the system. The deepest fluid inclusions (proto-fluid) are liquid-rich with low salinity, with NaCl dominant fluid type and ice melting temperatures (T_mi) near zero (0 °C), and salinities of 0.8 wt% NaCl equivalent. The homogenization temperature (T_h)  = 325 ± 5 °C. The boiling zone shows T_h = ±300 °C and apparent salinities between 1 and 4.9 wt% NaCl equivalent, implying a vaporization process and a very important participation of non-condensable gases (NCGs), mostly CO₂. Positive clathrate melting temperatures (fusion) with T_h = 150 °C are observed in the upper part of the geothermal reservoir (from 0 to 700 m depth). These could well be the evidence of a high gas concentration. The current water produced at the geothermal wells is NaCl rich (geothermal brine) and is fully equilibrated with the host rock at temperatures between T = 300 and 340 °C. The hot spring waters are acid-sulfate, indicating that they are derived from meteoric water heated by geothermal steam. The NCGs related to the steam dominant zone are composed mostly of CO₂ (80–98% of all the gases). The gases represent between 2 and 9 wt% of the total mass of the fluid of the reservoir.The authors interpret the evolution of this system as deep liquid water boiling when ascending through fractures connected to the surface. Boiling is caused by a drop of pressure, which favors an increase in the steam phase within the brine ascending towards the surface. During this ascent, the fluid becomes steam-dominant in the shallowest zone, and mixes with meteoric water in perched aquifers. Stable isotope compositions (δ¹⁸O–δD) of the geothermal brine indicate mixing between meteoric water and a minor magmatic component. The enrichment in δ¹⁸O is due to the rock–water interaction at relatively high temperatures. δ¹³C stable isotope data show a magmatic source with a minor meteoric contribution for CO₂. The initial isotopic value δ³⁴S_RES = −2.3‰, which implies a magmatic source. More negative values are observed for shallow pyrite and range from δ³⁴S (FeS₂) = −4‰ to −4.9‰, indicating boiling. The same fractionation tendencies are observed for fluids in the reservoir from results for δ¹⁸O.     

Changing moisture sources over the last 330,000 years in Northern Oman from fluid-inclusion evidence in speleothems

Speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods over the last 330,000 yr. Periods of rapid speleothem deposition occurred from 6000 to 10,500, 78,000 to 82,000, 120,000 to 135,000, 180,000 to 200,000, and 300,000 to 330,000 yr ago, with little or no growth during the intervening periods. During each of these five pluvial periods, δD values of water extracted from speleothem fluid inclusions (δD_FI) are between −60 and −20‰ (VSMOW) and δ¹⁸O values of speleothem calcite (δ¹⁸O_C) are between −12 and −4‰ to (VPDB). These values are much more negative than modern rainfall (for δD) or modern stalagmites (for δ¹⁸O). Previous work on the isotopic composition of rainfall in Oman has shown that northern and southern moisture sources are isotopically distinct. Combined measurements of the δD values of fluid-inclusion water with calculated δ¹⁸O values from peak interglacial speleothems indicate that groundwater was predominantly recharged by the southern (Indian Ocean) moisture source, when the monsoon rainfall belt moved northward and reached Northern Oman during each of these periods.     

Calcite fracture fillings as indicators of paleohydrology at Laxemar at the Äspö Hard Rock Laboratory,southern Sweden

Isotopic compositions of C (δ¹³C), O (δ¹⁸O) and Sr (δ⁸⁷Sr) were determined for calcite fracture fillings in the crystalline rock penetrated by a 1.6 km drill hole at Laxemar, near the Äspö Hard Rock Laboratory (ÄHRL) in southern Sweden. These calcites precipitated from groundwater some time in the past, and their δ¹³C, δ¹⁸O and δ⁸⁷Sr values reflect those of the source waters. The present-day groundwater system is hydrochemically stratified with highly saline water underlying more shallow brackish and fresh water. The origin of this stratified system is probably related to past glaciations although the ultimate origin of the deep, highly saline water is still problematical. None of the calcite fracture fillings sampled below 900 m could have precipitated from any of the present-day ground waters which in view of the glacial history of the region is not surprising. However, several shallow calcite fracture fillings are formed by precipitation from the present-day groundwater. Coupled variations in δ¹³C, δ¹⁸O and δ⁸⁷Sr isotopes at depths in excess of 900 m suggest that these isotope systems in calcite are recording a time-dependent evolution of groundwater composition.     

