No description available.

This study was undertaken to explain why silica is preserved in the oceans, which are undersaturated in silica. Ion probe, electron probe and SEM analysis of the surface chemistry of radiolarians (a siliceous microfossil) and analysis of data about their geographic distribution were performed to answer this question. It was found that a number of factors contribute to radiolarian preservation. A metallic coating consisting of Al, Ni, Mg, Cu, Fe, Co, Mn, Mo, Cr, Zn and Ba exists on the surface of all radiolarians. The composition of this coating varies with geographic location and depth in the water column. It also varies between live and dead water column samples. High aluminum content in the coating of sediment samples may inhibit silica dissolution. Anoxic basin conditions also aid silica preservation. Analysis of the geographic distribution of radiolarians which are indicators of particular oceanographic conditions showed that they are preserved in restricted zones in the oceans. Surface, warm water dwelling radiolarians are shown to be preserved in the equatorial region. Intermediate and deep, cold water dwelling radiolarians exhibit cosmopolitan distributions in the sediments with enhancement under oceanographic convergences and divergences.

With increasing demands for nuclear power supplies, more efficient means of evaluation and extraction of sedimentary uranium ore deposits, which form 96% of the U.S. reserves, are required. The fluvial, deltaic, and near-shore marine sands and associated lignites of the Texas coastal plain uranium deposits can assist in the need for increased nuclear power supplies. Tetravalent uranium is essentially immobile, while hexavalent uranium is easily complexed and very mobile in sedimentary aqueous systems. Fixation of uranium in the sediments seemed to be controlled by sorption and/or reduction by organic matter, H(,2)S, clays, zeolites, and carbonates. Texas uranium ore typically occurs in roll-front type deposits and these are discussed in conjunction with fixation and mobilization mechanisms. The basic strategies of the carbonate and acid leach systems are discussed. By monitoring effluent uranium and Rn-222 and cumulative uranium and Rn-222 extracted, it is shown that predictions can be made concerning mining efficiency, degree of secular equilibrium, future profitability, and mining duration. Dissolution Eh-pH diagrams constructed by assuming an infinite source of uraninite in water with various complexing agents are shown to agree more accurately with kinetic data of uraninite dissolution than conventional stability Eh-pH diagrams. At pH values between 6.0 and 9.0, the reduced reaction rate in this pH range, the first order dependence of the oxygen partial pressure and the hydrogen ion concentration, and the first order dependence of the carbonate at low concentration and zero dependence at higher concentration can be explained by the formation of (UO(,2))(,3)(OH)(,5)('+) predicted by the dissolution diagram. It is proposed that carbonate leach systems be operated at pH values between 9.0 and 10.0 and sulfate acid leach systems may be operated at a pH as high as 3.0. Utilizing evidence that uraninite dissolution is not diffusion limited and that in-situ leach solutions are quite undersaturated with respect to uranium, it is shown that uraninite dissolution is independent of hydrological parameters with the exception of the flow rate which regulates oxidant introduction to the ore body. The La Place source and sink equation is utilized to gain insight into parameters which can be adjusted to maximize mining efficiency while constraining the leach system. The optimum well spacing in an in-situ leach system is found to be a complex function of flow rate, the mobilization inhibiting factor (MIF), the rate of total oxidation of the aquifer, and the ability of the well system and the aquifer to confine the system. The results of the research are used to generate models to optimize parameters in the in-situ leach. The models are found to predict values in good agreement with literature values for uranium in-situ leach operations.

The generation, distribution, and migration process of helium-4 and radon-222 in the upper lithosphere and their fluxes at the soil-air interface were extensively reviewed. Global baseline concentration values were obtained calculated from more than 11,000 natural gas analyses of helium and 1,600 for radon from different places reported in the literature. These values are compatible with the lognormal hypothesis for the distribution of trace elements in the crust. The large volume of helium data in natural gases permitted an estimate of the global scale helium production rate from sediments to be between 1 x 10('5) to 6.7 x 10('5) atoms per cubic meter of sediment per second with an average helium concentration in sediments of about 67 ppm. These values are in close agreement with existing estimates for uranium and thorium concentrations in rather average sediments. The theoretical analysis and modeling of helium and radon production in subsurface radioactive deposits have shown that radon is unlikely to produce superficial anomalies of analytical significance unless some special mechanisms exist that are capable of providing fluid transport velocities larger than about 10('-7) meter per second. Some of the more probable mechanisms are discussed. The use of helium as a geophysical tool to locate uranium deposits is shown to be hampered by the very high atmospheric background and the lack of appropriate instrumentation for its measurement. The development of a quadrupole mass filter especially designed to monitor helium at the soil-air interface is described. Preliminary laboratory tests using author's prepared helium standards with several concentrations down to 12 ppm are also reported.

