GRANTEE: STATE UNIVERSITY OF NEW YORK, STONY BROOK

Department of Earth and Space Sciences

Stony Brook, New York 11794-2100

GRANT: DE-FG02-94ER14449

TITLE: High Precision Radiometric Dating of Sedimentary Materials

PERSONS IN CHARGE: G.N. Hanson (515-632-8210; Fax 516 632 8240; E-mail gil@pbisotopes.ess.sunysb.edu) and W.J. Meyers

Objectives: To develop field, petrographic and geochemical criteria to allow high precision U-Pb dating of calcretes or caliches (paleosol calcite) relatively enriched in uranium at paleo-exposure surfaces within rapidly deposited sequences of carbonate and clastic rocks.

Project Description: The ultimate goal was to obtain radiometric ages for caliche with uncertainties of three million years or less to date the times of sedimentation. Such uncertainties are much less than those for most epoch or period boundaries within the Paleozoic or early Mesozoic. For this dating it is essential that the duration of soil development be less than the uncertainty in the ages, that is less than about one million years. Caliche is commonly developed in both terrestrial clastic sequences and on marine carbonates. The two most obvious applications for precise ages on erosion surfaces and times of sedimentation are: 1) providing more precise duration's for unconformity bound genetic packages in sequence stratigraphy and 2) more precisely calibrating the geologic time scale for the early Mesozoic and Paleozoic.

Results: We began our studies on caliches developed in paleosol horizons in the Triassic fluvial clastic rocks of the of the Hartford Basin, Connecticut, and in the Pennsylvanian-Permian marine carbonates rocks of the Sacramento Mountains, New Mexico and the Central Basin Platform of the Permian Basin of West Texas. The results are very encouraging. It is possible to obtain U-Pb ages for paleosol calcite with uncertainties of less than 3 years. As a result it is possible to more precisely constrain the age of Pennsylvanian-Permian boundary and the duration of Pennsylvanian-Permian cycles. In these studies the U-Pb age for a sample is based on an isochron using at least five aliquots of the sample. Each aliquot is 10's to 100's of milligrams.

U-Pb ages for Caliche Developed in Triassic Clastic Fluvial Rocks. The paragenesis of calcite samples from the Triassic New Haven Arkose (Norian) was determined using optical and cathodoluminesence microscopy, uranium fission track analysis, as well as trace element and stable isotope geochemistry. Three generations of calcite were found. The first-generation micritic calcite and second-generation blocky calcite cement have characteristics consistent with their being paleosol calcite. The third generation blocky calcite is a later diagenetic calcite.

Massive first generation micritic calcite gives a U-Pb isochron age of 212 ±2 (2 sigma) Ma. This 212 Ma age and the stratigraphic relations for thissample are in excellent agreement with the ages proposed by Gradstein et al. (1994) for the upper Norian boundary of 209.6 ± 4.1 Ma and the lower Norian boundary of 220.7 ± 4.4 Ma.

The U-Pb data for two samples of first generation micrite in rhizoliths with about 15% insoluble residues give ages of 7 ± 66 Ma and 20 ± 36 Ma. These results suggest that relatively recent events disturbed the U-Pb system of these detrital rich samples, perhaps due to redistribution of U during weathering. The U-Pb data for a sample of pure third generation blocky calcite cement in a rhizolith yields an age of 81 ± 11 Ma. This age suggests that this later diagenetic blocky calcite precipitated during the late Cretaceous. These results suggest that by carefully selecting samples using optical and geochemical characteristics U-Pb dating of pure calcite found in paleosol horizons in clastic sediments can be used to precisely date the times of sedimentation and later diagenesis.

U-Pb ages for Caliche Developed in Pennsylvanian-Permian Marine Carbonates. Seven samples of paleosol calcite from six horizons across the Pennsylvanian-Permian boundary in both the Sacramento Mountains and the Central Basin Platform of the Permian Basin of West Texas give U-Pb ages consistent with their stratigraphic positions. These data place the age of the Carboniferous-Permian boundary at 300.2 ± 1.1 Ma and the Pennsylvanian-Permian boundary at 301.2 ± 1.4 Ma. The uncertainty in the boundary ages is low as a result of the high precision of the U-Pb ages and because the boundaries are well constrained by fusulinid biostratigraphy in the two areas studied. Based on 46 cycles between two precisely dated paleosol samples at 298 ± 1.4 Ma and 306 ± 2.6 Ma the average duration for each cycle is 161 ± 58 Ky. This period is unlike any recognized orbital forcing parameter. Thus, either the cycles are not of the same duration or other factors may be affecting the period of the cyclicity.

