Calcite Core Research Articles

The impact of grain size heterogeneity on the performance of low-salinity waterflooding in carbonate formations

Low-salinity waterflooding (LSWF) is a proven enhanced oil recovery method for many target fields. The efficiency of this technique depends on both operational factors as well as reservoir settings such as heterogeneity. Micro-scale rock heterogeneity can critically affect the performance of LSWF as it governs the flow path of the injected water, oil-brine-rock contacts and trapping mechanisms, thus sweep efficiency. However, there exist lack of experimental data and detailed understanding of the effect of grain size heterogeneity on LSWF performance. To address this gap, a systematic series of coreflooding experiments were designed using artificial calcite core plugs to investigate the effect of micro-scale heterogeneity, such as grain size and grain heterogeneity, on oil recovery factor by high-salinity waterflooding (HSWF) and LSWF. High salinity formation water, seawater, and diluted seawater were used as injection brines. For this purpose, two relevant grain size ranges (a) from 38 to 75 µm and (b) from 75 to 125 µm were considered to fabricate homogeneous plugs. Heterogeneous core plugs were made by mixing (a) and (b) grain samples. The artificial cores were initialized with the formation brine and crude oil, and aged to restore wettability. Then a set of coreflooding experiments were performed using the mentioned brine samples, in both secondary and tertiary modes by considering a shut-in period.The results reveal that grains size (or micro-scale) heterogeneity has a significant negative impact on both HSWF and LSWF performance and reduces the oil recovery efficiency of HSWF by approximately 15%. This is primarily attributed to the bypassing and entrapment of oil in heterogeneous cores. Moreover, the results show that regardless of the grain size, LSWF in homogeneous plugs result in almost similar recovery factors. The key finding of this study is that LSWF in heterogeneous plugs is more effective and recovers almost 10% more oil compared to HSWF. This is attributed to wettability alteration as indicated by the reduction of water relative permeability endpoint and delay of breakthrough time. The improvement in oil recovery was consistent with the results obtained from the flotation tests and dynamic IFT measurements. However, although LSWF outperforms HSWF in heterogeneous cores, the ultimate recovery factor becomes less in these plugs. Besides, superior performance of secondary flooding was observed compared to tertiary mode for LSWF, especially in cores with heterogeneous grain sizes, leading to almost 10% higher recovery factor.The novel insights gained through this research help identify the proper targets for LSWF in terms of small-scale heterogeneity and can be potentially applied in other water-based EOR techniques.

Influence of Host Marble Rocks on the Formation of Intrusive Alkaline Rocks and Carbonatites of Sangilen (E. Siberia, Russia)

The study of the O and C isotope composition of calcite from nepheline syenites, ijolites and carbonatites of the Chik intrusion and the intrusions of the Erzin–Tarbagatay group of Sangilen (Eastern Siberia, Russia) showed derivation from alkaline melts enriched with a carbonate component from the host marbleized sedimentary rocks. The calculations showed that about 40% of the initial mass of carbonates involved in the interaction with silicate melts have remained after decarbonation. During the assimilation of the carbonate, an oxygen isotope exchange took place between the residual carbonate material and the silicate phase. Crystallization products of such hybrid magmas are carbonatite veins, calcite-rich nepheline rocks and their pegmatites with a calcite core.

Bimineralic Middle Triassic ooids from Hydra Island: Diagenetic pathways and implications for ancient seawater geochemistry