The Austin Chalk is an impure "onshore" chalk that was deposited on a ramp marginal to the Gulf of Mexico during the Late Cretaceous. Basinal chalks are organic rich, commonly containing 0.5-5.0% amorphous, sapropelic kerogen derived from marine organic matter with only trace amounts of terrestial kerogen. Less organic matter was deposited and perserved in oxygenated shallow water, and fresh-water diagenesis oxidized the organic matter on outcrop. In each sample, the kerogen is concentrated in microstylolites, with organic fluids segregated in micropores in the chalk. The bitumen from the Austin Chalk contains geochemical fossils characteristic of plankton and algae: steranes, triterpanes, isoprenoids (particularly pristane and phytane), and even-carbon numbered C(,15)-C(,22) n-alkanes. These geochemical fossils become less abundant with increasing burial depth, due to the generation of hydrocarbons during catagenesis. The kerogen darkens and becomes condensed and aromatic as the H/C atomic ratio decreases and hydrocarbons are liberated from the kerogen and from asphaltenes. Saturated hydrocarbons, particularly C(,12)-C(,26) n-alkanes and isoalkanes and single-ring cycloalkanes, increase more rapidly than aromatic hydrocarbons and resins. Kerogen alteration and hydrocarbon generation are unaffected by the mineral composition of the chalk, suggesting that the dominant controls on the rate of alteration and the composition of hydrocarbons generated are the type of organic matter and thermal history of the rocks. The principal zone of oil generation for the Austin Chalk occurs at the same depth as for shales of the same age. The Austin Chalk reservoirs petroleum sourced within the formation and petroleum that has migrated into the chalk from elsewhere in the chalk and the underlying Eagleford Formation. The hydrocarbons are reservoired in interparticulate pores and micro- and macrofractures. Migration results in an enrichment in saturated hydrocarbons and, to a lesser extent, in aromatic hydrocarbons and resins in porous, reservior chalks. Crude oil produced from fractured chalks tends to have a high API gravity and to be enriched in saturated hydrocarbons, probably due to migration. Biodegradation may lower the gravity and hydrocarbon content of crude oils reservoired in shallow chalks.

The solubility of molybdenum trioxide in liquid-undersaturated water vapour has been investigated experimentally at 300, 320, and 360°C and 48 to 163 bars. Results of these experiments show that the solubility of MoO3 in the vapour phase is between 1 and 23 ppm, which is 19-20 orders of magnitude higher than that in a water-free system. Molybdenum solubilities increase linearly with fH2O , indicating that the metal forms a gaseous hydrated complex of the type MoO3·nH2O by the reaction: MoO3g+nH 2Og=MoO3· nH2Og A1 The hydration number, n, is interpreted to have a value of 2.6 at 300ºC, 2.5 at 320ºC, and 3.0 at 360ºC. Values of log K for this reaction are 16.9 at 300ºC, 16.5 at 320ºC, and 12.5 at 360ºC. / Calculations based on the extrapolated solubility of MoO 3 in equilibrium with molybdenite at 600ºC and 500 bars, using average H2O and total S fluxes of actively degassing volcanoes, with fO2 and fS2 controlled by the assemblage hematite-magnetite-pyrite, indicate that the vapour phase can transport sufficient Mo in about 900,000 years (within the life of some geothermal systems) to form a deposit of 336 Mt, with an average grade of 0.087% Mo (e.g., the Endako Mo-porphyry deposit, Canada).