Climatic and environmental controls on speleothem oxygen-isotope values

Variations in speleothem oxygen-isotope values (δ¹⁸O) result from a complicated interplay of environmental controls and processes in the ocean, atmosphere, soil zone, epikarst, and cave system. As such, the controls on speleothem δ¹⁸O values are extremely complex. An understanding of the processes that control equilibrium and kinetic fractionation of oxygen isotopes in water and carbonate species is essential for the proper interpretation of speleothem δ¹⁸O as paleoclimate and paleoenvironmental proxies, and is best complemented by study of site-specific cave processes such as infiltration, flow routing, drip seasonality and saturation state, and cave microclimate, among others. This review is a process-based summary of the multiple controls on δ¹⁸O in the atmosphere, soil, epikarst, and speleothem calcite, illustrated with case studies. Primary controls of δ¹⁸O in the atmosphere include temperature and relative humidity through their role in the multiple isotope “effects”. Variability and modifications of water δ¹⁸O values in the soil and epikarst zones are dominated by evaporation, mixing, and infiltration of source waters. The isotopically effective recharge into a cave system consists of those waters that participate in precipitation of CaCO₃, resulting in calcite deposition rates which may be biased to time periods with optimal dripwater saturation state. Recent modeling, experimental, and observational data yield insight into the significance of kinetic fractionation between dissolved carbonate phases and solid CaCO₃, and have implications for the ‘Hendy’ test. To assist interpretation of speleothem δ¹⁸O time series, quantitative and semi-quantitative δ¹⁸O-climate calibrations are discussed with an emphasis on some of the difficulties inherent in using modern spatial and temporal isotope gradients to interpret speleothems as paleoclimate proxy records. Finally, several case studies of globally significant speleothem paleoclimate records are discussed that show the utility of δ¹⁸O to reconstruct past climate changes in regions that have been typically poorly represented in paleoclimate records, such as tropical and subtropical terrestrial locations. The new approach to speleothem paleoclimatology emphasizes climate teleconnections between regions and attribution of forcing mechanisms. Such investigations allow paleoclimatologists to infer regional to global-scale climate dynamics.     

The lateral boundary of a metamorphic core complex: The Moutsounas shear zone on Naxos,Cyclades, Greece

We describe the structure, microstructures, texture and paleopiezometry of quartz-rich phyllites and marbles along N-trending Moutsounas shear zone at the eastern margin of the Naxos metamorphic core complex (MCC). Fabrics consistently indicate a top-to-the-NNE non-coaxial shear and formed during the main stage of updoming and exhumation between ca. 14 and 11 Ma of the Naxos MCC. The main stage of exhumation postdates the deposition of overlying Miocene sedimentary successions and predates the overlying Upper Miocene/Pliocene conglomerates. Detailed microstructural and textural analysis reveals that the movement along the Moutsounas shear zone is associated with a retrograde greenschist to subgreenschist facies overprint of the early higher-temperature rocks. Paleopiezometry on recrystallized quartz and calcite yields differential stresses of 20–77 MPa and a strain rate of 10⁻¹⁵–10⁻¹³ s⁻¹ at 350 °C for quartz and ca. 300 °C for calcite. Chlorite geothermometry of the shear zone yields two temperature regimes, 300–360 °C, and 200–250 °C. The lower temperature group is interpreted to result from late-stage hydrothermal overprint.     

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.     

Effect of temperature on the dissolution and thermal alteration of combined amino acids fixed in natural sediment under simulated hydrothermal conditions

Seafloor sediment containing biogenic amino acids was heated with NaCl solutions at 50–200 °C for 240 h to investigate the dissolution process of amino acids and evaluate their stabilities under hydrothermal conditions. Dissolved amino acids in the combined phase (dissolved combined amino acids, DCAAs) and free phase (dissolved free amino acids, DFAAs) were rapidly released into the solution during heating. The amount of DCAAs in the solutions was 4–9 times higher than the amount of DFAAs at each temperature. When heated at ⩽ 100 °C, most of the total dissolved hydrolyzable amino acids (TDHAAs) were in the combined form (DCAAs/TDHAAs ratios > 0.9). The compositions of the DCAAs in solutions heated at ⩽ 100 °C were similar to that of the total hydrolyzable amino acids (THAAs) of the initial sediment, indicating that the DCAAs, which are derived from organisms and biodebris in the sediment, are barely altered during the hydrothermal reaction at these temperatures. On the other hand, the DCAAs/TDHAAs ratios were 0.72 and 0.57 at 150 and 200 °C, respectively, and the compositions of the DCAAs at 150 and 200 °C were significantly different from that of the initial THAAs. In addition, non-protein amino acids (β-alanine and γ-aminobutyric acid), which are sensitive biochemical indicators of the diagenetic alteration of natural organic matter, drastically increased to 80.9% of the DCAAs after heating at 200 °C. These results suggests that DCAAs are thermally unstable in the hydrothermal solutions at ⩾ 150 °C. These DCAA would be transformed into thermally stable geo-polymers such as humic-like substances and hydrolyzable kerogens.     