GRANTEE: STATE UNIVERSITY OF NEW YORK, STONY BROOK

Department of Applied Mathematics and Statistics

Stony Brook, New York 11794-3600

GRANT: DE-FG02-92ER14261

TITLE: Medial Axis Analysis of Porous Media

PERSON IN CHARGE: W. B. Lindquist (516-632-8361; Fax 516-632-8491; E-mail lindquis@ams.sunysb.edu)

Objectives: The goal of this work is to develop a package of software tools to extract quantitative information on the geometry of the void and grain microstructure of rock starting from high resolution, three dimensional (e.g. X-Ray microtomographic or laser scanning confocal microscopic) images.

Project Description: High resolution (1 to 5 micron), three dimensional images of rock samples are segmented to provide specific grain/pore identification for each voxel in the image. Appropriate transforms are then applied to the segmented, digitized images to produce medial axis (co-dimension two) representations of the void (or grain) structure. The geometric properties of the medial axis are investigated statistically to develop predictive stochastic distributions characteristic of real, three dimensional porous media geometry.

Results: A computer package, 3DMA, written in C, has been developed which takes as input three dimensional tomographic images and, under user menu control, performs a variety of stochastic and geometric analyses of the image. Major analyses include: segmentation of the object; construction of medial axis of either void or grain phase; and computations of pore size distribution, distribution of connected volumes, geometric tortuosity of shortest pathways through the medial axis, two point correlation measurements, and specific surface area. During this last year major analyses have been extended to include coordination number (ie. branching ratio), path length and throat size distributions. To date this package has been applied to analyse three dimensional images of sandstones (Berea, Fontainebleau), basalts, glass bead packs, carbonates, and cellulose fiber networks.

GRANTEE: STATE UNIVERSITY OF NEW YORK, STONY BROOK

Department of Earth and Space Sciences

Stony Brook, New York 11794-2100

GRANT: DE-FG029-ER14633

TITLE: Surface Chemistry of Pyrite: An Interdisciplinary Approach

PERSONS IN CHARGE: Martin A.A. Schoonen (Department of Geosciences, SUNY-Stony Brook; 516-632-8007; Fax 516-632-8240; E-mail mschoonen@notes.cc.ess.sunysb.edu, Web http://sbmp97.ess.sunsb.edu), and Daniel R. Strongin (Department of Chemistry, Temple University; 215-204-7119; Fax 215-204-1532; E-mail dstrongi@nimbus.ocis.temple.edu)

Objective: The objective of this research program is to understand the surface chemistry of pyrite, the most abundant metal sulfide on Earth. Determining the charge development on pyrite surfaces and evaluating the interaction of the pyrite surface with an array of simple inorganic and organic molecules are the immediate goals. Through a combination of macroscopic observations and observations at the atomic/molecular level new insights are gained that will lead to a better understanding how pyrite reacts in a range of chemical environments.

Project Description: The charge development, interaction with inorganic and organic constituents, and the reactivity of pyrite are being investigated. Emphasis is placed on integrating macroscopic information from low-temperature techniques such as electrophoresis and microscopic information from modern surface science techniques that are used in the ultra high vacuum (UHV) environment. To obtain the former information, electrophoretic mobility measurements are used to determine the charge development on the pyrite surface as a function of solution composition. The sorption kinetics and equilibrium partitioning between pyrite and selected inorganic species are studied under anaerobic conditions in batch experiments. Using model, atomically clean "as-grown" surfaces of pyrite, electron spectroscopies in UHV are used to understand the atomic composition and the nature of the functional groups on pyrite after exposure to the aqueous solutions. A second set of UHV experiments investigate the interaction of environmentally relevant molecules on pyrite in vacuum and are designed to elucidate the effects of surface defects on the surface reactivity of pyrite. The integration of the UHV and low-temperature studies will provide a complete picture of the type of surface functional groups at the pyrite surface, their acid-base behavior, and interaction with selected aqueous constituents.

Results: In this first year two issues have been addressed by combining and integrating aqueous geochemistry studies and vacuum-based surface science studies. First, by using electrophoresis the charge development onto pyrite surfaces as a function of pH, H₂S, and Fe²⁺(aq) concentration has been determined. Second, using UHV surface science techniques, the structure of the binding sites of pyrite that we believe control this charge development behavior have been investigated. This UHV work has led to a novel way to characterize the binding sites of pyrite by using adsorbed xenon as a probe of mineral structure. A brief summary of results obtained so far from this research is presented below.

Electrophoresis Experiments of Pyrite. Electrophoresis have been used to determine the charge development onto pyrite surfaces as a function of pH, H₂S, and Fe²⁺(aq) concentration. Starting material in all these studies have been carefully cleaned, crushed Huanzala pyrite. It is well known that the charge development on oxides is largely controlled by the nature of the cation in the solid; hence, additional experiments were conducted on a range of pyrite analogs and related metal sulfides. In brief, the charge development on pyrite and related metal sulfides is strongly dependent on Fe(aq) or the addition of H₂S. Without added Fe(aq) or H₂S, pyrite is negatively charged at pH values above pH 1 to 2. With increasing Fe(aq) the zetapotential-pH curves show that in acid solutions the charge may be reversed. Addition of H₂S leads to a negative zetapotential over the entire pH range. Studies with other metal sulfides show also a isoelectric point at low pH in the absence of free metal ions. These studies suggest that the charge development of sulfides, unlike oxides, are only weakly dependent on the nature of the metal cation. To our knowledge this is the most comprehensive study on charge development on metal sulfides and pyrite in particular to date.