AbstractThis study reports on the occurrence of formerly bimineralic, ancient, calcitic coated grains, including half‐moon ooids and micritic to microsparitic ooids in the Middle Anisian (Middle Triassic) grain flow from Hydra Island (Greece), and discusses their importance for palaeoseawater geochemical interpretations. The bi‐partite fabric in half‐moon ooids resulted from early pre‐compaction rapid dissolution of both aragonitic cortices and the more‐soluble components of the high‐Mg calcite core in the primary ooid, as well as the collapse of less‐dissolved remnants to the bottom of the oomouldic cavity which was later filled with calcite cement. Zonal, micritic to microsparitic ooids formed through intracortical dissolution of primary aragonite lamellae which were later replaced by (micro)sparry calcite. At the same time, recrystallization of original high‐Mg calcite layers to micrite occurred. The stratigraphic occurrence of the investigated bimineralic ooids conforms to the global distribution of carbonate ooids through the Middle Triassic period. The cementation history of the grain flow hosting coated grains is well documented by the paragenetic sequence established in half‐moon ooids which predicts that calcite precipitated under burial (shallow to deep) conditions. The main control on precipitation of bimineralic ooids was the Mg/Ca ratio of seawater, with lesser influences from seawater temperature, salinity and ρCO2 which promoted the degree of carbonate saturation, leading to higher precipitation rates for both aragonite and high‐Mg calcite. The studied bimineralic ooids represent a change in the global carbonate factory during the Middle Triassic in response to secular variations in the chemistry of palaeoseawater.

Impact of media coating on simultaneous manganese removal and remineralization of soft water via calcite contactor

The aim of this study was to investigate the negative impact of a newly-formed manganese (Mn)-layer on calcite dissolution in the long-term operation of a calcite contactor. Simultaneous removal of Mn and remineralization of soft water in an up-flow calcite contactor was conducted and led to a progressive loading of Mn into the calcite matrix. The calcite contactor demonstrated high Mn removal; however, the hardness release decreased from 32 to 20 mg CaCO3 L−1 after 600 h of operation on a high Mn concentration (5 mg L−1) feed. For an elevated Mn concentration (i.e. 5 mg Mn L−1) in the feed water, the coated layer was mainly composed of Mn which inhibits the mass transfer from the calcite core to the liquid phase. The superficial layer was identified as 5.2% Mn oxides (MnOx) by X-ray photoelectron spectroscopy (XPS). Therefore, it is postulated that Mn removal starts with an ion exchange sorption reaction between soluble Mn2+ from aqueous phase and Ca2+ from the CaCO3 matrix which is followed by a slow recrystallization of MnCO3 into MnO2. On the other hand, when the Mn content in the feed water was lower (i.e. 0.5 mg Mn L−1), a considerably lower amount of MnOx was detected on the coated media. For all the examined conditions, the formation of this coating improved Mn removal due to the autocatalytic nature of the adsorption/oxidation of dissolved manganese by MnOx. A mechanistic model based on calcite dissolution and the progressive formation of a MnO2 layer was implemented in PHREEQC software to predict the reduction in hardness release expected in long-term operation. The model was calibrated with experimental data and resulted in realistic breakthrough curves. In order to accurately predict the pH of the effluent stream, a slow-rate recrystallization of MnCO3 into MnO2 was implemented (compared to the fast precipitation of MnO2 or the absence of MnO2 formation).

Chemical mechanisms of dissolution of calcite by HCl in porous media: Simulations and experiment

Abstract We use a pore-scale dissolution model to simulate the dissolution of calcite by HCl in two different systems and compare with experiment. The model couples flow and transport with chemical reactions at the mineral surface and in the fluid bulk. Firstly, we inject HCl through a single channel drilled through a solid calcite core as a simple validation case, and as a model system with which to elucidate the chemical mechanisms of the dissolution process. The overall dissolution rate is compared to a corresponding experiment. Close agreement with experimental and simulated dissolution rates is found, which also serves to validate the model. We also define a new form of effective Damkohler number which can be obtained from simulated chemical distributions, and show how this gives a more precise measure of the balance of transport and reaction. Secondly, we inject HCl into a Ketton carbonate rock core at high flow rate, which leads to wormhole formation, and compare to experiment. The simulation matches the experimental mass dissolution rate extracted from the micro-CT images, and predicts the resulting morphological changes reasonably well. The permeability change though is greater in the experiment than in the simulation, and this is shown to be due to more elongated wormhole formation in experiment. Possible reasons for this are discussed, including uncertainties in diffusion coefficients, and calcite density variations and micro-porosity in the Ketton grains. The distribution of chemical species from the simulation then permits a detailed understanding of the rate-controlling mechanisms at work, including the relative importance of the H+–calcite and H2CO3–calcite dissolution pathways.

Replacement of Calcite (CaCO3) by Cerussite (PbCO3).

