Laboratory Heating Experiments Research Articles

Heating experiments on glass inclusions in Allende (CV3) olivines: Clues to the formation conditions of chondrules?

Several pieces of the Allende CV3 chondrite were heated up to different final temperatures (1100, 1250, 1450 °C) with the aim to study glassy and glass-bearing inclusions in olivines as well as the glass mesostasis of chondrules and aggregates. The experiments were performed in a Pt-Pt90Rh10 heating stage at 1 bar pressure. The oxygen fugacity is estimated to have been between 10 −9 and 10 −10 atm at 1200 °C. The variation of the chemical composition of the heated glasses gives information concerning the behavior of the incompatible elements (with respect to the host) Al, Ca and Na. The chemical variation in the heated mesostasis glass shows that Ca exchange between the gas and condensed phases at sub-solidus temperatures can occur in a short time. Laboratory heating experiments show that glass inclusions will behave as closed systems and therefore preserved the alkalis they acquired. On the other hand, the mesostasis glass can loose them when heated to temperatures higher than 1100 °C. Evidently, the presence of Na-rich glasses, in chondrules and aggregates available to us, indicate that if there was a thermal process that did affect them, it must have been a low temperature one.

Temperature activation of organic matter and minerals during burial has the potential to sustain the deep biosphere over geological timescales

Recent research has shown that sub-surface sedimentary bacteria are widespread and that these bacteria, far from any photosynthetic energy supply, may be globally the majority of bacteria on Earth. However, it is unclear how they obtain energy over great depths and geological timescales, given that only recalcitrant organic matter would be expected to be available. We have investigated this question by conducting long term heating experiments (up to ∼500 days) with surface sediment slurries to model burial and the associated temperature increase in sub-surface sediments. In both static (thermal gradient system, 0–100 °C) and sequential heating (30–90 °C at different heating rates) experiments bacteria produced considerable concentrations of H 2, CH 4 and other hydrocarbons, as well as organic acids, thereby providing energy for themselves and other bacteria. With respect to deep sediments these compounds have previously been interpreted as being solely products of thermogenic origin, but the use of irradiated controls showed that they were predominantly of bacterial origin in the heating experiments. Even after 300 days at constant temperature (30 or 60 °C) incremental heating to 90 °C continuously stimulated bacterial activity, indicating temperature related activation of recalcitrant organic matter. Aromatization of organic matter is proposed as a source of the H 2. Addition of magnetite to slurries at 60 °C stimulated H 2 production, suggesting reduction of water coupled to oxidation of the ferrous iron in magnetite. Therefore, magnetite and related iron oxide minerals may be a potential inorganic source of H 2 in sub-seafloor sediments. Iron oxide minerals may, in addition, be involved in sufate formation, which also occurred above ∼60 °C. Deep, hot sediments from the Nankai Trough also showed increases in acetate, H 2 and sulfate, similar to the laboratory heating experiments. H 2 formation at the higher temperatures in the experiments stimulated autotrophic bacterial populations and a similar situation may occur in deep sediments.

Predicting oil and gas compositional yields via chemical structure–chemical yield modeling (CS–CYM): Part 2 – Application under laboratory and geologic conditions

We have developed a method for calculating from first principles the amounts and composition of products resulting from the thermal decomposition of a solid complex carbonaceous material. Advanced solid state analysis provides chemical and property measurements for kerogen that are used to construct a representative model of its chemical structure (CS). These chemical structural models are then coupled with a chemical yields (CY) model that is based primarily on free radical reaction mechanisms. The combined model, CS–CYM offers the ability to predict the thermal conversion of kerogen under a wide range of heating rates and temperature. Results from laboratory heating experiments are compared with those calculated with CS–CYM and show excellent agreement with a variety of bulk physical and chemical properties, open and closed system pyrolysis yields and product compositions, and open system kinetics. The reliability of CS–CYM in predicting the thermal reactions that occur under laboratory conditions indicates that we have captured a significant portion of the thermal reaction mechanisms and pathways that occur in nature. Hence, we believe that CS–CYM approach provides a better prediction of the thermal chemistry that occurs under geologic conditions than extrapolation of experimental results conducted at much higher temperature and faster heating rates where reactions, product distributions and modes of expulsion may differ significantly from basinal processes.