An absolute paleotemperature record from 10 to 6 Ka inferred from fluid inclusion D/H ratios of a stalagmite from Vancouver Island, British Columbia, Canada

By using continuous helium flow during the crushing of calcite speleothem samples, we are able to recover liberated inclusion waters without isotopic fractionation. A paleotemperature record for the Jacklah Jill Cave locality, Vancouver Island, BC, was obtained from a 30-cm tall stalagmite that grew 10.3-6.3 Ka ago, using δ¹⁸O values of the crushed calcite and of the inclusion water as inferred from its δD. It is found that the locality experienced mean annual temperature variations up to 11 °C over a 4-Ka period in the early Holocene. At the beginning of the period, local temperature quickly increased from a minimum of ∼1 °C to around 10 °C, but this early climate optimum, about 3 °C warmer than today, only lasted for ∼1200 years. About 8.6 Ka ago, temperature had declined to ∼7 °C, approximately the same as the modern cave temperature. Since then, the study area has experienced only minor temperature fluctuations, but there was a brief fall to ∼4 °C at around 7 Ka ago, which might be caused by a short lived expansion of local alpine glaciers. The long-term T-dependence of δD was 1.47‰/°C, identical to the value in modern precipitation.     

Intermediate sulfidation epithermal gold-base metal deposits in Tertiary subaerial volcanic rocks,Sahinli/Tespih Dere (Lapseki/Western Turkey)

Gold in the Sahinli and Tespih Dere intermediate sulfidation gold-base metal deposits in Western Turkey occurs in relatively deep epithermal quartz veins along with base metal minerals which have epithermal textures, including plumose quartz, vug infills, comb and cockade textures and matrix-supported milled breccias. The total sulfide content of the veins in the area is variable ranging from < 1% to 60% and is dominated by pyrite, galena, sphalerite and chalcopyrite. Sphalerite is Fe-poor (0.6 to 1.4 mol% FeS). Minor amounts of Ag-rich tetrahedrite are present. Primary hydrothermal alteration minerals include illite/muscovite, mixed-layer illite/smectite (11.6 Å) and clinochlore towards the east and, alunite, dickite/nacrite and pyrophyllite towards the west at Sahinli; major illite/muscovite and dickite occur at Tespih Dere and Sarioluk, respectively.Fluid inclusions in main-stage quartz at Sahinli are only liquid-rich, with homogenization temperatures ranging from 220 to 322 °C and the majority of T_h values between 250 and 300 °C. Salinity ranges from 4.3 to 6.9 wt.% NaCl equiv. First ice-melting temperatures (T_mf) between ?24.5 and ?19.0 °C indicate that the fluids were dominated by NaCl – H₂O during mineralization. The relatively higher average T_h at the Tespih Dere deposit (295 °C) is attributed to a relatively deeper level of exposure.Calculated δ¹⁸O values indicate that ore-forming hydrothermal fluids in the study area had δ¹⁸O_H2O ranging from + 1.1 to + 9.7‰ (average = 3.8‰), strongly ¹⁸O-enriched compared with present-day hydrothermal meteoric water in the area (δ¹⁸O = ?8.5‰). δD values of fluid inclusions in quartz range from ?58 to ?93‰ and δD values of clay minerals and alunite from ?40 to ?119‰. δD values from intermediate argillic alteration (average = ?68‰) in the study area are very similar to δD values of the present-day local geothermal system (average δD = ?54‰) whereas δD values from advanced-argillic alteration (average δD = ?33‰) are very different from the present-day local geothermal system.The δ³⁴S values in samples from the Sahinli and Tespih Dere deposits average ?2.9‰ for pyrite; ?3.3‰ for chalcopyrite; ?5.4‰ for sphalerite and ?7.6‰ for galena. These data are consistent with derivation of the sulfur from either igneous rocks or possibly from local wallrock.     