The sulfur concentration in silicate melts at sulfide saturation (SCSS) was experimentally investigated in a temperature range from 1250ºC to 1450ºC and a pressure range from 500 MPa to 1 GPa in a piston-cylinder apparatus. The investigated melt compositions varied from rhyolitic to basaltic. All experiments were saturated with a FeS melt. Temperature was confirmed to have a positive effect on the SCSS and no measurable pressure effect was observed. Oxygen fugacity was controlled to be either near the carbon-carbon monoxide buffer or one log unit above the nickel-nickel oxide buffer, and found to positively affect the SCSS. A series of models were constructed to predict the SCSS as a function of temperature, pressure, melt composition, oxygen fugacity and sulfur fugacity of the system. The coefficients were obtained by the regression of experimental data from this study and from data in the literature. The best model found for the prediction of the SCSS is: lnSppm =-996T+9.875+0.997lnMFM+0.1901lnf O2-0.0722&parl0;PT &parr0;-0.115lnfS2 where P is in bar, T is in K, and MFM is a compositional parameter describing the melt based upon cation mole fractions: MFM=Na+K+2 Ca+Mg+Fe2+Six Al+Fe3+. / This model predicts the SCSS in anhydrous silicate melts from rhyolitic to basaltic compositions at crustal conditions from 1 bar to 1.25 GPa, temperatures from ~1200 to 1400ºC, and oxygen fugacities between approximately two log units below the fayalite-quartz-magnetite buffer and one log unit above the nickel-nickel oxide buffer. For cases where the oxygen and sulfur fugacities cannot be adequately estimated a simpler model also works acceptably: lnSppm =-5328T+8.431+1.244 lnMFM-0.01704P T+lnaFeS where aFes is the activity of FeS in the sulfide melt and is well approximated by a value of 1. Additional experiments were performed on other basalts in a temperature range from 1250ºC to 1450ºC at 1 GPa to test the models. The model predictions and the measurements of the SCSS agree within 5%. Although I cannot fix exactly the stoichiometric coefficients of the reaction controlling sulfur dissolution, my experiments and models suggest that the solution reaction for sulfur in melts saturated with sulfide is similar to: 8FeSsulfide +3FeOsilicate+4O2-silicat e+2O2gas ⇔4S2-silicate+2S 2gas+11FeOsulfide where the subscripts indicate the phase and O 2- represents "free" oxygens in the silicate melt. / Keywords. sulfur, solubility model, dissolution mechanism, silicate melts

The Amba Dongar complex, India, consists of calciocarbonatite, ankerite-dominated ferrocarbonatite and numerous nephelinitic bodies intruded into Late Cretaceous quartz sandstones and Deccan basalts. / Ankeritic (mottled) and calcite +/- barite +/- ankerite (multiphase) melt inclusions were observed in apatite hosted by calciocarbonatite. Mottled inclusions start melting at temperatures of 610°C and are completely molten at temperatures >800°C. The onset of melting, of multiphase inclusions, is at 680°C, and final melting occurs above 1100°C. Based an the coexistence of mottled (ankeritic) and multiphase (calcitic) inclusions, a model is proposed for the genesis of calciocarbonatite and ferrocarbonatite that calls upon separation of immiscible ankeritic and calcitic liquids from a parent carbonate melt. / Primary fluid inclusions, in apatite hosted by calciocarbonatite, and in quartz of the surrounding fenitized sandstones, record a complex evolution of aqueous fluids. The fluids (orthomagmatic) were trapped at temperatures between 1000 and 260°C, at pressures ranging from 12.5 kbars to 500 bars, respectively, while fenitizing fluids ranged in temperature 260 to 120°C, and were trapped at <500 bars pressure. The low temperature phase behaviour of all inclusions indicates that the fluids are NaCl-, KCl-bearing brines with salinities between 15 and 1.5 wt.%. The compositions of decrepitate residues indicate that early fluids were dominated by SO4 2- and HCO3-, and had low Na/Na+K ratios (<0.5) More evolved fluids display an increasing dependence an Cl -, over SO42- and HCO3 -, and are increasingly depleted in K relative to Na. / The replacement of quartz by potassium feldspar in fenites was accompanied by significant losses of Si and additions of K, Al, Ca, Ba, Fe, La, Ce, F, Rb, Sr, Y and HREE. The water-rock ratio during fenitization was &ap;722. / The Phalaborwa complex, South Africa (2060 Ma) consists of a large intrusion of pyroxenite which was later cored by phoscorite and subsequently intruded by banded (early) and transgressive (late) carbonatites. The carbonate rocks are host to 400 Mt of early bornite and late chalcopyrite ores. / Solid-vapour and solid-liquid-vapour melt inclusions are present in phoscorite and transgressive carbonatite. The solids comprise calcite, a magnesian silicate and magnetite +/- Cu-Fe sulphide, while the fluid is a saline Mg-, Fe- and S-bearing NaCl-KCl brine (≈22 wt.% NaCl eq.). Solid-vapour inclusions in phoscorite produce immiscible carbonate and silicate liquids between 680 and 800°C. Melt inclusions in transgressive carbonatites produce only one liquid between 550°C to 670°C and homogenize above 750°C. Liquid immiscibilty was important in removing Si from the carbonate-rich melt, however, silicate and carbonate rocks are not related by liquid immiscibility. Fractionation is considered to have been the dominant process in forming the complex / Two episodes of fenitization were discerned in surrounding granites and gneisses, from chemical and mineralogical changes, and were associated with emplacement of pyroxenite and carbonatite. Both events were accompanied by significant gains in K, Ca, Ba, Mg, Fe, Sr, Th and LREE, and losses in Si and Na. / The presence of Cu-bearing sulphides in solid-vapour inclusions hosted by phoscorite indicates that Phalaborwa magmas were enriched in Cu before emplacement Solid-liquid-vapour inclusions provide the first evidence of a separate magmatic aqueous fluid phase at Phalaborwa and support interpretation of a hydrothermal origin for copper mineralization in transgressive carbonatite.

