Attila Demény

Carbon isotope anomaly and other geochemical changes at the Triassic-Jurassic boundary from a marine section in Hungary

Most mass extinctions are linked with carbon isotope excursions, implying that biotic crises are coupled with changes in the global carbon cycle. The isotopic evolution during the end-Triassic extinction is far less documented than that for the other major Phanerozoic extinctions. Here we report a sharp and short-lived 23.5‰ excursion in carbon isotope values for carbonate (d 13 Ccarb) corresponding to a 22‰ excursion in the isotopic composition of marine organic matter (d 13 Corg) and other geochemical

Crustal contamination and fluid/rock interaction in the carbonatites of Fuerteventura (Canary Islands, Spain): A C, O, H isotope study

Abstract Fuerteventura—the second largest of the Canary Islands consists of Mesozoic sediments, submarine volcanic rocks, dike swarms and plutons of the Basal Complex, and younger subaerial basaltic and trachytic series. Carbonatites are found in two Basal Complex exposures: the Betancuria Massif in the central part of the island and the Esquinzo area in the north. δ 13 C values of the carbonatites increase progressively from south to north of the island. This phenomenon is attributed to different degrees of assimilation of sedimentary carbonate. Homogeneous, typically magmatic δ 18 O values for carbonatites which have preserved primary igneous textures and minerals suggest a well-mixed reservoir where changes in δ 13 C values result from the storage of carbonate magmas at different structural levels. The magma storage allowed assimilation of sediment to varying degrees before final emplacement of carbonatites. Shifts in δ 18 O towards more positive and negative values from presumed primary compositions are observed in the carbonatites. On the basis of the oxygen isotope compositions of calcite, mica and K-feldspar, and the hydrogen isotope compositions of micas, the changes in the δ 18 O values of the carbonatites can be related to fluid/rock interactions.

Genesis and transformations of monazite, florencite and rhabdophane during medium grade metamorphism: examples from the Sopron Hills, Eastern Alps

Electron microprobe studies on the age, mineral chemistry and alteration on accessory LREE-phosphate minerals have been carried out in medium-grade metamorphic rocks of the Sopron Hills belonging to the Lower Austroalpine tectonic unit. Monazite (and xenotime) is relatively common, whereas rhabdophane and florencite are restricted to certain rock types. A first generation of monazite was formed in mica schists during the pre-Alpine, Hercynian metamorphism at 575–700 jC and 1.8– 3.8 kbar as evidenced by P–T data from the literature, their mineral paragenetic and textural characteristics and supported by Th–U–total Pb ages of ca. 300 Ma. In orthogneisses, monazite is rare and of igneous origin. Kyanite quartzites and leucophyllites that were formed by Mg metasomatism contain inherited monazite from the precursor rocks. A new generation of monazite was also formed during the Alpine metamorphism at V550 jC, 13 kbar according to the literature data, giving ages around 75 Ma. Pronounced negative Eu anomalies were found in the igneous monazites (Eu/Eu* 0.4). Small differences have been observed in Yand HREE contents, whereas the LREE sections of the rare-earth element (REE) patterns nearly coincide. Th and Ca enter the monazite structure at the expense of REE, nearly according to the brabantitic replacement 2REE 3+ XTh 4+ +Ca 2+ . In some mica schists, monazite is altered to rhabdophane. Rhabdophane, distinguished from monazite by quantitative electron microprobe analysis by low-oxide total, is found in many mica schists and orthogneisses. It forms fine-grained aggregates, often attached to apatite or monazite. It usually has higher Yand Ca contents and a less pronounced negative Eu anomaly than that of coexisting monazite. It may have been formed either by crystallization from REE-containing hydrous solutions or from monazite reacting with Y–Ca-containing solutions. Florencite appears only in some leuchtenbergite-bearing leucophyllites, kyanite quartzites and REE-rich clasts. It is often idioblastic and may be grown on apatite or monazite. It is chemically close to its ideal composition, but Ca, Sr and Th may replace REE in minor amounts. In some grains, ThO2 may reach 10 wt.%. The data indicate that the charge balance is maintained by different mechanisms in low- and high-thorian florencite. No Yor HREE (above Gd) could be measured in florencite. No fractionation was observed between coexisting monazite and florencite; however, monazite inclusions in florencite are depleted in La–Ce and enriched in HREE. D 2002 Elsevier Science B.V. All rights reserved.

