成岩过程中碳酸盐-二氧化碳平衡体系的热力学模拟

碳酸盐矿物的溶解与沉淀在沉积岩成岩作用中具有非常重要的意义,在与之有关的地层流体中,CO_2组分受到了广泛关注.本文选取CaCO_3(方解石)-H_2O-CO_2与CaMg(CO_3)2(白云石)-H_2O-CO_2体系作为研究对象,利用热力学方法在假设体系达到平衡的前提下对100～7000m深度范围的温度压力条件下的CaCO_3-H_2O-CO_2与CaMg(CO_3)2-H_2O-CO_2体系中各组分的平衡浓度([H~+]、[Ca~(2+)]、[Mg~(2+)]……)进行了数值模拟,并对不同埋藏深度的温度压力下方解石、白云石的溶解与沉淀,流体对CO_2的吸收与释放,pH变化等相关地质过程进行了讨论.研究表明:随埋深增加,体系的pH值越来越趋于酸性,但碳酸盐并没有持续溶解,而是呈先溶解后沉淀的趋势.对比CaCO_3-H_2O-CO_2体系中碳酸盐的溶解量和沉淀量,可将小于700m的深度范围定义为碳酸盐溶解带,700～2500m的深度范围定义为补偿带,大于2500m的深度范围定义为沉淀带.对比两个体系的固体相体积变化发现,含CO_2的溶蚀性流体虽然不能在白云岩地层中溶出较灰岩地层更多的阳离子,但会产生更高的孔隙度.     

Compositional homogeneity in low-temperature chlorites

Electron microprobe analyses of small chlorite grains (10–20 μm width) in diagenetically altered rocks from three deep wells and a series of outcrop samples ranging in maximum age from 4 to 1300 Ma show an increasingly smaller range in grain-to-grain composition as a function of the average age of the specimens in a series. The scatter of composition in a sample in each series changes little with depth despite present day, or estimated maximum temperature differences of 70° C to 100° C from top to bottom. Comparison of these chlorite compositions with those published for geothermal and metamorphic rocks formed under conditions of 300–450° C indicates that the homogeneity of chlorite composition is a function of both the temperature of equilibration and its duration. Apparently total homogenization of Fe/(Fe+Mg), Al/(Al+Mg+Fe) content will occur after 10¹⁰ years at low temperatures. Simple calculations indicate that a slow process such as solid state diffusion could be responsible for the eventual homogenization of mineral grain composition to reach chemical equilibrium after phase equilibrium has been reached.     

On simulating reactive chemical transport with dominating precipitated species controlling chemical equilibrium

We present an approach developed to compute chemical equilibrium and its corresponding reactive chemical transport when dominating precipitated species (DPS) exist. In computing chemical equilibrium, most models take the concentrations or activities of component species and precipitated species as the master variables. However, when the amount of a precipitated species is much larger than those of other species, small computational errors on this DPS concentration might introduce large errors on the concentrations of other species and would cause non‐mass‐conserved numerical results. To deal with the existence of DPS, we pick as master variables the concentration change, rather than the concentration, of DPS to compute chemical equilibrium. Since the concentration changes of DPS will no longer be much larger than the concentrations of other species in determining equilibrium, our approach is able to provide correct numerical results. We also employ the modified total analytical concentrations, rather than the total analytical concentrations, of aqueous components as the dependent variables in presenting and solving corresponding transport equations. Several examples are given to reveal the numerical problems associated with DPS and to demonstrate the success of our approach. This revised version was published online in July 2006 with corrections to the Cover Date.     