The effects of fluid-rock interaction at hydrothermal conditions were investigated by a detailed study of altered quartz diorite around a single gold-bearing quartz vein, and by experiments reacting quartz diorite with water, using a single-pass, continuous-flow system. Alteration in the natural environment produced ankerite and albite distal to the vein, and albite adjacent to it. Mass balance calculations show that there was a progressive mass loss (up to 30%) towards the vein, due primarily to the dissolution of quartz. In experiments at 200$ sp circ$C, SiO$ sb2$, Na and CO$ sb2$ concentrations were controlled by the dissolution of quartz, albite and ankerite. SiO$ sb2$ reached a steady state concentration below quartz saturation. The concentrations of Ca, Mg, Fe, and Al were buffered by the precipitation of smectite, boehmite and calcite. These experiments show that a fluid may not reach quartz saturation at 200$ sp circ$C, especially if the infiltrating fluid contains dissolved components which can promote the formation of secondary, quartz-consuming minerals. In the experiment at 350$ sp circ$C, the fluid reached quartz saturation almost immediately. The secondary minerals were anorthite, chlorite, boehmite, and titanite, which formed at the expense of albite, calcite, and ankerite, thereby increasing pH and releasing Na, SiO$ sb2$, CO$ sb2$ and Al to the fluid. Quartz was completely dissolved within 500 hours. The most significant conclusion from this experiment is that the progressive loss of quartz to a quartz-undersaturated fluid increases the surface area of more resistant minerals and promotes their dissolution. Investigations of both the natural and experimental systems emphasize the importance of quartz dissolution in creating porosity, and mineral reaction kinetics in controlling wall-rock alteration.

The incorporation of divalent metals into calcite grown from low-temperature (~25°C) aqueous solutions is quantified by partition coefficients (DMe). However, DMe values are commonly derived from bulk analyses of calcite seeds, a practice that underestimates the effect of differential incorporation at symmetrically nonequivalent sectors (i.e, sector zoning) and limits the application of DMe in diagenetic and environmental studies. This study sheds some light on the effect of sector zoning on DMe by investigating the growth kinetics and morphology of calcite and the incorporation trends of divalent Cd, Mn and Zn. / Despite their similar ionic radii, Mn and Zn were found to exhibit opposite surface site preferences over a large range of chloride concentration. Zn was preferentially incorporated at geometrically less constrained surface sites of (101&macr;4) faces of calcite when coprecipitated from stepwise diluted solutions. The anomalous incorporation trend of Zn is not due to the prevalence of "large" aqueous complexes (e.g., ZnCln2-n ) but is probably controlled by its electronic configuration. The increase in Cl incorporation into calcite observed in parallel with its stepwise dilution in solution correlates also with an increase in surface roughness, suggesting that the latter is a factor contributing to impurity incorporation during crystal growth. / Investigation of the spontaneous nucleation and growth morphology of calcite from aqueous solutions prepared with different (Ca2+):( CO2-3 ) activity ratios and NaCl and NH4Cl concentrations demonstrated the potential role that the amine functional group in organic molecules may play in influencing biomineralisation and the role of the (Ca2+):( CO2-3 ) activity ratio as an abiotic habit modifier. / Cd2+, Mn2+ and Zn2+ respectively inhibited the growth of the (101&macr;0), (011&macr;2) and (0001) surfaces of calcite and were preferentially incorporated in the stabilized corresponding growth sectors. DMe values unaffected by growth kinetics approach DMe,ideal when calculated from analyses of the stabilized growth sectors and may represent equilibrium values. DCd,eq = 2235 +/- 400 at {101&macr;0}, DMn,eq = 97 +/- 6 at {011&macr;2} and DZn,eq = 190 +/- 32 at (0001). While at {101&macr;4}, D Cd and DMn reached only 500 +/- 120 and 21 +/- 3.4, respectively. These findings demonstrate that equilibrium partitioning is attained only when (1) calcite growth reaches stoichiometric saturation and "morphological equilibrium" with the aqueous solution and (2) if metal incorporation occurs entirely at surface sites offering optimal geometry and coordination for ion incorporation. A conceptual model is proposed to quantify the contribution of sector zoning on DMe values derived from bulk analyses of precipitates.