Burbankite group minerals and their alteration in rare earth carbonatites—source of elements and fluids (evidence from C–O and Sr–Nd isotopic data)

The 370–380 Ma Khibina and Vuoriyarvi complexes on the Kola Peninsula, Russia, which form part of the Palaeozoic Kola Alkaline Province, contain REE-rich carbonatites with burbankite (Na,Ca)3(Sr,Ca,REE,Ba)3(CO3)5 or calcioburbankite (Na,Ca)3(Ca,Sr,REE,Ba)3(CO3)5 as the principal primary REE mineral. Within each complex the C–O and Sr–Nd isotopic data are similar for burbankite group minerals and co-existing calcite or dolomite (Khibina: δ13C(V-PDB)=−6.4 to−5.8‰, δ18O(V-SMOW)=7.3–7.7‰, (87Sr/86Sr)370=0.70390–0.70404 and (143Nd/144Nd)370=0.51230–0.51235; Vuoriyarvi: δ13C=−4.2 to −3.0‰, δ18O=8.1–9.4‰, (87Sr/86Sr)370=0.70313–0.70315 and (143Nd/144Nd)370=0.51243–0.51245). This indicates that the REE mineralization and its host carbonatites in each complex are derived from the same source and are co-genetic. There is, however, a great difference between the Sr, Nd and C isotopic signatures from Khibina and Vuoriyarvi, whereas the δ18O ranges are similar. This suggests that the REE carbonatites of the two complexes originate from sources with different isotopic signatures. At least three mantle components are needed to explain the variations in Sr and Nd compositions in the carbonatites from Kola. The δ13C ranges of primary carbonatites with low δ18O values are quite different for Khibina and Vuoriyarvi and show correlation with the radiogenic isotope compositions. The data may be best explained by subduction-related source contamination that caused δ13C variations in different mantle components. During late-stage processes burbankite and calcioburbankite have been replaced by various assemblages of REE–Sr–Ba minerals. The alteration of burbankite group minerals is an open-system hydrothermal process leading to multiple element transfer. It has produced mineral assemblages which are characterized by high δ18O values (Khibina: δ18O(V-SMOW)=11.4–13.9‰ and Vuoriyarvi: δ18O=17.1–18.0‰) compared to primary burbankite and calcioburbankite. Co-existing calcite and dolomite have retained their original C and O isotope compositions, and one calcite sample from Khibina shows strong positive δ13C–δ18O shifts similar to those of the pseudomorph. The high δ18O and sometimes high δ13C values can be attributed to low-temperature isotope exchange between minerals and fluid with variable CO2/H2O ratio taking place during and/or after crystallization as usually observed in carbonatites. The Sr and Nd isotope compositions of pseudomorphs and associated calcite/dolomite in general are identical to those of burbankite/calcioburbankite and associated carbonates suggesting that the fluids which caused burbankite alteration are from the same source, i.e. carbonatitic. Small variations in the Sr and Nd isotope signatures point to interaction of the pseudomorph-forming fluid with alkali silicate wall rocks.

