Lynda B. Williams

Boron isotope geochemistry during diagenesis; Part I, Experimental determination of fractionation during illitization of smectite

Experiments were performed to measure the isotopic fractionation of boron between illite/smectite (I/S) clay minerals and water as a function of temperature (300° and 350°C) and degree of illitization. Corresponding changes in the oxygen isotopes were monitored as an indication of the approach to equilibrium. The kinetics of the B-isotope exchange follows the mineralogical restructuring of smectite as it recrystallizes to illite. An initial decline in δ11BI/S occurs when the I/S is randomly ordered (RO). The δ11BI/S values reach a plateau during R1 ordering of the I/S, representing a metastable condition. The greatest change in δ11BI/S is observed during long-range (R3) ordering of the I/S when neoformation occurs. Values of δ11BI/S measured on the equilibrium reaction products were used to construct a B-isotope fractionation curve. There is a linear correlation among data from these experiments and 1100°C basaltic melt-fluid fractionation experiments (Hervig and Moore, 2000) that can be extrapolated to include adsorption experiments at 25°C (Palmer et al., 1987). Unlike other stable isotopic systems (e.g., oxygen) there is no mineral-specific fractionation of B-isotopes, but rather a coordination dependence of the fractionation. Under diagenetic conditions B is predominantly in trigonal coordination in fluids but substitutes in tetrahedral sites of silicates. The preference of 10B for tetrahedral bonds is the major fractionating factor of B in silicates.

Isotopic and elemental partitioning of boron between hydrous fluid and silicate melt

Abstract The fractionation of B and its isotopes between aqueous fluid and silicate melt has been studied from 550 to 1100 °C and 100-500 MPa. Fluid-melt partition coefficients are <1 for basaltic melt and >1 for rhyolite melt. This shows that B is not always strongly extracted from melts into hydrous fluids. Boron isotopic fractionation is large compared with the carbon and oxygen stable isotopic systems (especially at high T) and is most simply explained by differences in coordination (trigonal vs. tetrahedral) among coexisting phases. Combined with earlier measurements on illite-water (300- 350 °C), B isotopic fractionation defines a temperature-dependent trend from 300 to 1100 °C. Because of the large magnitude and apparent low sensitivity to bulk composition, B isotopic fractionation can be readily applied to studies of diagenesis, hydrothermal alteration of planetary bodies, subduction- zone processing and arc magma generation, and magma chamber evolution.

Nitrogen isotope geochemistry of organic matter and minerals during diagenesis and hydrocarbon migration

The magnitude of isotopic variations between organic and inorganic nitrogen was examined in samples from three stacked hydrocarbon reservoirs in the Fordoche Field (Louisiana Gulf Coast Basin, USA). Measurements were made of δ 15N in kerogen, bitumen, oil, formation water, and fixed-NH4 extracted from mudstones, nonproductive sandstones, and productive sandstones. Nitrogen isotope fractionation occurs because 14N is released preferentially to 15N from organic molecules during thermal maturation. Released 14N goes into solution, or may be adsorbed by minerals, leaving crude oil enriched in 15N. Diagenetic clay minerals (e.g., illite) commonly form in the temperature range of hydrocarbon generation, and NH4+ may be fixed in clay interlayers with an isotopic ratio similar to that of the migrating fluids. Results indicate that the influence of organic matter on mineral δ 15N depends on the timing of authigenic mineral formation relative to fluid migration. The average δ 15N of kerogen (3.2 ± 0.3‰) and fixed-NH4 from mudstones (3.0 ± 1.4) is similar, while bitumen increases from +3.5 to +5.1‰ with depth. In deep reservoir sandstones (>100°C), the δ 15N of crude oil averages +5.2 ± 0.4‰, similar to the δ 15N of bitumen in the proposed source rocks. Formation waters are 14N-enriched with an average δ 15N of −2.2 ± 2.6‰. Fixed-NH4 δ 15N values lie between that of the oil and water. The average δ 15N of fixed-NH4 is 3.0 ± 1.2‰ in productive sandstones, and 0.2 ± 2.4‰ innonproductive sandstones. In the shallower reservoir sandstones (<90°C) fixed-NH4 is apparently not influenced by the presently associated fluids. Productive and nonproductive sandstones have distinctly low average δ 15N values (−1.2 ± 0.8‰), yet crude oil (+11.1 ± 0.3‰) and water (+3.8 ± 0.1‰) have been 15N-enriched by ∼6‰ relative to the deeper reservoirs. This suggests that the present fluids migrated into the reservoir after authigenic illite had formed. Fluids become enriched in 15N during migration and the amount of enrichment may be a function of the amount of interaction with argillaceous sediments.