Temperature sensitivity of aspartic and glutamic acid racemization in the foraminifera Pulleniatina

The temperature sensitivity of amino acid racemization was quantified for aspartic acid (Asp) and glutamic acid (Glu) in tests of the foraminifera genus, Pulleniatina. More than 800 tests from the top 1 cm of a box core sample were heated for up to 499 days at four temperatures ranging from 90 to 140 °C, and were analyzed individually to evaluate the rate of amino acid racemization. The foraminifera were heated in a seawater solution, which was determined to be a superior pore fluid compared to heating in unbuffered water. A three-step screening procedure was developed to systematically identify and exclude outliers. Three empirical models were evaluated for their goodness of fit to the pattern of increasing d/ l with time for a given temperature, and to the pattern of long-term racemization under deep-sea conditions. A simple power-law function ( t ∝ D / L n ) provides the best fit to the progression of Asp and Glu d/ l under isothermal conditions, with coefficients ( n values) optimized at 3.0 for Asp and 2.4 for Glu. The data from laboratory-heating experiments were combined with data from 14C-dated Holocene Pulleniatina, which extended the evaluation of racemization kinetics to long-term rates at ambient temperatures, to calculate the Arrhenius parameters for racemization of Asp and Glu. Pulleniatina racemizes relatively slowly compared with other carbonate fossils, with E a values of 31.5±0.2 and 31.2±0.3 kcal mol −1 (131.0±0.8 and 131.9±1.3 kJ mol −1) for Asp and Glu , respectively. Errors associated with the age and paleotemperature calculations were derived using a Monte Carlo procedure that propagated the integrated effect of all input variables. Assuming a temperature uncertainty of ±0.5 °C results in an overall age error of about ±15% for Asp and 20% for Glu; realistic uncertainties in effective diagenetic temperature are about ±1 °C.

Short-term annealing characteristics of spontaneous fission tracks in zircon: A qualitative description

Thermal annealing behavior of fission tracks (FT) has not yet been documented under short-term (< 4 min) heating condition. We performed a new series of 17 laboratory heating experiments on spontaneous FTs in zircon at 550–910 °C for ∼4, 10 and 100 s under strict controls of time and temperature using a graphite furnace. FTs were almost completely annealed at 912 °C for 3.9 s, at 858 °C for 10.4 s and at 805 °C for 100.6 s. These results suggest that FTs in zircon are totally annealed at time–temperature conditions of ordinary pseudotachylyte formation (i.e. at 1000 °C for 5 s). The measured FT lengths in ∼4 and 10 s heating runs are shorter than that predicted by previous annealing kinetic models, suggesting a more rapid annealing therein. The component of short tracks in these samples increases suddenly as the annealing proceeds, which has not been observed for longer heating runs including the results of previous studies. Such rapid shortening for ∼4 and 10 s heating is caused by earlier emergence of the annealing process of segmentation.

Experimental study on hydrothermal annealing of fission tracks in zircon

Increasing temperature causes fading of fission tracks (FTs) in minerals, which has been quantified by many laboratory heating experiments in atmosphere. In order to elucidate whether or not FT annealing at hydrothermally pressured conditions is the same as that at atmospheric condition, we performed zircon heating experiments using a hydrothermal synthetic apparatus. For reliable comparison, we annealed the same zircon samples using the same temperature monitor and analyzed them using the same experimental procedure as the previous experiment at atmosphere [Yamada et al., Chem. Geol. (Isot. Geosci. Sect.) 122 (1995) 249]. The observed FT annealing characteristics are indistinguishable between atmospheric and hydrothermally heating. It is therefore implied that the annealing kinetics of FT in zircon previously established can be applied to thermal history analysis of rocks that were subjected to hydrothermal conditions in nature, such as those in fault and subduction zones.

