Geochemistry of oceanic carbonatites compared with continental carbonatites: mantle recycling of oceanic crustal carbonate

Major and trace element and Sr-Nd-Pb-O-C isotopic compositions are presented for carbonatites from the Cape Verde (Brava, Fogo, Sáo Tiago, Maio and Sáo Vicente) and Canary (Fuerteventura) Islands. Carbonatites show pronounced enrichment in Ba, Th, REE, Sr and Pb in comparison to most silicate volcanic rocks and relative depletion in Ti, Zr, Hf, K and Rb. Calcio (calcitic)-carbonatites have primary (mantle-like) stable isotopic compositions and radiogenic isotopic compositions similar to HIMU-type ocean island basalts. Cape Verde carbonatites, however, have more radiogenic Pb isotope ratios (e.g. ²⁰⁶Pb/²⁰⁴Pb=19.3-20.4) than reported for silicate volcanic rocks from these islands (18.7-19.9; Gerlach et al. 1988; Kokfelt 1998). We interpret calcio-carbonatites to be derived from the melting of recycled carbonated oceanic crust (eclogite) with a recycling age of ~1.6 Ga. Because of the degree of recrystallization, replacement of calcite by secondary dolomite and elevated ‘¹³C and ‘¹⁸O, the major and trace element compositions of the magnesio (dolomitic)-carbonatites are likely to reflect secondary processes. Compared with Cape Verde calcio-carbonatites, the less radiogenic Nd and Pb isotopic ratios and the negative Ɨ/4 of the magnesio-carbonatites (also observed in silicate volcanic rocks from the Canary and Cape Verde Islands) cannot be explained through secondary processes or through the assimilation of Cape Verde crust. These isotopic characteristics require the involvement of a mantle component that has thus far only been found in the Smoky Butte lamproites from Montana, which are believed to be derived from subcontinental lithospheric sources. Continental carbonatites show much greater variation in radiogenic isotopic composition than oceanic carbonatites, requiring a HIMU-like component similar to that observed in the oceanic carbonatites and enriched components. We interpret the enriched components to be Phanerozoic through Proterozoic marine carbonate (e.g. limestone) recycled through shallow, subcontinental-lithospheric-mantle and deep, lower-mantle sources.     

Occurrence of naphthalenesulfonates and their condensates with formaldehyde in a landfill leachate and their transport behavior in groundwater of the Upper Rhine Valley, Germany

. Naphthalenesulfonates and their condensation products with formaldehyde are manufactured in the chemical industry for broad application as tanning agents, dispersing agents, and as superplasticizers for concrete. With the disposal of waste they have found their way into the aquatic environment. Five monomeric naphthalenesulfonates and a group of dimeric sulfonated naphthalene-formaldehyde condensates (SNFC) were identified in the leachate and in groundwater observation wells downstream in the plume of the Karlsruhe-West landfill, situated in the Upper Rhine Valley in southwest Germany. For the main monomer of technical SNFC products, 2-naphthalenesulfonate (2-NS), the concentration in the leachate was 170 µg l^–1, which decreased to 9.5 µg l^–1 at a distance of about 300 m from the boundary of the landfill. In the Merdingen tracer test field, the transport of these compounds was studied under more controlled conditions in two tracer experiments with (1) two model compounds, and (2) a technical SNFC product and uranine as a reference tracer. In these experiments, the monomeric naphthalenesulfonates and the SNFC with n=2 behave as conservative tracers. Thus, the findings of the landfill study were supported by these results. Higher molecular SNFC were strongly retarded, which is attributed to adsorption to soil particles. The results of the second tracer experiment suggest a degradation of 2-NS and 2,6-naphthalenedisulfonate (2,6-NDS) after adaptation of the microorganisms in the groundwater aquifer as a consequence of the first tracer experiment.     