Evaluation of the medicinal use of clay minerals as antibacterial agents

Natural clays have been used to heal skin infections since the earliest recorded history. Recently, our attention was drawn to a clinical use of French green clay (rich in Fe‐smectite) for healing Buruli ulcer, a necrotizing fasciitis (‘flesh-eating’ infection) caused by Mycobacterium ulcerans. These clays and others like them are interesting as they may reveal an antibacterial mechanism that could provide an inexpensive treatment for this and other skin infections, especially in global areas with limited hospitals and medical resources. Microbiological testing of two French green clays and other clays used traditionally for healing identified three samples that were effective at killing a broad spectrum of human pathogens. A clear distinction must be made between ‘healing clays’ and those we have identified as antibacterial clays. The highly adsorptive properties of many clays may contribute to healing a variety of ailments, although they are not antibacterial. The antibacterial process displayed by the three identified clays is unknown. Therefore, we have investigated the mineralogical and chemical compositions of the antibacterial clays for comparison with non-antibacterial clays in an attempt to elucidate differences that may lead to identification of the antibacterial mechanism(s). The two French green clays used to treat Buruli ulcer, while similar in mineralogy, crystal size, and major element chemistry, have opposite effects on the bacterial populations tested. One clay deposit promoted bacterial growth whereas another killed the bacteria. The reasons for the difference in antibacterial properties thus far show that the bactericidal mechanism is not physical (e.g. an attraction between clay and bacteria), but by a chemical transfer or reaction. The chemical variables are still under investigation. Cation exchange experiments showed that the antibacterial component of the clay can be removed, implicating exchangeable cations in the antibacterial process. Furthermore, aqueous leachates of the antibacterial clays effectively kill the bacteria. Progressively heating the clay leads first to dehydration (200°C), then dehydroxylation (550°C or more), and finally to destruction of the clay mineral structure (∼900°C). By identifying the elements lost after each heating step, and testing the bactericidal effect of the heated product, we eliminated many toxins from consideration (e.g. microbes, organic compounds, volatile elements) and identified several redox-sensitive refractory metals that are common among antibacterial clays. We conclude that the pH and oxidation state buffered by the clay mineral surfaces is key to controlling the solution chemistry and redox-related reactions occurring at the bacterial cell wall.

What Makes a Natural Clay Antibacterial

Natural clays have been used in ancient and modern medicine, but the mechanism(s) that make certain clays lethal against bacterial pathogens has not been identified. We have compared the depositional environments, mineralogies, and chemistries of clays that exhibit antibacterial effects on a broad spectrum of human pathogens including antibiotic resistant strains. Natural antibacterial clays contain nanoscale (<200 nm), illite-smectite and reduced iron phases. The role of clay minerals in the bactericidal process is to buffer the aqueous pH and oxidation state to conditions that promote Fe(2+) solubility. Chemical analyses of E. coli killed by aqueous leachates of an antibacterial clay show that intracellular concentrations of Fe and P are elevated relative to controls. Phosphorus uptake by the cells supports a regulatory role of polyphosphate or phospholipids in controlling Fe(2+). Fenton reaction products can degrade critical cell components, but we deduce that extracellular processes do not cause cell death. Rather, Fe(2+) overwhelms outer membrane regulatory proteins and is oxidized when it enters the cell, precipitating Fe(3+) and producing lethal hydroxyl radicals.

AMMONIUM SUBSTITUTION IN ILLITE DURING MATURATION OF ORGANIC MATTER

Pierre shale samples from a thin stratigraphic zone within the contact aureole of the lampro-phyric Waisen dike record changes due to thermal effects that are not influenced by detrital differences. Analyses of fixed-NH4, mineralogy, and Rock-Eval pyrolysis indicators of organic matter maturity provide new insights on the fixation process. Fixed-NH4 increases with the quantity of authigenic illite formed from illite/smectite, but the maximum fixation per unit of illite formed occurs within the “oil window” where thermal breakdown of organic matter is rapid. Extrapolation of these results to the burial diagenetic regime supports the potential use of fixed-NH4 as an indicator of organic maturity and hydrocarbon migration pathways.

The influence of organic matter on the boron isotope geochemistry of the gulf coast sedimentary basin, USA