The mobility of toxic elements, such as lead (Pb) can be attenuated by adsorption, incorporation, and precipitation on carbonate minerals in subsurface environments. Here, we report a study of the bulk transformation of single-crystal calcite (CaCO3) into polycrystalline cerussite (PbCO3) through reaction with acidic Pb-bearing solutions. This reaction began with the growth of a cerussite shell on top of calcite surfaces followed by the replacement of the remaining calcite core. The external shape of the original calcite was preserved by a balance between calcite dissolution and cerussite growth controlled by adjusting the Pb2+ concentration and pH. The relation between the rounded calcite core and the surrounding lath-shaped cerussite aggregates was imaged by transmission X-ray microscopy, which revealed preferentially elongated cerussite crystals parallel to the surface and edge directions of calcite. The replacement reaction involved concurrent development of ∼100 nm wide pores parallel to calcite c-glide or (12̅0) planes, which may have provided permeability for chemical exchange during the reaction. X-ray reflectivity measurements showed no clear epitaxial relation of cerussite to the calcite (104) surface. These results demonstrate Pb sequestration through mineral replacement reactions and the critical role of nanoporosity (3% by volume) on the solid phase transformation through a dissolution-recrystallization mechanism.

A eukaryote assemblage intercalated with Marinoan glacial deposits in South Australia

Composite hematite–silica structures recovered from a siltstone bed in the Elatina Formation of South Australia include (1) sub-circular to whorl-shaped forms, (2) elongate to half-moon-shaped forms and (3) and lozenge-shaped forms locally linked into chains. They range from 200 to 500 µm in diameter and are interpreted as eukaryote tests. Evidence for internal etching of a calcite core of some tests indicates that at least some of the hematite–silica fabrics were acquired through replacement. Carbon isotope values of −20‰ δ 13 C are suggestive of precipitation by microbial activity, and imply a change in ambient fluid chemistry associated with a pH reduction. The tests occur within sandstone beds that were deposited on a tidally modulated braidplain during the Marinoan glaciation at the end of the Cryogenian. The quartz grains in the sandstone sample lack the typical textures (surface striae, internal fractures or irregular grain boundaries) expected for glacially transported material. Thus, on textural grounds we argue that the eukaryote tests represent a proglacial ecosystem during a late Cryogenian snowball Earth event. Supplementary material: Video files of digital X-ray tomographs (μCT) in the longitudinal and transverse planes are available at: https://doi.org/10.6084/m9.figshare.c.2209723 .

Preparation and formation mechanism of ZnO supported hollow SiO2 nanoparticle by an interfacial reaction through micropores

Abstract A ZnO supported hollow SiO 2 (ZnO/h-SiO 2 ) composite nanoparticle with a strong yellow emission has been prepared using the Zn(OAc) 2 precursor with a catalytic amount of NaOH. A single nano-sized ZnO having defective oxygen homogeneously dispersed on the SiO 2 shell. How this desired structure was achieved is discussed by comparison of three carrier nanoparticles of h-SiO 2 , calcite core–SiO 2 shell (CS), and dense SiO 2 (d-SiO 2 ) which have several spaces where the precursor solution was trapped. That is, h-SiO 2 has a micropore/hollow interior/interspace among the particles, CS has a micropore/interspace, and d-SiO 2 only has an interspace. On the CS carrier, similar ZnO single nanoparticles were dispersed and the ZnO/CS composite showed a strong yellow emission. On the d-SiO 2 carrier, aggregated ZnO nanoparticles were localized on part of the SiO 2 surface. Consequently, the existence of micropores is the key structure to obtain the ZnO nanoparticle dispersion. During penetration of the ZnO precursor into the hollow interior through the micropores, dissociated Zn 2+ ions adsorb on the SiO 2 surface. Hydroxide ions (OH − ) of NaOH then make contact with the adsorbed Zn 2+ on the SiO 2 shell to form ZnO. The micropore could work as a mold to prevent excess growth of the ZnO. The strong emission was also contributed by defects among the siloxane bonds in the SiO 2 shell due to its light scattering ability. The obtained ZnO/h-SiO 2 can be useful as a yellow phosphor of a white light emitting diode (LED).