Amino acid dating of recent intertidal sediments in the Wadden Sea, Germany

The extent of aspartic acid racemization was used to date intertidal deposits of the Swinnplate, Wadden Sea, southern North Sea. Ratios of D- and L-aspartic acid for the common cockleCerastoderma edule were related to calibrated14C ages. Cross-calibration based on laboratory heating experiments allowed to also include the soft clamMya arenaria. Based on the determined ages, a sediment accumulation rate of 30–35 cm/100 years is estimated. Although no cross-calibration was carried out forMytilus edulis, the aspartic acid racemization rates confirm the trend of the other two species. The amino acid dating technique appears to be applicable to highly dynamic tidal sedimentary environments at time scales exceeding decades.

Determination of activation energy and pre-exponential factor for individual compounds on release from kerogen by a laboratory heating experiment.

Kerogen recovered from Neogene Shinjo sediments was heated from room temperature to 800°C using a thermogravimetric analyzer (TG). Organic compounds released from kerogen during heating were trapped and analyzed by a gas chromatograph (GC)-mass spectrometer (MS). Among these compounds, benzene, hexane, toluene, phenol, heptane, and indene were selected to determine temperatures at which maximum release of these compounds occurred based on TG-MS peak profiles. This analysis was carried out using programmed heating rates (Hr) of 5, 10, 20, 30, and 60°C min-1 and five temperatures at peak tops (Tmax in absolute temperature) were determined for each compound. The plot of 1/Tmax vs. ln(Hr/(Tmax)2) for each compound showed a good straight line with a high correlation coefficiency. Accordingly, a mean activation energy (E0) and pre-exponential factor (A) at E0 for each compound on the release from kerogen were calculated, using the equation of the approximate method (or Gaussian E-distribution method) by Braun and Burnham (1987), ln(Hr/(Tmax)2) = −E0/RTmax + ln(AR/E0) where R is the ideal gas constant. The activation energies and pre-exponential factors of these compounds were in a range of 43.1 to 60.9 kcal/mol and of 3.56 × 1010 to 1.21 × 1016 s-1, respectively. The averages of the activation energies and pre-exponential factors obtained from the five kerogens had decomposition rates in order of indene > benzene, toluene > phenol > hexane, heptane, which indicated that hexane and heptane were more strongly bound to kerogen than the three aromatic hydrocarbons.

Mass-dependent fractionation of Mg, Si, and Fe isotopes in five stony cosmic spherules

We have measured with an electron microprobe the Mg, Al, Si, Ca, Ti, Mn, and Fe contents of five strongly heated stony cosmic spherules (sCS) from the South Pole water well. We have also measured the isotopic compositions of Si, and when possible of Mg and of Fe in these objects by ion microprobe. Except for iron, the measured elemental compositions are chondritic within a factor of 2. In four samples, the ratio of 57Fe/56Fe exceeds the terrestrial value by 3.5‰ to 48‰. Mass-dependent fractionation of the isotopes of Si ranges from ∼2 to ∼8 ‰/AMU in three samples. Mg is clearly fractionated in only one sample, for which δ25Mg = ∼8 ‰. The extent of mass-dependent fractionation of the isotopes and, by implication, of evaporative loss generally follows a trend Mg < Si < Fe. The trend is similar to that found in laboratory heating experiments of charges with solar composition. Although the observed isotopic inhomogeneities within some samples call into question the strict validity of the Rayleigh equation for the sCS, its approximate application to our new and to previously published results for Mg suggests that evaporative losses of greater than 40 wt.% occur rarely from sCS, and that the precursor grains of the sCS had a CM-carbonaceous-chondrite-like complement of Mg, Si, Ca, and Al. Low Fe contents relative to CM abundances could reflect an unusual precursor composition, or, more probably, losses by processes that did not fractionate isotopes, i.e., ejection of immiscible FeS and FeNi beads from the melt or rapid, complete separation and decomposition of FeS at the surface.