Exhumation of the Rechnitz Window at the border of the Eastern Alps and Pannonian Basin during Neogene extension

Abstract The Rechnitz Window is the easternmost Penninic window of the Alps and the only one that is partly covered by Neogene sediments. Zircon and apatite fission-track ages have been measured on the Penninic metasediments of the Rechnitz series to better understand the exhumation of the window at the Alpine-Pannonian border. The zircon FT ages range from 21.9 to 13.4 Ma, similar to the white mica K/Ar ages (19 and 23 Ma). The exhumation rate during the Early Miocene extension was high (∼40°C/Ma). Apatite samples display FT ages of 7.3 and 9.7 Ma, thus the cooling rate was significantly less (7–11°C/Ma) during the post-rift uplift uplift in Late Miocene-Pliocene than in the Early Miocene escape period. Zircon FT ages decrease eastward due to the gradual southeastward slide of the Austroalpine cover along low-angle normal faults. The exhumation of the Rechnitz metamorphic core complex is younger than the unroofing of the eastern Tauern Window. Morphology of detrital zircon crystals of the Penninic metasediments was used for the differentiation of tectonic subunits and to trace the internal structure of the window. One population was probably derived from undifferentiated calc-alkaline granitoids and tholeiitic granitoid sources, while the other derived from evolved calc-alkaline granitoids and alkaline granitoids of anorogenic complexes. The areal distribution of zircon populations is in harmony with the eastward shift of zircon cooling ages. These data indicate the exhumation of deep levels of Penninic metasediments in the centre of the window.

Stable isotope studies and processes of carbonate formation in Hungarian alkali basalts and lamprophyres: evolution of magmatic fluids and magma-sediment interactions

Processes of carbonate formation have been related to C and O isotopic compositions in the Mesozoic alkali basalt (Mecsek Mts.) and lamprophyre (Transdanubian Range) suites of Hungary. In the studied magrnatic rocks, carbonates are present as ocelli, amygdales, xenoliths, veins and groundmass carbonate. C and O isotope studies of these types of carbonate have yielded information on the origin of the carbonates and indicated the following processes of formation that determined the δ13C and δ18O values of the carbonates:(1)Crystallization of magmatic carbonate. Textural characteristics and δ13C values suggest formation of magmatic carbonate in alkali basalt and lamprophyre dikes, whereas the δ18O compositions of these carbonates indicate low temperature oxygen isotope exchange with magmatic fluids.(2) Assimilation of sedimentary carbonate by silicate magmas. Even completely recrystallized amygdales and ocelli of basalts and lamprophyres have preserved their sedimentary δ13C values. In contrast, variations in the extent of mobilization and isotope exchange with magmatic fluids are reflected in differences in the ranges of the δ18O values of amygdales, ocelli and veins, and can be attributed to different amounts of fluids involved in the magmatic events.(3) Low temperature alteration of magmatic rocks caused only 18O-enrichment in the carbonate amygdales of basalts and the groundmass carbonates of lamprophyres, indicating that no externally-derived CO2 was present in the alteration fluids.(4) Degassing of magma and magmatic fluid. Correlations between δ13C and δ18O data, magma crystallization depths and amygdale sizes in the alkali basalts suggest that CO2 degassing has been responsible for the negative δ13C and positive δ18O shifts observed. A similar trend was found in the lamprophyres, but the extent of the δ18O shift indicates that in these rocks H2O degassing also played an important role.

Hydrogen index as reflecting intensity of sulphidic diagenesis in non-bioturbated, shaly sediments

Abstract Based on 13 published porewater H 2 S and sulphate profiles the amount of H 2 S escaping from non-bioturbated shales varies between some few % to 45% of the amount of bacterially generated H 2 S. This finding permits calculation of the original organic carbon (TOC or ) content of immature nonbioturbated shales using TOC and sulphur content data. In two immature non-bioturbated sequences from Hungary (Toarcian and Oligocene) the first-order correlation between HI and TOC/TOC or was found to be stronger than that between HI and TOC, indicating that sulphate reduction was the leading process both in decrease in TOC content and degradation of kerogen source potential.