A critical analysis of calcium carbonate mesocrystals.

The term mesocrystal has been widely used to describe crystals that form by oriented assembly, and that exhibit nanoparticle substructures. Using calcite crystals co-precipitated with polymers as a suitable test case, this article looks critically at the concept of mesocrystals. Here we demonstrate that the data commonly used to assign mesocrystal structure may be frequently misinterpreted, and that these calcite/polymer crystals do not have nanoparticle substructures. Although morphologies suggest the presence of nanoparticles, these are only present on the crystal surface. High surface areas are only recorded for crystals freshly removed from solution and are again attributed to a thin shell of nanoparticles on a solid calcite core. Line broadening in powder X-ray diffraction spectra is due to lattice strain only, precluding the existence of a nanoparticle sub-structure. Finally, study of the formation mechanism provides no evidence for crystalline precursor particles. A re-evaluation of existing literature on some mesocrystals may therefore be required.

Theoretical and experimental insights on the true impact of C12TAC cationic surfactant in enhanced oil recovery for heavy oil carbonate reservoirs

Herein were performed theoretical and experimental studies about the performance of the cationic surfactant docecyltrimethylammonium chloride (C12TAC) on the enhancement of the oil recovery factor (ORF) from heavy oil-impregnated calcite cores. The interaction energies between C12TAC and either of some molecules present in oil such as an asphaltene (Asph), a resin (Res) or an octanoic acid, and among these and a calcite surface were theoretically determined within the framework of the Density Functional Theory. Experimental analysis based on Fourier transform infrared (FTIR) spectroscopy as well as 1H and 13C nuclear magnetic resonance (NMR) spectra were made on the heavy crude oil recovered by spontaneous imbibition tests from the calcite cores. Results evidenced that, by using brine containing 2.9ppm×105ppm of total ion, it were obtained ORF values of 8.9% at 90°C temperature and atmospheric pressure, and 36% at 150°C temperature and 145psi pressure. Meanwhile with the inclusion of C12TAC at the same temperature and pressure conditions used in pure brine experiments, noticeable increased ORF values of 36% and 44%, respectively, were found. From the molecular interaction strengths between C12TAC and the fractions present in the oil, it has been evidenced also the oil disaggregation as well as the subsequent oil viscosity reduction, but finally without significant wettability alteration of the calcite core. A saturated, aromatics, resins and asphaltenes (SARA) component analysis revealed a significant change in composition moving from their original values of 20.6, 21.0, 35.5 and 22.9%, respectively, to 31.0, 23.2, 28.7 and 17.2%, accordingly. The lack of wettability alteration of C12TAC in such process is due to a weak interaction in C12TAC:Res and C12TAC:Asph molecular pairs followed by a clear diminishment of Res and Asph fractions in SARA, which reveals Asph and Res cannot be removed from rock surface. The quantum nature of the supramolecular assembling between the cationic surfactant and oil molecules is described by molecular orbitals analysis.

Modeling and Simulation of Alternative Injections of CO2 and Water into Porous Carbonate Formations

Water alternating gas (WAG) technique is used in the petroleum industry to inject carbon dioxide (CO2) into underground formations either for sequestration or enhanced oil recovery (EOR) processes. CO2 injection causes reactions with formation brine or aquifer and produces carbonic acid, the acid dissolves calcite and changes flow behavior significantly. Modeling and investigating effects of CO2 injection into carbonate formations during WAG processes, investigating parameters related to chemical reactions between reservoir rock and injecting fluid and also better understanding of the process theory for future experiments are the most important goals of this paper. To achieve these experimental data were used. Changes of output calcium concentration from a calcite core sample during three WAG cycles have been studied in laboratory works. The sample is modeled as a medium consisting of a set of capillary pipes and two pore size distribution models are used. Plug flow model and mass conservation law are used for modeling and Darcy law and Hagen-Poiseuille equation are also used to determine characteristics of the porous model. The model is built for linear, miscible and one-dimensional flow. The results show that experimental and model data coincide well in the first and second cycles of both porous models however; they are not coincided in the third cycle. It is because of precipitation and dissolution that cause permeability alternations. Results of the two porous models are compared also.