Mathematical modeling of stable carbon isotope ratios in natural gases

A new approach is presented for mathematical modeling of stable carbon isotope ratios in hydrocarbon gases based on both theoretical and experimental data. The kinetic model uses a set of parallel first-order gas generation reactions in which the relative cracking rates of isotopically substituted (k∗) and unsubstituted (k) bonds are represented by the equation k∗/k=(A f ∗/A f ) exp(−ΔEa/RT), where R is the gas constant and T is temperature. Quantum chemistry calculations have been used to estimate the entropic (A f∗/A f) and enthalpic (ΔEa) terms for homolytic bond cleavage in a variety of simple molecules. For loss of a methyl group from a short-chain n-alkane (≤ C 6), for example, we obtain an average ΔEa of 42.0 cal/mol and an average A f∗/A f of 1.021. Expressed differently, 13C-methane generation is predicted to be 2.4% (24‰) slower than 12C-methane generation (from a short-chain n-alkane) in a sedimentary basin at 200°C but only 0.7% (7‰) slower in a laboratory heating experiment at 500°C. Similar calculations carried out for homolytic bond cleavage in other molecules show that with few exceptions, ΔEa varies between 0 and 60 cal/mol and A f∗/A f between 1.00 and 1.04. Examination of this larger data set reveals: (1) a weak sigmoid relationship between ΔEa and bond dissociation energy; and (2) a strong positive correlation between ΔEa and A f∗/A f. The significance of these findings is illustrated by fitting a kinetic model to chemical and isotopic data for the generation of methane from n-octadecane under isothermal closed-system conditions. For a specific temperature history, the fitted model provides quantitative relationships among methane carbon isotope composition, total methane yield and methane generation rate which may have relevance to the cracking of oil-prone kerogens and crude oil. The observed variability of the kinetic reactivity of various methane source rocks highlights the need to apply and adequately calibrate such models with laboratory data for specific study areas. With this approach isotope data of natural gases can be used not only to estimate the time of gas generation in a sedimentary basin, but also to evaluate the source rock maturities at which specific accumulations were generated, and place constraints on trap charging histories.

An experimental study of organic-inorganic interactions during vitrinite maturation

Vitrinite reflectance is a parameter widely used to determine the extent of thermal maturation in sedimentary basins. Laboratory heating experiments were conducted to examine the role of aqueous fluid and sediment composition, temperature, and time on the development of vitrinite reflectance at elevated temperatures and pressures. Samples of organic-lean Middle Valley sediment containing dispersed vitrinite and Wyodak Coal composed almost entirely of vitrinite were heated in the presence of compositionally variable aqueous fluids at 225° to 375°C and 350 bars. Inorganic fluid composition was monitored as a function of time, and vitrinite reflectance was measured at the termination of each experiment. The rate of vitrinite maturation was influenced by inorganic fluid and vitrinite composition, in addition to time and temperature during the experiments. In particular, the rate at which vitrinite reflectance increased for a given temperature correlated positively with in situ H + activity. This observation suggests that acid catalyzed ionic reaction mechanisms and/or catalytically active transition metals and sulfur species play an important role in organic transformations responsible for increased vitrinite reflectance. The activity of other inorganic ionic species such as Mg ++, Ca ++, Na +, K +, and SO 4 =, and the redox state of the fluid did not influence the rate of vitrinite maturation during these experiments. Results of this study demonstrate that the geochemical environment surrounding vitrinite influences the rate of maturation. The presence of water may facilitate reaction pathways that are not readily available in dry systems. Vitrinite maturation profiles that deviate from what are considered to be “normal” trends may reflect subsurface variations in the composition of pore fluids or the presence or absence of an aqueous phase in contact with vitrinite particles. Accordingly, kinetic models that consider organic reactions responsible for increased vitrinite reflectance in terms of time and temperature alone may not accurately account for geochemical processes occurring in natural systems.