A new chlorite geothermometer for diagenetic to low-grade metamorphic conditions

The evolution of chlorite composition with temperature (and pressure) serves as basis to a number of chlorite chemical thermometers, for which the oxidation state of iron has been recognised as a recurrent issue, especially at low temperature (T). A new chlorite geothermometer that does not require prior Fe³⁺ knowledge is formulated, calibrated on 161 analyses with well-constrained T data covering a wide range of geological contexts and tested here for low-T chlorites (T < 350 °C and pressures below 4 kbar). The new solid-solution model used involves six end-member components (the Mg and Fe end-members of ‘Al-free chlorite S’, sudoite and amesite) and so accounts for all low-T chlorite compositions; ideal mixing on site is assumed, with an ordered cationic distribution in tetrahedral and octahedral sites. Applied to chlorite analyses from three distinct low-T environments for which independent T data are available (Gulf Coast, Texas; Saint Martin, Lesser Antilles; Toyoha, Hokkaido), the new pure-Fe²⁺ thermometer performs at least as well as the recent models, which require an estimate of Fe³⁺ content. This relief from the ferric iron issue, combined with the simple formulation of the semi-empirical approach, makes the present thermometer a very practical tool, well suited for, for example, the handling of large analytical datasets—provided it is used in the calibration range (T < 350 °C, P < 4 kbar).     

A method for computing multicomponent chemical equilibria based on equilibrium constants

The equilibrium activities and concentrations of N chemical species in a multicomponent system may be calculated given M independent equilibrium constants relating these species and N-M mass balances. These N equations are solved by Newton-Raphson iteration. Where initial concentration estimates are poor or the number of species large, an additional curve crawler technique introduces components by stepwise infinitesimal increments.     

Fluid inclusions in sedimentary and diagenetic systems

Some of the major problems in sedimentary geology can be solved by using fluid inclusions in sedimentary and diagenetic minerals. Important fluids in the sedimentary realm include atmospheric gases, fresh water of meteoric origin, lake water, seawater, mixed water, evaporated water, formation waters deep in basins, oil, and natural gas. Preserving a record of the distribution and composition of these fluids from the past should contribute significantly to studies of paleoclimate and global-change research, is essential for improving understanding of diagenetic systems, and provides useful information in petroleum geology. Applications of fluid inclusions to sedimentary systems are not without their complexities. Some fluid inclusions exposed to natural conditions of increasing temperature may be altered by thermal reequilibration, which results in stretching, or leakage and refilling, of some fluid inclusions. Similarly, overheating in the laboratory can also cause reequilibration of fluid inclusions, so fluid inclusions from the sedimentary realm must be handled carefully and protected from overheating. Natural overheating of fluid inclusions must be evaluated through analysis of the most finely discriminated events of fluid inclusion entrapment, fluid inclusion assemblages (FIA). Consistency in homogenization temperatures within a fluid inclusion assemblage, consisting of variably sized and shaped inclusions, is the hallmark of a data set that has not been altered through thermal reequilibration. In contrast, fluid inclusion assemblages yielding variable data may have been altered through thermal reequilibration. If a fluid inclusion assemblage has not been altered by thermal reequilibration, its fluid inclusions may be useful as geothermometers for low- and high-temperature systems, or useful as geobarometers applicable throughout the sedimentary realm. If a fluid inclusion assemblage has been altered partially by thermal reequilibration, techniques for distinguishing between altered and unaltered fluid inclusions may be applied.

In studies of global change, fluid inclusions can be used as sensitive indicators of paleotemperature of surface environments. Fluid inclusions also preserve microsamples of ancient seawater and atmosphere, the analysis of which could figure prominently into discussions of past changes in chemistry of the atmosphere and oceans. In petroleum geology, fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons; they can delineate the evolution of the chemistry of hydrocarbons; and they remain important in understanding the thermal history of basins and relating fluid migration events to evolution of reservoir systems. In studies of diagenesis, fluid inclusions can be the most definitive record. Most diagenetic systems are closely linked to temperature and salinity of the fluid. Thus, fluid inclusions are sensitive indicators of diagenetic environments.     