The Characterization Of CaCO3 in a Geothermal Environment: A Sem/Tem-Eels Study

AbstractMineralization of microbial biomass is a common phenomenon in geothermal habitats, but knowledge of the structure of the minerals formed in these environments is limited. A combination of spectroscopic, microscopic, and stable isotopic methods, as well as the chemical analysis of spring water, were employed in the present study to characterize calcium carbonate minerals deposited in filamentous cyanobacterial mats in different locations of La Duke hot spring, a circumneutral thermal feature near the north entrance of Yellowstone National Park, Montana, USA. Calcite was the primary crystalline mineral phase associated with biofilm-containing deposits closest to the source of the spring and the suspended microbial biomass in a pool further from the source. The carbonate minerals at all sites occurred as aggregated granules, ~2 μm in diameter, in close association with the microbial biomass. Only in the deposits closest to the source were the granules organized as laminated structures interspersed with microbial biomass. The calcium carbonate grains contained two distinct regions: a dense monolithic calcite core and a porous dendritic periphery containing organic matter (OM). Electron energy loss spectroscopy (EELS) indicated that the voids were infilled with OM and carbonates. The EELS technique was employed to distinguish the source of carbon in the organic matter and carbonate mixture. The studies of carbon isotope compositions of the calcium carbonates and the saturation indices for calcite in the spring waters suggest that processes (abiotic vs. biotic) controlling the carbonate formation may vary among the sampling sites.

Fabrication of solid and hollow carbonate apatite microspheres as bone substitutes using calcite microspheres as a precursor

Spherical carbonate apatite (CO3Ap) microspheres approximately 1 mm in diameter were fabricated by granulation of calcium hydroxide around a core followed by carbonation and phosphatization through dissolution-precipitation reaction. CO3Ap microspheres with high uniformity could not be achieved without using a core. Solid CO3Ap microspheres were obtained using a calcite core whereas hollow CO3Ap microspheres were obtained using a NaCl core. The obtained microsphere was identified as B-type CO3Ap by Fourier transform infrared analysis and the carbonate content was approximately 7-8 wt% regardless of the type of core used for sample preparation. The mechanical strength of both the solid and hollow CO3Ap microspheres was sufficient for practical use as a bone substitute.

Fabrication of Size-Tunable TiO2 Tubes Using Rod-Shaped Calcite Templates

Titania tubes with tunable wall thickness were produced by the sol-gel reaction of titanium(IV) n-butoxide in the presence of rod-shaped calcite particles that act as templates. A shell of amorphous titania was deposited around the calcite particles by sol-gel synthesis. The titania was crystallized to the anatase or rutile phase by sintering at different temperatures, and then acid etching was used to remove the calcite core, leaving hollow titania tubes. The influences of several parameters on the final particle formation were investigated, including calcite templates, surfactant, the method of adding reagents, and catalyst. The average width of the prepared titania tubes ranges from nearly 100 nm to 1 microm, with wall thickness ranging from approximately 70 to 300 nm. A possible growth mechanism of the titania tubes is presented. The ability to control titania tube size and crystal structure is important for photocatalysis, photovoltaics, and other applications. The fabrication approach presented is applicable to form tubes of other oxide materials by sol-gel synthesis.