OXYGENIC KEROGENIZATION OF ASPHALTENES FROM THE DEAD SEA BASIN (ISRAEL)

A kerogen‐like material was generated during a laboratory heating experiment with asphaltenes in the presence of O2. The asphaltenes had been extracted from a floating block of Dead Sea asphalt, and heated at 100 ‐ 200 d̀C for 1–12 days. This oxygenic kerogenization reaction follows first‐order kinetics. The half‐life (t1/2) of the asphaltenes (i.e. the time in which 50% of the asphaltenes had been converted to artificial kerogen) varied from 2,300 days at Od̀C, to 2 days at 100 d̀C. Similar results were obtained with asphaltenes which had been extracted from a number of bitumen‐bearing rocks in the Dead Sea area (from the Amiaz, Ein Boqeq, Nebi Mussa and Ef'e boreholes), and also from sandstones from the Heimar and IPRG boreholes which were impregnated with heavy or asphaltic crudes.These and other results suggest that low‐temperature (≤100d̀C) oxygenic kerogenization of Dead Sea asphaltenes (with or without the mediation of meteoric waters) may be a pathway for the formation of kerogen. Asphaltenes extracted from other bituminous rocks — the La Luna limestone (U. Cretaceous, Venezuela); the Serpiano marl (M. Triassic, Switzerland); the Messel shale (Tertiary, Germany); and the Aleksinac shale (Tertiary, Yugoslavia) — underwent laboratory kerogenization in a similar fashion to the Dead Sea materials. This suggests that oxygenic kerogenization may be widespread in porous and fractured, bitumen‐bearing rocks.

Comparison of isoleucine epimerization and leaching potential in the molluskan genera Astarte, Macoma, and Mya

Amino acid racemization geochronology is widely applied to provide relative age control for carbonate fossils, however, there are significant chemical differences between genera which affect diagenetic reactions in proteins such as hydrolysis and racemization. In order to evaluate intergeneric influences on isoleucine epimerization, laboratory heating experiments were undertaken to compare the rate of epimerization of L-isoleucine to D-alloisoleucine and the potential for leaching of amino acids in three molluskan genera: Astarte, Macoma, and Mya. Shell fragments from these genera were heated for up to 816 h at 100°C to simulate aging. After 96 h of heating, the three genera experienced similar degrees of epimerization in the total acid hydrolysate, but with additional heating, Astarte and Macoma epimerized more extensively than Mya. In the free amino acid fraction, the relative rates of epimerization were reversed: Mya shows the fastest apparent rate and Astarte the slowest.All three genera lost amino acids from both the free and bound fractions by leaching, but more significantly, alloisoleucine leached proportionately much faster than isoleucine. If leaching of amino acids under natural conditions continues after significant alloisoleucine is produced, then the preferential loss of alloisoleucine over isoleucine will result in lowering of the alloisoleucine/isoleucine ratio and, therefore, lead to significant underestimation of ages. An important implication for all studies utilizing amino acid racemization geochronology is that even coeval shells of the same genus may give different alle/Ile values if different individual shells experienced different burial conditions and degrees of leaching.

The metamorphic history of eucrites and eucrite‐related meteorites and the case for late metamorphism