Pleistocene speleothems of Mallorca: implications for palaeoclimate and carbonate diagenesis in mixing zones

The Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, are excellent indicators of palaeoclimate variations, and are samples that allow evaluation of the products and processes of mixing‐zone diagenesis in an open‐water cave system. Integrated stratigraphic, petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates: meteoric‐marine mixing zone and meteoric‐vadose zone. Mixing‐zone precipitates formed at and just below the water–air interface of cave pools during interglacial times, when the cave was flooded as a result of highstand sea‐level. Mixing‐zone precipitates include bladed and dendritic high‐Mg calcite, microporous‐bladed calcite with variable Mg content, and acicular aragonite; their presence suggests that calcium‐carbonate cementation is significant in the studied mixing‐zone system. Fluid inclusion salinities, δ¹³C and δ¹⁸O compositions of the mixing‐zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred, rather, the proximity to the water table and degassing of CO₂ at the interface, were the major controls on precipitation. Thus, simple two‐end‐member mixing models may apply only in mixing zones well below the water table. Meteoric‐vadose speleothems include calcite and high‐Mg calcite with columnar and bladed morphologies. Vadose speleothems precipitated during glacial stages when sea level was lower than present. Progressive increase in δ¹³C and δ¹⁸O of the vadose speleothems resulted from cooling temperatures and more positive seawater δ¹⁸O associated with glacial buildup. Such covariation could be considered as a valid alternative to models predicting invariant δ¹⁸O and highly variable δ¹³C in meteoric calcite. Glacio‐eustatic oscillations of sea‐level are recorded as alternating vadose and mixing‐zone speleothems. Short‐term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone. Fluid‐inclusion and stable‐isotope data suggest that aragonite, as opposed to calcite, precipitated during times of reduced meteoric recharge.     

Textural and carbon isotopic evidence of monsoonal changes recorded in a composite-type speleothem from Korea since MIS 5a

Textural and stable isotopic records of a composite-type speleothem from Gwaneum Cave in the eastern part of the Korean peninsula show prominent paleoenvironmental changes since MIS (marine oxygen isotope stage) 5a. Based on ²³⁰Th/²³⁴U dating, the speleothem experienced growth from 90.9 ± 6.5 ka to 1.2 ± 0.5 ka with several hiatuses. Four growth phases (A, B, C and D) are recognized based on speleothem type and texture. Very irregular and laterally discontinuous growth laminae in Phases B and C indicate that the cave coralloids grew over the stalagmite (Phase A) when the supply of dripping water became limited. Variations within the δ¹³C time series of Phase A are interpreted as responses to millennial-scale fluctuations of the East Asian monsoon intensity during MIS 5a. The monsoonal interpretation is based on the idea that δ¹³C values reflect the isotopic composition of soil-derived CO₂, which, in turn, should relate to monsoon-driven changes in terrestrial productivity above the cave during the MIS 5a. Our reconstruction reveals that the significant monsoonal changes on the Korean peninsula occurred in conjunction with changes in sea level and/or oceanic circulations during the transition period from MIS 5a to MIS 4.     

The Cipoeiro gold deposit,Gurupi Belt,Brazil: Geology,chlorite geochemistry,and stable isotope study

The Cipoeiro gold deposit, located in the Gurupi Belt, northern Brazil, is hosted by tonalites of 2148 Ma. The deposit is controlled by splays related to the major strike-slip Tentugal shear zone, and at the deposit scale, the mineralization is confined to ductile–brittle shear zones. Mineralization style comprises thick quartz veins and narrow and discontinuous quartz-carbonate veinlets associated with disseminations in altered host rocks. The postmetamorphic hydrothermal paragenesis is composed of quartz, calcite, chlorite, white mica (phengite), pyrite, and minor albite. Electron microprobe analysis of chlorites reveals a relatively uniform chemical composition at depths of more than 100 m. The chlorites are characterized by (Fe + Mg) ratios between 0.37 and 0.47 and Al^IV ranging between 2.22 and 2.59 a.p.f.u. and are classified as Fe-chlinochlore. Temperatures calculated by applying the Al^IV contents of chlorites yield a relatively narrow interval of 305 ± 15°C. Stable isotope (O, H, C, S) compositions have been determined in silicate, carbonate, and sulfide minerals. The δ¹⁸O and δD values of the mineralizing fluid range from +2.4 to +5.7 and from −43‰ to −20‰, respectively, and are interpreted as having a metamorphic origin. The δ¹³C values of fluid CO₂ are in the range −10.7‰ to −3.9‰, whereas the fluid δ³⁴S is around 0‰. Carbon and sulfur compositions are not diagnostic of their sources, compatible as they are with mantle, magmatic, or average crustal reservoirs. The hydrothermal paragenesis, chlorite–pyrite coexistence, temperature of ore formation, and sulfur isotope evidence indicate relatively reduced fO₂ conditions for the mineralizing fluid. Geologic, chemical, and isotopic characteristics of the Cipoeiro deposit are compatible with the class of orogenic gold deposits.     