Characteristics and composition of fly ash from Canadian coal-fired power plants

Fly ashes were collected from the electrostatic precipitator (ESPs) and/or the baghouse of seven coal-fired power plants. The fly ashes were sampled from power plants that use pulverized subbituminous and bituminous feed coals. Fly ash from bituminous coals and limestone feed coals from fluidized-bed power plant were also sampled. The fly ashes were examined for their mineralogies and elemental compositions. The fly ashes from pulverized low sulfur coals are ferrocalsialic, those from high sulfur coals are ferrosialic and the fly ashes from the fluidized bed coals are ferrocalcic. The concentrations of As, Cd, Hg, Mo, Ni, and Pb in fly ash are related to the S content of the coal. Generally, those feed coals with a high S content contain higher concentrations of these elements. The concentrations of these elements are also greater for baghouse fly ash compared to ESP fly ash for the same station. The S content of fly ash from high S coal is 0.1% for pulverized ESP fly ash and 7% for baghouse fly ash from the fluidized bed, indicating that most of the S is captured by fly ash in the fluidized bed. The baghouse fly ash from the fluidized bed has the highest content of Cd, Hg, Mo, Pb, and Se, indicating that CaO, for the most part, captures them. Arsenic is captured by calcium-bearing minerals and hematite, and forms a stable complex of calcium or a transition metal of iron hydroxy arsenate hydrate [(M 2+) 2Fe 3(AsO 4) 3(OH) 4·10H 2O] in the fly ash. Most elements in fly ash have enrichment indices of greater than 0.7 indicating that they are more enriched in the fly ash than in the feed coal, except for Hg in all ESP ashes. Mercury is an exception; it is more enriched in baghouse fly ash compared to ESP. Fly ash collected from a station equipped with hot side ESP has a lower concentration of Hg compared to stations equipped with cold side ESP using feed coals of similar rank and mercury content. Fly ash particles from fluidized bed coal are angular and subangular with cores of quartz and calcite. The quartz core is encased in layer(s) of calcium-rich aluminosilicates, and/or calcium/iron oxides. The calcite core is usually encased in an anhydrite shell.

Comparative effect of Mn(II) and Fe(III) as activators and inhibitors of the adsorption of other heavy metals on calcite

The adsorption of Cu(II) and Cr(III), and their mixtures with Mn(II) and Fe(III) on marble dust has been investigated. Total cation concentrations was 0.05 M, and marble dust (+95% calcite) content in solution was 10 g L −1 (equivalent to a nominal concentration 0.1 M in CaCO 3). Mn(II), slightly adsorbing on the active centres of calcite, blocks them and slows down the adsorption of Cr(III). However, somehow it catalyses the precipitation of CuCO 3under such conditions. Fe(III) accelerates the adsorption of Cr(III) and slows down the adsorption of Cu(II). Fe(III) adsorbs very fast as hydroxide in a porous layer around the original marble particle, and can act as ion exchange medium for another hydroxide-precipitating heavy metal, e.g. Cr(III). If the co-adsorbing cation precipitates as carbonate, e.g. Cu(II), then the Fe(III)-hydroxide layer seems to act as an enlarged molecular diffusion path between the bulk phase and the surface of the calcite core, slowing down Cu adsorption. These antagonistic blocking/catalytic effects of Mn(II) and Fe(III) at high heavy metal concentrations are very interesting for the selective separation of cations, for example from partially concentrated industrial waste waters, and has never been reported before.

Synthesis and characterization of CaCO 3@SiO 2 core–shell nanoparticles

This paper reports on a new type of composite nanoparticle consisting of a calcite (CaCO 3) core and a silica (SiO 2) shell. The cubic core–shell particles were created using a surface precipitation procedure using cubic calcite as seeds on which a silica layer was grown. Moreover, the calcite of core–shell nanoparticles can be dissolved leaving silica shells. The characterizations of composite and silica shells were made by using TEM, BET, XRD, IR, XRF, and TGA. The results show that the silica layer was continuously coated on the surface of CaCO 3 core with a thickness of about 5 nm. After coated with silica shells, the thermal stability decreases for its reaction with silica. The shape and size of pores of silica shells were similar to that of the CaCO 3 cores.

An investigation of closure temperature of the biotite Rb-Sr system: The importance of cation exchange