Estimation of primary productivity in the Toarcian tethys- A novel approach based on TOC, reduced sulphur and manganese contents

Abstract About 60 samples from the non-bioturbated Urkut Manganese Ore Formation (Bakony Mountains, Hungary), representing the early Toarcian anoxic event, were studied for: (1) TOC content, Rock-Eval parameters and kerogen δ 13 C; (2) S content and δ 34 S and (3) Al, Ca, Mn, Si and Ti contents. Using the example of the formation, a novel approach for estimating paleoproductivity is presented, taking into account reduced sulphur and manganese contents, which reflect the amount of organic carbon lost during early burial. Drastic changes in Mn content (between 0.7% and 37%) during deposition of the formation resulted in very variable early diagenetic processes, with sulphate reduction dominating in low-Mn sediments, and Mn reduction dominating in high-Mn sediments. The amounts of reduced sulphur and manganese allowed calculation of the original organic carbon (TOC or ) content throughout the formation. Based on stratigraphic variations in chemical composition, changes in the rate of sedimentation were assessed for the three main members of the formation. Next, using the formula describing the relationship between measured productivity, rate of sedimentation and carbon flux reaching the sea-floor, developed by Suess (1980) , past productivity was calculated. The values obtained, lower than those prevailing in present-day upwelling systems, are probably underestimated because of uncertainties of chronostratigraphic and water-depth data. However, relative differences between productivity values obtained for the three members are considered as realistic and suggest an increase in productivity during deposition of the formation. This increase is not related to stratigraphic variations in Mn content and is accompanied by an increase in the growth of calcareous plankton.

Empirical equations for the temperature dependence of calcite-water oxygen isotope fractionation from 10 to 70°C

Although the temperature dependence of calcite-water oxygen isotope fractionation seems to have been well established by numerous empirical, experimental and theoretical studies, it is still being discussed, especially due to the demand for increased accuracy of paleotemperature calculations. Experimentally determined equations are available and have been verified by theoretical calculations (considered as representative of isotopic equilibrium); however, many natural formations do not seem to follow these relationships implying either that existing fractionation equations should be revised, or that carbonate deposits are seriously affected by kinetic and solution chemistry effects, or late-stage alterations. In order to test if existing fractionation-temperature relationships can be used for natural deposits, we have studied calcite formations precipitated in various environments by means of stable isotope mass spectrometry: travertines (freshwater limestones) precipitating from hot and warm waters in open-air or quasi-closed environments, as well as cave deposits formed in closed systems. Physical and chemical parameters as well as oxygen isotope composition of water were monitored for all the investigated sites. Measuring precipitation temperatures along with oxygen isotope compositions of waters and calcites yielded empirical environment-specific fractionation-temperature equations: [1] 1000 · lnα = 17599/T - 29.64 [for travertines with a temperature range of 30 to 70°C] and [2] 1000 · lnα = 17500/T - 29.89 [for cave deposits for the range 10 to 25°C]. Finally, based on the comparison of literature data and our results, the use of distinct calcite-water oxygen isotopic fractionation relationships and application strategies to obtain the most reliable paleoclimate information are evaluated.

H isotope fractionation due to hydrogen-zinc reactions and its implications on D H analysis of water samples

After conversion of water to H2 gas with zinc for DH ratio determination, hydrogen gas can be absorbed by the zinc during cooling, resulting in H isotope fractionation. Thermal release and subsequent DH ratio determination of hydrogen bound to zinc revealed that the fractionation between the bound H and the H2 gas is extremely large with an empirical α-value of ∼ 1.62. Differences in the fractionations observed using stopcock-sealed vessels and break-seal tubes indicate that the conversion method and H2Zn ratios should be the same for both samples and standards. The effect of hydrogen absorption can be eliminated (or lowered) by re-heating the zinc during transfer of hydrogen gas into the mass spectrometer. For measurement of hydrogen isotope composition of geological samples with varying and sometimes unpredictable water content (e.g., fluid inclusions of minerals) recording a calibration curve of varying water/zinc ratios and δD-values is proposed.