Dissolution channels in quartz and the role of pressure changes in gold and sulfide deposition in the Archean, greenstone-hosted, Hutti gold deposit, Karnataka, India

The pressure, temperature and composition of ore fluids that resulted in gold deposition in the Archean, greenstone-hosted Hutti deposit have been studied using fluid inclusions and the compositions of arsenopyrite and chlorite. Five types of fluids have been identified in fluid inclusions in quartz veins associated with mineralization. They are (1) monophase CO ₂-rich fluid; (2) low-salinity (0 to 14 wt% NaCl equivalent) and high-salinity (16 to 23 wt% NaCl equiv.) aqueous fluids; (3) high-salinity (28 to 40 wt% NaCl equiv.), polyphase aqueous fluids; (4) CO ₂–H ₂O–NaCl fluids of low salinity (0–8 wt% NaCl equiv.); and (5) a few carbonic inclusions with halite±nahcolite. The diversity of entrapped fluid composition is explained in terms of changes in fluid pressure and temperature which affect a more or less uniform supply of primary low-salinity CO ₂–H ₂O–NaCl fluid to the shear zone. Geothermobarometric studies indicate that during mineralization temperature ranged between 360 and 240 °C, and fluid pressure between 3,600 and 1,600 bar. The data are interpreted in terms of the cyclic fault-valve mechanism for active shear zones. Deposition of gold and sulfides has been studied on the basis of constraints from the composition of wall-rock chlorite, ore-mineral assemblages, and textural features. Tubular channels, 20 to 100 µm wide and up to 500 µm long that arise from fractures and C-planes in sheared quartz veins are reported for the first time. The channels have pyrrhotite, arsenopyrite, pyrite and gold at their distal ends, with calcite filling up the remaining part. These channels form in response to increases in T and P, by dissolution of quartz grains, guided by dislocations in them. At the P– T conditions of interest, gold and sulfide deposition takes place in the shears and fractures of quartz veins from CO ₂–H ₂O–NaCl ore fluid of low salinity and pH due to changes in phase compositions that occur during the process of shear failure of the enclosing rocks. In the wall rock where pH is buffered, gold deposition takes place from the predominant Au(HS) ₂ ^- species with progressive sulfide deposition and decrease in SS, from 0.01 to 0.001 mol/kg as T falls from 360 to 240 °C.     

Morphologic and spectral investigation of exceptionally well-preserved bacterial biofilms from the Oligocene Enspel formation, Germany

The fossilised soft tissues of a tadpole and an associated coprolitic structure from the organic-rich volcanoclastic lacustrine Upper Oligocene Enspel sediments (Germany) were investigated using high-resolution imaging techniques and nondestructive in situ surface analysis. Total organic carbon analysis of the coprolite and the sediment revealed values of 28.9 and 8.9% respectively. The soft tissues from the tadpole and the coprolite were found to be composed of 0.5 to 1 μm-sized spheres and rod shapes. These features are interpreted as the fossil remains of bacterial biofilms consisting probably of heterotrophic bacteria and fossilised extracellular polymeric substances. They became fossilised while in the process of degrading the organic matter of the organism and the coprolite. Comparison with a modern marine biofilm revealed morphologic details identical to those observed in the fossil bacterial biofilms. Although the fossil biofilms on both macrofossils exhibited identical microtextures, their mode of preservation was inhomogeneous and varied between calcium phosphate and an as yet unidentified mineral phase consisting mainly of Si, Ca, Ti, P, and S, but also showing the presence of Mg, Al, and Fe. The coprolite consists purely of fossilised bacterial cells in a densely packed arrangement and associated fossilised extracellular polymeric substances.In addition to preliminary imaging and energy-dispersive X-ray analysis, both the fossil biofilms and the sediment were investigated by nondestructive in situ analysis using time of flight-secondary ion mass spectroscopy (ToF-SIMS). The mass spectra obtained on the coprolite in mass-resolved chemical mapping mode allowed the tentative identification of a number of organic secondary ion species. Some spectra appear to indicate the presence of bacterial hopanoids, but further work using standard techniques such as gas chromatography mass spectroscopy is needed to conclusively verify the presence of these substances. Nevertheless, ToF-SIMS chemical maps were successfully correlated with electron microscopy images, allowing the correlation of molecular spectra, the spatial distribution of individual organic species, and specific morphologic features to demonstrate the potential of this approach in the analysis of microfossils.     