Factors controlling closure in the biotite Rb-Sr system were investigated in a detailed study of an amphibolite-facies metacarbonate from the central Swiss Alps. Oxygen isotope data suggest that the rock cooled as a closed system. Calcite-dolomite thermometry temperatures of ∼450°C to 500°C and feldspar thermometry temperatures of ∼300°C to 400°C provide evidence of extensive Ca-Mg and Na-K exchange during cooling. Biotite in the sample is 90 mol.% phlogopite and has high Rb (∼900 ppm) compared to Sr (∼0.3 ppm), giving precise Rb-Sr ages. Carefully separated and sized phlogopite shows a range of Rb-Sr ages that do not simply decrease with grain size as predicted by current models of closure temperature. Rb-Sr ages decrease from 18.1 Ma to 16.6 Ma with a decrease in mean grain diameter from 1.16 mm to 0.74 mm, but grains with mean diameter of 0.54 mm show an increase again to 17.6 Ma. This contrasts with Ar-Ar data for single phlogopites, which do show a decrease in age with decreasing grain size. The Rb-Sr age pattern is due to Rb-loss during cooling, which is most pronounced in the finest fraction. The phlogopites are restricted to a 2-cm-thick layer in calcite marble; 87Sr/ 86Sr of the calcite decreases away from the phlogopite band over 4 cm, indicating that the calcite was moving towards Sr-isotope equilibration with the phlogopites over this distance and that the phlogopite was not equilibrating with an “infinite reservoir.” Ion microprobe traverses across grains of different minerals reveal systematic core-rim variations in major and trace element concentrations. In particular, Sr decreases from calcite core to rim, but increases from core to rim in K-feldspar, whereas Rb decreases from core to rim in phlogopite but also increases from core to rim in K-feldspar. These gradients are interpreted as indicating the direction of transport of elements during cooling as a result of cation exchange reactions; calcite and phlogopite were sources for Sr and Rb, respectively, whereas K-feldspar acted as a sink for both elements. This chemical equilibration was taking place at the same time as isotopic equilibration during cooling, and was equally important in controlling the apparent ages recorded by the mica grains. In contrast, closure temperature calculations for geochronological systems based on classic Dodson-type models assume parent and daughter element concentrations are homogeneous across grains and do not change with time, only isotopic exchange is modeled. Closure in mica Rb-Sr systems will depend both on the factors that control isotopic exchange (grain size, mode, 87Sr diffusion coefficients) and those that control chemical exchange (grain size, mode, Rb and Sr diffusion coefficients, Rb and Sr contents of phases and their partition coefficients).

Pseudomorphs of impact spherules from a Cretaceous-Tertiary boundary section at Shell Creek, Alabama

During a biostratigraphic investigation of a Cretaceous-Tertiary boundary section at Shell Creek, Alabama dark green spherules with distinctive morphologies were serendipitously found in the lower Danian basal Clayton sand. With the exception of in the upper 30 cm, they were found throughout the sand, upwards from an erosional contact separating the sand from the underlying upper Maastrichtian Prairie Bluff Chalk. Additional studies revealed that the spherules exhibit the morphologies of splash-form tektites and that their original composition has been altered to a smectite external layer with a calcite core. The presence of spherules with similar morphologies and size distribution in K-T boundary sections at Beloc (Haiti) and in Arroyo El Mimbral (northeastern Mexico) leads to the deduction that the Shell Creek spherules represent altered impact ejecta. Impact glasses, Ni spinels and shocked mineral grains were searched for extensively, but intense alteration and dilution by detrital quartz sand have made it unlikely that such supporting evidence of an impact origin will be found at this site. Nonetheless, the large sizes of the microtektite pseudomorphs, which are well over 1 mm in diameter, and abundance (the total fluence is about 2 g/cm 2), makes this location an important corroboration of a proximal site for the K-T impact.

500,000-Year Stable Carbon Isotopic Record from Devils Hole, Nevada

The record of carbon-13 (delta(13)C) variations in DH-11 vein calcite core from Devils Hole, Nevada, shows four prominent minima near glacial terminations (glacial-interglacial transitions) V to II. The delta(13)C time series is inversely correlated with the DH-11 oxygen isotope ratio time series and leads it by as much as 7000 years. The delta(13)C variations likely record fluctuations in the delta(13)C of dissolved inorganic carbon of water recharging the aquifer. How such variations are transported 80 kilometers to Devils Hole without obliteration by water-rock reaction remains an enigma. The record may reflect (i) global variations in the delta(13)C of atmospheric CO(2) and, hence, the delta(13)C of continental biomass or (ii) variations in extent and density of vegetation in the southern Great Basin. In the latter case, delta(13)C minima at 414, 334, 246, and 133 thousand years ago mark times of maximum vegetation.