Abstract— We report induced thermoluminescence (TL) data for separates from three howardite, eucrite and diogenite (HED) meteorites and the Vaca Muerta mesosiderite. The results of thermal modeling of the surface of their parent body are also described. The TL sensitivities for matrix samples from the LEW 85300, 302 and 303 paired eucrites and the Bholghati howardite are lower than the TL sensitivities for the clasts, which is consistent with regolith working of the matrix in fairly mature regoliths. Within an isochemical series of HED meteorites, TL sensitivity reflects metamorphic intensity, but clast‐to‐clast variations in the TL sensitivities of the Vaca Muerta mesosiderite and clasts in the EET 87509, 513 and 531 paired howardite primarily reflect differences in mineralogy and petrology. Thermoluminescence peak temperatures indicate that all the components from the LEW 85300, 302 and 303 paired eucrites experienced a reheating event involving temperatures &gt;800 °C, which is thought to have been due to impact heating, and therefore that the event was concurrent with or postdated brecciation. The Vaca Muerta clasts are essentially unmetamorphosed, but the induced TL data indicate that the remaining howardite, eucrite, dioenite and mesosiderite (HEDM) meteorites experienced metamorphism to a variety of intensities but involving temperatures &lt;800 °C. Laboratory heating experiments show that temperatures &gt;800 °C cause a change in TL peak temperature. Feldspars from a variety of terrestrial and extraterrestrial sources show this behavior, and x‐ray diffraction and kinetic studies suggest that it is indirectly related to Al, Si disordering.Cooling rates are not consistent with autometamorphism following the initial igneous event or with heating by subsequent eruptions of lava onto the surface of the HED parent body. Instead, our thermal models suggest that the metamorphism occurred within a regolith ejecta blanket of up to a few kilometers thick, with different levels of metamorphism corresponding to different thicknesses of blanket, between essentially 0 and ∼2 km, rather than different burial depths in a regolith of uniform thickness. We argue that metamorphism occurred 3.9 Ga ago and was associated with the resetting of the Ar‐Ar system for the HED meteorites.

An assessment of a model of heat and moisture transfer in unsaturated soil

An assessment of the validity of a theoretical formulation of coupled heat and moisture in non-deforming unsaturated soil is presented. This was achieved by comparison of results from the theoretical formulation with those obtained from controlled laboratory heating experiments. The work centred on one material, a highly compacted medium sand, for which the thermophysical parameters had been established independently of the heating experiments. A solution of the theoretical formulation was achieved by means of a numerical analysis, using the finite difference method. Very good correlations were achieved between experimental and numerical results. Confidence in the ability of the theoretical formulation to describe fully the physical processes involved is thus enhanced. Application of these conclusions to problems of heat and moisture transfer in non-deforming, cohesionless soils in general is now considered possible. The numerical results were achieved using parallel computing techniques. This methodology was shown to perform very efficiently, yielding a simulation of the full range of the transient behaviour for all the experiments considered. The potential of parallel computing systems for the solution of a computationally demanding problem of this type is thus revealed. Ľexposé évalue la validité d'une formulation théorique du couplage chaleur-humidité dans un sol non déformant non saturé. Cette évaluation repose sur la comparaison des résultats de la formulation théorique aux résultats d'essais contrôlés de chauffage en laboratoire. Les travaux se sont concentrés sur un matériau: un sable moyen fortement tassé, dont les paramètres thermo-physiques avaient été établis indépendamment des essais de chauffage. On a résolu la formulation théorique à ľaide analyse numérique en utilisant la méthode des différences finies. On a obtenu de triés bonnes corrélations entre les valeurs expérimentales et les valeurs calculées. On a maintenant une plus grande certitude que les formules théoriques peuvent décrire complétement les processus physiques qui entrent en jeu. On pense qu'il est maintenant possible d'appliquer ces conclusions en général aux problèmes du transfert de chaleur et d'humidité dans des sols non déformants sans cohésion. On a eu recours à des techniques de calcul parallèles pour obtenir les valeurs théoriques. Cette méthodologie s'est avérée triès efficace et a permis de simuler la gamme complète des comportements transitoires dans tons les essais envisagés. Cela montre qu'il est possible d'utiliser des systèmes de calcul parallèles pour résoudre un problème de ce type, qui est fort exigeant sur le plan du calcul.