Oxygen fugacity and geochemical variations in the martian basalts: implications for martian basalt petrogenesis and the oxidation state of the upper mantle of Mars

The oxygen fugacity of the Dar al Gani 476 martian basalt is determined to be quartz-fayalite-magnetite (QFM) −2.3 ± 0.4 through analysis of olivine, low-Ca pyroxene, and Cr-spinel and is in good agreement with revised results from Fe-Ti oxides that yield QFM −2.5 ± 0.7. This estimate falls within the range of oxygen fugacity for the other martian basalts, QFM −3 to QFM −1. Oxygen fugacity in martian basalts correlates with ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and La/Yb ratios, indicating that the mantle source of the basalts is reduced and that assimilation of crust-like material controls the oxygen fugacity. This allows constraints to be placed on the oxidation state of the martian mantle and on the nature of assimilated crustal material. The assimilated material may be the product of early and extensive hydrothermal alteration of the martian crust, or it may be amphibole- or phlogopite-bearing basaltic rock within the crust. In either case, water may play a significant role in the oxidation of basaltic magmas on Mars, although it may be secondary to assimilation of ferric iron-rich material.     

The major-ion composition of silurian seawater

One-hundred fluid inclusions in Silurian marine halite were analyzed in order to determine the major-ion composition of Silurian seawater. The samples analyzed were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and one formation in the Early Silurian Canning Basin (Australia), the Mallowa Salt of the Carribuddy Group. The results indicate that the major-ion composition of Silurian seawater was not the same as present-day seawater. The Silurian ocean had lower concentrations of Mg²⁺, Na⁺, and SO₄²⁻, and much higher concentrations of Ca²⁺ relative to the ocean’s present-day composition. Furthermore, Silurian seawater had Ca²⁺ in excess of SO₄²⁻. Evaporation of Silurian seawater of the composition determined in this study produces KCl-type potash minerals that lack the MgSO₄-type late stage salts formed during the evaporation of present-day seawater. The relatively low Na⁺ concentrations in Silurian seawater support the hypothesis that oscillations in the major-ion composition of the oceans are primarily controlled by changes in the flux of mid-ocean ridge brine and riverine inputs and not global or basin-scale, seawater-driven dolomitization. The Mg²⁺/Ca²⁺ ratio of Silurian seawater was ∼1.4, and the K⁺/Ca²⁺ ratio was ∼0.3, both of which differ from the present-day counterparts of 5 and 1, respectively. Seawaters with Mg²⁺/Ca²⁺ <2 facilitate the precipitation of low-magnesian calcite (mol % Mg < 4) marine ooids and submarine carbonate cements whereas seawaters with Mg²⁺/Ca²⁺ >2 (e.g., modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg²⁺/Ca²⁺ ratio of seawater, not the pCO₂ of the Silurian atmosphere.     

Microbial respiration and diffusive transport of O2, O2, and OO in unsaturated soils: a mesocosm experiment

Although the flux of molecular O₂ between the atmosphere and the subsurface is intrinsically linked to the net soil production of greenhouse gasses, few studies have focused on the controls affecting the isotopic composition of O₂ in the subsurface. Here, we developed and tested a stable oxygen isotope tracer technique and gas transport modeling approach to evaluate O₂ cycling and fluxes from the subsurface that used an environmentally controlled soil mescosm. We measured the O₂ and δ¹⁸O₂ profiles in a model unsaturated soil zone and quantified the O₂ consumption rates and the O₂ isotope fractionation factors resulting from the combined processes of subsurface microbial (including bacteria, fungi, and protozoa) consumption of O₂ and diffusive influx of O₂ from the atmosphere. We found that at high respiration rates in the mesocosm, there appeared to be very little isotope fractionation of O₂ by soil microorganisms. Although the mesocosm respiration rates are not typical of natural soils in northern temperate climes, they may be more representative of soils in warm and moist tropical environments. Our findings caution against the indiscriminate application of laboratory-determined oxygen isotope fractionation factors to field settings. The oxygen isotope tracer and modeling approach demonstrated here may be applied to gain a better understanding of biogenic gas production and O₂ cycling in subsurface systems and soils.     