Paleohydrogeological events recorded by stable isotopes,fluid inclusions and trace elements in fracture minerals in crystalline rock,Simpevarp area,SE Sweden

Fracture minerals calcite, pyrite, gypsum, barite and quartz, formed during several events have been analysed for δ¹³C, δ¹⁸O, δ³⁴S, ⁸⁷Sr/⁸⁶Sr, trace element chemistry and fluid inclusions in order to gain knowledge of the paleohydrogeological evolution of the Simpevarp area, south-eastern Sweden. This area is dominated by Proterozoic crystalline rocks and is currently being investigated by the Swedish Nuclear Fuel and Waste Management Co. (SKB) in order to find a suitable location for a deep-seated repository for spent nuclear fuel. Knowledge of the paleohydrogeological evolution is essential to understand the stability or evolution of the groundwater system over a time scale relevant to the performance assessment for a spent nuclear fuel repository. The ages of the minerals analysed range from the Proterozoic to possibly the Quaternary. The Proterozoic calcite and pyrite show inorganic and hydrothermal-magmatic stable isotope signatures and were probably formed during a long time period as indicated by the large span in temperatures (c. 200–360 °C) and salinities (0–24 wt.% eq. CaCl₂), obtained from fluid inclusion analyses. The Paleozoic minerals were formed from organically influenced brine-type fluids at temperatures of 80–145 °C. The isotopic results indicate that low temperature calcite and pyrite may have formed during different events ranging in time possibly from the end of the Paleozoic until the Quaternary. Formation conditions ranging from fresh to brackish and saline waters have been distinguished based on calcite crystal morphologies. The combination of δ¹⁸O and crystal morphologies show that the fresh–saline water interface has changed considerably over time, and water similar to the present meteoric water and brackish seawater at the site, have most probably earlier been residing in the bedrock. Organic influence and closed system in situ microbial activity causing disequilibrium are indicated by extremely low δ¹³C (down to −99.7‰), extreme variation in δ³⁴S (−42.5‰ to +60.8‰) and trace element compositions. The frequency of calcite low in δ¹³C and high in Mn, as well as pyrite with biogenically modified δ³⁴S decreases with depth. Strontium isotopes have been useful to separate the different generations and the Sr isotope ratios in the groundwaters have been determined mainly by in situ water–rock interaction processes. The difficulty of separating late Paleozoic calcite from possibly recent calcite, and the fact that these calcites are usually found in the same fracture systems indicate that water conducting structures have been intermittently conductive from the Paleozoic and onwards. The methodology used has been successful in separating the different generations and characterising their formation conditions.     

Water determination in iron oxyhydroxides and iron ores by Karl Fischer titration

Protohematite (Fe_2?x/3(OH) _x O_3?x 1 ≤ x < 0.5) and hydrohematite (Fe_2?x/3(OH) _x O_3?x 0.5 ≤ x < 0) are iron-defective phases containing hydroxyl groups in their structures. These species were described in prior studies mainly with the aid of X-ray diffraction and Infrared spectroscopy. The existence of these phases in soils might have influence in redox processes, and they were considered as a possible water reservoir in Martian soils. In this study, we have used for the first time the Karl Fischer titration method to determine the amount of water released after heating several synthetic samples of goethite, hematite and natural iron ores at 105, 400, 600 and 900 °C. It was found that heating at 105 °C did not remove all moisture from the samples, and higher temperatures were necessary to completely remove all the absorbed water. The water contents determined at 400, 600 and 900 °C were found to be the same within the experimental errors, suggesting the inexistence of both protohematite and hydrohematite in the investigated samples. Therefore, the above-mentioned effects of these phases in soils might have to be reevaluated.     