A PARALLEL COMPUTING SOLUTION OF HEAT AND MOISTURE TRANSFER IN UNSATURATED SOIL

New parallel software for the analysis of coupled heat and moisture transfer in unsaturated soil is developed. The model, written in a two-dimensional polar co-ordinate formulation, is based on a finite difference self-implicit method. The code is programmed in FORTRAN with message passing libraries PARMACS and executed on a ‘Paramid’ parallel supercomputer. The validity of the parallel code by comparison of simulation results with experimentally measured values obtained from a laboratory heating experiment is examined. An assessment of the algorithm's performance on a large network of processors is also explored. It was found that the simulation results compared very well with the experimental measurements. The efficiency of the parallel code was also revealed leading to the conclusion that the algorithm was highly efficient in nature. The new parallel code was also found to be more efficient when dealing with larger problems requiring more finite difference nodal points, on a larger network of processors.

Geochemical alteration of pyrochlore group minerals; betafite subgroup

Other| October 01, 1996 Geochemical alteration of pyrochlore group minerals; betafite subgroup Gregory R. Lumpkin; Gregory R. Lumpkin Australian Nuclear Science and Technology Organisation, Materials Division, Menai, N.S.W., Australia Search for other works by this author on: GSW Google Scholar Rodney C. Ewing Rodney C. Ewing University of New Mexico, United States Search for other works by this author on: GSW Google Scholar American Mineralogist (1996) 81 (9-10): 1237–1248. https://doi.org/10.2138/am-1996-9-1022 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Gregory R. Lumpkin, Rodney C. Ewing; Geochemical alteration of pyrochlore group minerals; betafite subgroup. American Mineralogist 1996;; 81 (9-10): 1237–1248. doi: https://doi.org/10.2138/am-1996-9-1022 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Relatively low-temperature, secondary alteration is common in samples of metamict betafite and initially proceeds by the substitution mechanisms ANaYF → A□Y□, ACaYO → A□Y□, and ACaxO → A□X□ Alteration is usually accompanied by hydration (∼10-15 wt% H2O) together with minor increases in Al, K, Mn, Fe, Sr, and Ba. At this stage, U and Th remain relatively unaffected by the alteration process. Once Na and F are removed and the Ca content drops below about 0.2-0.3 atoms per formula unit (∼2.5-3.5 wt% CaO), betafite bulk compositions fall within the stability field of liandratite + uranpyrochlore + rutile (or anatase), thus promoting major element mobility (including Th, U, Pb, and B-site cations), incipient recrystallization, and partial dehydration. New phase assemblages occur as a function of decreasing bulk U content in the order liandratite + rutile, liandratite + uranpyrochlore + rutile, and uranpyrochlore + rutile. The same phase assemblages also occur in laboratory heating experiments performed in an inert atmosphere at 1000 °C. Up to 20-30% of the original amount of U may be lost during severe secondary alteration and recrystallization of betafite. Part of this U is retained by liandratite crystallized in the adjacent host rock. Loss of radiogenic Pb results from both long-term diffusion and secondary alteration, aided by radiation damage-induced volume expansion and microfrac-turing. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Age of Pre-late-Wisconsin Glacial-Estuarine Sedimentation, Bristol Bay, Alaska