Consequences of diffuse and channelled porous melt migration on uranium series disequilibria

Magmas erupted at mid-ocean ridges (MORB) result from decompression melting of upwelling mantle. However, the mechanism of melt transport from the source region to the surface is poorly understood. It is debated whether melt is transported through melt-filled conduits or cracks on short time scales (<∼ 10³ yrs), or whether there is a significant component of slow, equilibrium porous flow on much longer time scales (>∼ 10³-10⁴ yrs). Radiogenic excess ²²⁶Ra in MORB indicates that melt is transported from the melting region on time scales less than the half life of ²²⁶Ra (∼1600 yrs), and has been used to argue for fast melt transport from the base of the melting column. However, excess ²²⁶Ra can be generated at the bottom of the melt column, during the onset of melting, and at the top of the melt column by reactive porous flow. Determining the depth at which ²²⁶Ra is generated is critical to interpreting the rate and mechanism of magma migration. A recent compilation of high quality U-series isotope data show that in many young basalts, ²²⁶Ra excess in MORB is negatively correlated with ²³⁰Th excess. The data suggest that ²²⁶Ra excess is generated independently of ²³⁰Th excess, and cannot be explained by “dynamic” or fractional melting, where observed radiogenic excesses are all generated at the base of the melt column. One explanation is that the negative correlation of activity ratios is a result of mixing of slow moving melt that has travelled through reactive, low-porosity pathways and relatively fast moving melt that has been transported in unreactive high-porosity channels. We investigate this possibility by calculating U-series disequilibria in a melting column in which high-porosity, unreactive channels form within a low-porosity matrix that is undergoing melting. The results show that the negative correlation of ²²⁶Ra and ²³⁰Th excesses observed in MORB can be produced if ∼60% of the total melt flux travels through the low-porosity matrix. This melt maintains ²²⁶Ra excesses via chromatographic fractionation of Ra and Th during equilibrium transport. Melt that travels through the unreactive, high-porosity channels is not able to maintain significant ²²⁶Ra excesses because Ra and Th are not fractionated from each other during transport and the transport time for melt in the channels to reach the top of the melt column is longer than the time scale for ²²⁶Ra excesses to decay. Mixing of melt from the high porosity channels with melt from the low-porosity matrix at the top of the melting column can produce a negative correlation of ²²⁶Ra and ²³⁰Th excesses with the slope and magnitude observed in MORB. This transport process can also account for other aspects of the geochemistry of MORB, such as correlations between La/Yb, α_Sm/Nd, and Th/U and ²²⁶Ra and ²³⁰Th excess.     

Physical and chemical properties of the waters of saline lakes and their importance for deep-water renewal: Lake Issyk-Kul, Kyrgyzstan

The relationships between electrical conductivity, temperature, salinity, and density are studied for brackish Lake Issyk-Kul. These studies are based on a newly determined major ion composition, which for the open lake shows a mean absolute salinity of 6.06 g kg⁻¹. The conductivity-temperature relationship of the lake water was determined experimentally showing that the lake water is about 1.25 times less conductive than seawater diluted to the same absolute salinity as that of the lake water. Based on these results, an algorithm is presented to calculate salinity from in-situ conductivity measurements. Applied to the field data, this shows small but important vertical salinity variations in the lake with a salinity maximum at 200 m and a freshening of the surface water with increasing proximity to the shores. The algorithm we adopt to calculate density agrees well with earlier measurements and shows that at 20°C and 1 atm Lake Issyk-Kul water is about 530 g m⁻³ denser than seawater at the same salinity. The temperature of maximum density at 1 atm is about 0.15°C lower than that for seawater diluted to the same salinity. Despite its small variations, salinity plays an important role, together with temperature changes, in the static stability and in the production of deep-water in this lake. Changes in salinity may have had important consequences on the mixing regime and the fate of inflowing river water over geological time. Uncharged silicic acid is negligible for the stability of the water column except near an ∼15 m thick nepheloid layer observed at the bottom of the deep basin.     

A Comparison of Two-Phase Dynamic Upscaling Methods Based on Fluid Potentials

Although a number of methods for calculating dynamic pseudo-functions have been developed over the years, there is still a lack of understanding as to why a certain method will succeed in some cases but fail in others. In this paper, we describe the results of an assessment of several upscaling methods, namely the Kyte and Berry (KB) method, the Stone method, the Hewett and Archer (HA) method and the Transmissibility-Weighted (TW) method. We have analyzed the equations for deriving the methods and investigated the results of numerical simulations of gas displacing oil, in a variety of models to enable us to gain new insights into these, and related, upscaling methods. In particular, some novel observations on methods based on fluid potential are presented and the issue of using predicted fluid mobilities as a criterion of accuracy of an upscaling method is clarified.