Hot orogens and supercontinent amalgamation: A Gondwanan example from southern India

The Southern Granulite Terrane in southern India preserves evidence for regional-scale high to ultrahigh temperature metamorphism related to the amalgamation of the supercontinent Gondwana. Here we present accessory mineral (zircon and monazite) geochronological and geochemical datasets linked to the petrological evolution of the rocks as determined by phase equilibria modelling. The results constrain the duration of high to ultrahigh temperature (> 900 °C) metamorphism in the Madurai Block to be c. 40 Ma with peak conditions achieved c. 60 Ma after the formation of an orogenic plateau related to the collision of the microcontinent Azania with East Africa at c. 610 Ma. A 1D numerical model demonstrates that the attainment of temperatures > 900 °C requires that the crust be moderately enriched in heat producing elements and that the duration of the orogenic event is sufficiently long to allow conductive heating through radioactive decay. Both of these conditions are met by the available data for the Madurai Block. Our results constrain the length of time it takes for the crust to evolve from collision to peak P–T (i.e. the prograde heating phase) then back to the solidus during retrogression. This evolution illustrates that not all metamorphic ages date sutures.     

Biomarkers record environmental changes along an altitudinal transect in the wettest place on Earth

The combined application of plant wax δD (δD_wax) and branched glycerol dialkyl glycerol tetraethers (brGDGTs) has been suggested as proxy for paleo-elevation. In some of the altitudinal transects studied so far, nonlinear precipitation gradients, large variations in seasonality, soil and vegetation types introduced substantial amounts of scatter in the relationship between these proxies and altitude. To further evaluate the principal functioning of the proxies, δD_wax and brGDGTs were analysed in surface soils along an altitude gradient (from 28 m up to 1865 m a.s.l.) in Meghalaya, India. The transect experiences limited seasonal temperature variation and receives very high monsoonal precipitation along the whole elevation gradient. The δD_wax show a significant relation with altitude (r² = 0.72). The additional fits with stream water δD (r² = 0.72) and modelled precipitation δD (r² = 0.72) indicate that δD_wax tracks the altitude effect on precipitation. Also the brGDGT distribution shows a correlation with altitude, reflecting the decrease in temperature with higher elevation (r² = 0.65, or r² = 0.66 using the original and recalibrated methylation of branched tetraethers-cyclisation of branched tetraethers (MBT–CBT) proxy). Application of the original MBT–CBT calibration generates calculated air temperatures that overestimate measured air temperature by ∼6 °C, whereas temperatures similar to measured are obtained with the revised calibration. These results indicate that δD_wax and brGDGTs may faithfully and accurately track environmental changes with altitude for transects where seasonal and diurnal temperature variability is relatively limited. Furthermore, proxy application to locations that experience high rainfall increases the suitability as climatic indicators, as it excludes soil moisture availability as a limiting factor.     

Thermoluminescence, electron paramagnetic resonance and optical absorption in natural and synthetic rhodonite crystals

Thermoluminescence, electron paramagnetic resonance and optical absorption properties of rhodonite, a natural silicate mineral, have been investigated and compared to those of synthetic crystal, pure and doped. The TL peaks grow linearly for radiation dose up to 4 kGy, and then saturate. In all the synthetic samples, 140 and 340°C TL peaks are observed; the difference occurs in their relative intensities, but only 340°C peak grows strongly for high doses. Al₂O₃ and Al₂O₃ + CaO-doped synthetic samples presented several decades intenser TL compared to that of synthetic samples doped with other impurities. A heating rate of 4°C/s has been used in all the TL readings. The EPR spectrum of natural rhodonite mineral has only one huge signal around g = 2.0 with width extending from 1,000 to 6,000 G. This is due to Mn dipolar interaction, a fact proved by numerical calculation based on Van Vleck dipolar broadening expression. The optical absorption spectrum is rich in absorption bands in near-UV, visible and near-IR intervals. Several bands in the region from 540 to 340 nm are interpreted as being due to Mn³⁺ in distorted octahedral environment. A broad and intense band around 1,040 nm is due to Fe²⁺. It decays under heating up to 900°C. At this temperature it is reduced by 80% of its original intensity. The pink, natural rhodonite, heated in air starts becoming black at approximately 600°C.