Surface Science Studies of Pyrite. Photoemission of adsorbed Xenon (PAX) has been used as a probe of the short range order of pyrite. This technique has yielded results that start to elucidate where on the mineral surface adsorbates bind and react. Recent research has investigated the striated (100) crystallographic plane of pyrite. Studies have shown that adsorbates, such as CH₃OH, H₂O, and H₂S shows preferentially to defect sites on FeS₂(100). These defects, at least in part, are thought to be sulfur anion vacancy sites. CH₃OH and H₂O show primarily molecular adsorption and desorption, but H₂S shows much more chemistry. Electron spectroscopies show that H₂S dissociates on defect sites on pyrite to form S-H and H surface species. Atomic H resulting from this dissociation diffuses form the dissociation site to the stoichiometric FeS₂ surface where it binds stongly to surface S forming SH functional groups. These studies are suggesting that there are intrinsically stable defect sites of high reactivity on pyrite that can dissociate adsorbate and then release fragments onto the less reactive stoichiometric surface. Guevremont J. Strongin D. R., and Schoonen M.A.A. (1997) Effects of surface imperfections on the binding of CH₃OH and H₂O on FeS₂(100): using adsorbed Xe as a probe of mineral surface structure. Surface Science in press. Schoonen M.A.A., Xu Y., and Strongin D.R. (1997) An introduction to geocatalysis. Journal of Exploration Geochemistry in press.

GRANTEE: STATE UNIVERSITY OF NEW YORK, STONY BROOK

Department of Earth and Space Sciences

Stony Brook, New York 11794-2100

GRANT: DE-FG0294-ER14455

TITLE: Micromechanics of Failure in Brittle Geomaterials

PERSONS IN CHARGE: Teng-Fong Wong (516-632-8212; Fax 516-632-8240; E-mail wong@seism1.ess.sunysb.edu) and Joanne T. Fredrich (Sandia National Laboratory)

Objectives: The objectives of this project are to provide a fundamental understanding of the effects of pore geometry and cementation, damage state, and load path on the deformation and failure mode of brittle porous and nonporous geologic materials by measurement of mechanical behavior under high pressure and deviatoric stress, quantitative microstructural characterization of pristine and deformed samples, and theoretical analysis.

Project Description: Knowledge of the failure behavior of rocks is important for several energy-related applications, including reservior engineering, oil and gas exploration and production, underground disposal of nuclear waste, and drilling technology. The experimental investigation will provide a detailed understanding of the micromechanical processes associated with the brittle failure of geomaterials and includes triaxial compression and extension tests following various load paths. Tests are conducted to various stages of failure and include measurement of strain and acoustic emission. The micromechanical failure process are further elucidated and characterized quantitatively using light microscopy, laser scanning confocal microcopy, and scanning electron microscopy. Work focuses on porous carbonate and siliciclastic rocks, although related experiments are also being performed on low-porosity crystalline rocks in order to study completely the effect of certain parameters. The results of the laboratory tests and microstructural studies are used to guide analyses using fracture mechanics and continuum plasticity theories.

Results: (1) Effect of water on brittle strength and compaction behavior of sandstone. Experimental techniques were developed to monitor the evolution of volumetric strain and acoustic emission activity in nominally dry and saturated samples under triaxial loading. Preliminary experiments were conducted on the Berea and Darley Dale sandtones. Both hydrostatic and nonhydrostatic compaction were significantly enhanced in the presence of water. Shear-enhanced compaction in nominally dry samples initiated at stress levels higher than those in water-saturated samples. Significant weakening due to the presence of water was observed in Darley Dale sandstone. Our data and other published data suggest that water-weakening effect in sandstone is controlled by subcritical crack growth in feldspar grains. Further experiments are being conducted to test this hypothesis. (2) Development of dilatancy and micromechanics of failure in anisotropic rock. Triaxial compression experiments were conducted on Four-mile gneiss, and microstructure of the deformed samples were studied in some detail using optical and scanning electron microscopy. The onset of dilatancy, initiation and propagation of wing cracks, and final failure by crack coalescence were controlled strongly by the biotite foliation in the gneiss. Previously damage mechanics models for brittle failure were formulated and applied primarily to isotropic rocks. We generalized these models to take into consideration anisotropic of dilatancy and wing crack nucleation. The anisotropic model is in good agreement with our gneiss data. (3) Preliminary experiments were conducted on failure of porous carbonates. Some of the deformed sample were prepared for quantitative characterization of damage.