Pleistocene glacial-estuarine sediment deposited in an intertidal environment of northeastern Bristol Bay, southwestern Alaska, was dated using a variety of approaches, including infrared stimulated and thermoluminescence (IRSL and TL) techniques. Analysis of modern and14C-dated Holocene tide-flat mud demonstrates that the bulk of sediment in this environment is reset by solar radiation, thereby lending confidence to ages obtained from similar Pleistocene deposits by luminescence techniques. IRSL seems to be especially well suited for dating, with resolution on time scales of <10,000 yr. The ages of tide-flat mud of the Nushagak Formation, derived from the Ahklun Mountains to the northwest of Bristol Bay, and of Halfmoon Bay drift, derived from the Alaska Peninsula to the southeast, suggest contemporaneous glacial-estuarine deposition related to independent glacial source areas about 75,000–80,000 yr ago. This age is consistent with other geochronological data that indicate a pre-late-Wisconsin and post-substage-5e age, including nonfinite14C ages, a lack of interglacial indicators, and Old Crow tephra (∼140,000 yr) atop the drift, normal paleomagnetic inclinations, and amino acid (isoleucine) epimerization ratios (aIle/Ile). AIle/Ile ratios inPortlandia arctica(0.052 ± 0.003) from a marine-lag horizon at South Naknek beach, which separates Halfmoon Bay drift above from older glacial-estuarine drift below, are only slightly higher than inMya truncata(0.041 ± 0.007) from last-interglacial Pelukian deposits at Nome. As laboratory heating experiments show that the two genera epimerize at similar rates, these data imply correlation of the marine lag at South Naknek beach with Pelukian deposits. Hence, glaciers on the Alaska Peninsula experienced major pre-late-Wisconsin advances both before and after the last interglaciation. Shells reworked into Halfmoon Bay drift yield aIle/Ile ratios of 0.028 ± 0.005 forPortlandiaat Second Point and 0.027 ± 0.001 forHiatella arcticaat Etolin Point. Together with assumptions about the postdepositional temperature history, these ratios indicate that the shells are at least 55,000 yr, and probably closer to ∼90,000 yr, although the uncertainty in this age estimate is broad. The amino acid and luminescence data converge on an age between about 75,000, and 90,000 yr, late during oxygen-isotope stage 5, for a major ice advance far beyond late-Wisconsin limits.

Geosynthesis of organic compounds: III. Formation of alkyltoluenes and alkylxylenes in sediments

Heating ortho-undecyltoluene with an acidic clay catalyst and hexamethylbenzene as a methylating agent resulted in the formation of hydrocarbon mixtures containing undecyltoluenes and dimethylundecylbenzenes. At short heating times, methylation at ring positions 4 and 5 forming 2,4- and 2,5-dimethylundecylbenzenes occurred predominantly, while at longer heating times, methylation and isomerization/transalkylation reactions resulted in higher concentrations of more stable alkylxylene isomers. The distributions of undecyltoluenes and dimethylundecylbenzenes in two mature crude oils were also compared to those formed in the laboratory heating experiments. One crude oil derived from carbonate sediments was shown to have a predominance of ortho-undecyltoluene and 2,4- and 2,5-dimethylundecylbenzenes, similar to the reaction products formed from short-term laboratory heating experiments. The other crude oil, derived from clastic sediments containing abundant clays, was shown to contain a mixture of the undecyltoluenes and dimethylundecylbenzenes similar to the reaction products generated from longer-term heating experiments. These observations suggest that alkylxylenes in crude oils result from sedimentary methylation of ortho-alkyltoluenes, as the ortho-alkyltoluenes isomers are often the most abundant isomers in low maturity sediments. The initial products formed under kinetic control then undergo clay-catalyzed isomerization/transalkylation reactions to yield mixtures with higher abundances of the more thermodynamically stable isomers.

The effect of maturity on the relative abundances of cadalene and isocadalene in sediments from the Gippsland Basin, Australia

Two sedimentary sequences from the Gippsland Basin, Australia have been analysed for cadelene and 3-isopropyl-1,6-dimethylnaphthalene (isocadalene). The relative abundance of these compounds changes with increasing depth (maturity) of the sediments and is associated with an increase in the abundance of 1,6-dimethylnaphthalene relative to other dimethylnaphthalene isomers. These changes have been attributed to dealkylation and isomerization reactions of cadalene in the sediments because analogous changes have been demonstrated in laboratory heating experiments in which cadalene was converted into a mixture containing mainly 1,6-dimethylnaphthalene and isocadalene when heated in the presence of aluminum smectite. The ratio of isocadalene and cadalene may be a useful maturity parameter for some sediments and crude oils which contain these compounds.