Large variations in the boron isotopic composition of sedimentary environments make boron an attractive monitor of fluid/rock interactions during diagenesis. Studies of B in marine sediments have shown that preferential adsorption of 10B on clay minerals leaves pore waters enriched in 11B. During diagenesis, clay minerals recrystallize and incorporate 10B into the mineral structure (Spivack, A.J., Palmer, M.R., Edmond, J.M., 1987. The sedimentary cycle of the boron isotopes. Geochim. Cosmochim. Acta 51, 1939–1949). This process should cause a depletion of B in the pore water with an increase in the δ11B. In the Gulf Coast sedimentary basin (USA), however, there is a general increase in B-content of formation waters (Land, L.S., Macpherson, G.L., 1992. Origin of saline formation waters. Cenozoic Section, Gulf of Mexico Sedimentary Basin. Geochim. Cosmochim. Acta 76, 1344–1362; Moldovanyi, E.P., Walter, L.M., 1992. Regional trends in water chemistry, Smackover Formation, Southwest Arkansas: Geochemical and physical controls. AAPG Bull. 76, 864–894.) and a decrease in δ11B with depth. This suggests that another source of 10B exists in deep basinal environments. We know that oil reservoir brines are commonly enriched in boron (Collins, A.G., 1975. Geochemistry of Oilfield Waters. Elsevier, New York, p. 496.), therefore this study examines organic matter as a possible source of boron during thermal maturation. Samples of water, oil, and cored sediments were collected from three stacked hydrocarbon reservoirs in the Gulf of Mexico sedimentary basin at a depth of 3500–4350 m. Extraction of boron from organic matter (oil and kerogen) was done by Parr Bomb volatilization, with mannitol used as a B-complexing agent. The isotope ratios were measured using negative thermal ionization and compared to in situ analyses using secondary ion mass spectrometry. The δ11B values of pore filling clays in sandstone reservoirs is −2±2‰. The B-content of the clay averages 144 ppm. Oil field waters show a range in B-content from 8–85 ppm and δ11B values from +28 to +37‰, increasing from the lowest to the uppermost reservoir. There is an apparent 11B-enrichment of fluids with progressive migration through clay-rich sediments. Very little B (ppb) was found in the oil, but kerogen extracted from the oil source rock (Sassen, R., 1990. Lower Tertiary and Upper Cretaceous source rocks in Lousiana and Mississippi: implications to Gulf of Mexico crude oil. AAPG Bull. 74, 857–878.) contains significant B (140 ppm) with a δ11B of −2±2‰, similar to the pore filling clay minerals in the sandstones. While kerogen comprises only ∼2% of the sedimentary basin, its influence can be significant if B with distinctly low δ11B is released over a specific temperature interval during thermal maturation. The release of B from organic compounds could cause the observed regional 10B enrichment in waters deep in the Gulf Coast basin.

Diagenesis of ammonium during hydrocarbon maturation and migration, Wilcox Group, Louisiana, U.S.A.

Abstract As organic matter matures, N is released in the reduced form as NH4+ and substitutes for K+ in clay minerals. As oil forms, an abundance of N is generated, coincident with the formation of authigenic illite. This study suggests that NH4+ released from organic matter during maturation migrates through sandstones together with the pore fluids and volatile phases generated from the migrating crude oil. This causes anomalously high NH4+ substitution in authigenic illites. Fixed-NH4 concentrations in illite are a product of both hydrocarbon maturation and migration. Concentrations of fixed-NH4 from organic-rich mudstones in two Wilcox Group oil fields show a general increase with depth, coincident with increasing organic maturation. Reservoir sandstones containing crude oils have fixed-NH4 concentrations as much as two times those of the mudstones, when values are normalized to the quantity of clay minerals. Fixed-NH4 concentrations in the gaseous portion of the reservoir are three times as high. Two out of five sandstones from a dry hole adjacent to a producing oil field contain anomalously high (>2σ above mean) concentrations of fixed-NH4 with respect to total organic centent. This, together with a high Production Index and Hydrogen Index in those units, suggests that migrating hydrocarbons may have influenced the fixed-NH4 concentrations.

CHEMICAL AND MINERALOGICAL CHARACTERISTICS OF FRENCH GREEN CLAYS USED FOR HEALING

The worldwide emergence of infectious diseases, together with the increasing incidence of antibiotic-resistant bacteria, elevate the need to properly detect, prevent, and effectively treat these infections. The overuse and misuse of common antibiotics in recent decades stimulates the need to identify new inhibitory agents. Therefore, natural products like clays, that display antibacterial properties, are of particular interest.The absorptive properties of clay minerals are well documented for healing skin and gastrointestinal ailments. However, the antibacterial properties of clays have received less scientific attention. French green clays have recently been shown to heal Buruli ulcer, a necrotic or ‘flesh-eating’ infection caused by Mycobacterium ulcerans. Assessing the antibacterial properties of these clays could provide an inexpensive treatment for Buruli ulcer and other skin infections.Antimicrobial testing of the two clays on a broad-spectrum of bacterial pathogens showed that one clay promotes bacterial growth (possibly provoking a response from the natural immune system), while another kills bacteria or significantly inhibits bacterial growth. This paper compares the mineralogy and chemical composition of the two French green clays used in the treatment of Buruli ulcer.Mineralogically, the two clays are dominated by 1Md illite and Fe-smectite. Comparing the chemistry of the clay minerals and exchangeable ions, we conclude that the chemistry of the clay, and the surface properties that affect pH and oxidation state, control the chemistry of the water used to moisten the clay poultices and contribute the critical antibacterial agent(s) that ultimately debilitate the bacteria.

Abundant ammonia in primitive asteroids and the case for a possible exobiology

Carbonaceous chondrites are asteroidal meteorites that contain abundant organic materials. Given that meteorites and comets have reached the Earth since it formed, it has been proposed that the exogenous influx from these bodies provided the organic inventories necessary for the emergence of life. The carbonaceous meteorites of the Renazzo-type family (CR) have recently revealed a composition that is particularly enriched in small soluble organic molecules, such as the amino acids glycine and alanine, which could support this possibility. We have now analyzed the insoluble and the largest organic component of the CR2 Grave Nunataks (GRA) 95229 meteorite and found it to be of more primitive composition than in other meteorites and to release abundant free ammonia upon hydrothermal treatment. The findings appear to trace CR2 meteorites’ origin to cosmochemical regimes where ammonia was pervasive, and we speculate that their delivery to the early Earth could have fostered prebiotic molecular evolution.