Approach to complete limit equilibrium analysis for rock wedges — the method of “artificial supports”

SummaryApproach to Complete Limit Equilibrium Analysis for Rock Wedges — The Method of Artificial Supports Since the movement of rock wedges or blocks is the primary phenomenon in rock mass failure, it is convenient to use limit equilibrium approaches for stability analysis. In spite of their wide use, limit equilibrium analysis is often restricted due to: 1) the usual procedural simplifications (regarding geometry, force application, failure modes), and 2) basic deficiencies of the limit equilibrium approach (the fundamentally indeterminate rigid body problem requires assumptions regarding the force reactions). The method of artificial supports intends to circumvent or reduce many of these restrictions. In particular, the problem of unknown force distribution on the supporting plane is reduced by specifically considering and optimizing a variety of reactions on these planes. All sliding modes of failure — i. e., translational, free and restrained rotational sliding — can be correctly modelled. Particularly significant is the result that the usual assumption oftranslational sliding can be incorrect andunsafe, if forces other than the weight are significant. The methodologies employed in the artificial support model can be extended to all modes of failure, i. e., lifting off and toppling in addition to the previously mentioned sliding modes; this is demonstrated with a procedure for a complete stability analysis for a 2-joint wedge.

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ZusammenfassungEin Weg zur vollständigen Gleichgewichtsberechnung für Felskörper — Die Methode der Künstlichen Auflager Da die Bewegung von Einzelblöcken das primäre Phänomen bei Felsstürzen ist, eignen sich Grenzgleichgewichtsbetrachtungen besonders gut für Stabilitätsuntersuchungen. Vor allem wegen der üblichen Vereinfachungen (Geometrie, Kräfteansätze, Versagensmechanismen) und grundlegender Unzulänglichkeiten (das grundsätzlich statisch unbestimmte Starrkörperproblem erfordert Annahmen bezüglich der Auflagerkräfte) sind solche Grenzgleichgewichtsbetrachtungen oft nur beschränkt anwendbar, obwohl sie häufig angewendet werden. Es wird versucht, mit der Methode der künstlichen Auflager viele dieser Einschränkungen zu umgehen. Vor allem wird das Problem der unbekannten Spannungsverteilung in den Auflageflächen dadurch reduziert, daß mögliche Kombinationen solcher Reaktionen in die Rechnung einbezogen und optimiert werden. Alle Gleitbewegungen, seien es Translationen oder freie bzw. beschränkte Rotationen auf den Auflageflächen, können korrekt behandelt werden. Besondere Bedeutung kommt aber der Tatsache zu, daß die übliche Annahme einer translatorischen Gleitung falsch ist bzw. daß man damit auf der unsicheren Seite liegt, wenn außer dem Eigengewicht noch andere Kräfte einwirken.Die Überlegungen, die der Modellvorstellung der künstlichen Auflager zugrunde gelegt wurden, können auf alle Instabilitätsfälle ausgedehnt werden, also auch auf Abheben und Kippen zusätzlich zu den vorhin genannten Gleitbewegungen; dies wird am Beispiel einer Rechenprozedur für eine vollständige Stabilitätsuntersuchung eines Gleitkeils gezeigt.

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RésuméTentative visant à compléter l'analyse par équilibre limite — la méthode des supports artificiels Le fait que le déplacement de blocs est le principal phénomène d'instabilité en massif rocheux suggère l'utilisation de méthodes d'équilibre limite à l'étude de sa stabilité. Bien qu'elles soient fréquemment utilisées, les analyses par équilibre limite sont souvent restreintes par suite 1) de simplifications opérationnelles (concernant la géométrie, les forces et les modes d'instabilité), et 2) des insuffisances fondamentales de cette approche (le problème des corps indéformables est en essence statiquement indéterminé et réclame en conséquence l'adoption d'hypothèses sur la teneur des réactions).L'intention de la méthode des supports artificiels est de réduire ou éviter plusieures de ces restrictions. En particulier le problème de la distribution inconnue des forces sur le plan d'assise est résolu en considérant et optimisant un certain nombre de réactions entre corps rigide et le plan de base. Tous les modes d'instabilité au glissement — à savoir translation, rotation restreinte et rotation libre — peuvent être correctement représentés. Spécialement digne d'intérêt est à cet égard le résultat concernant l'habituelle hypothèse de glissement en translation, puisque celui-ci peut s'avérer incorrect et exhiber des facteurs de sécurité supérieurs, par rapport à un glissement en rotation, lorsque des forces autres que le poids sont à considérer.La méthodologie servant de base à la méthode des supports artificiels peut être étendue à tous modes d'instabilité, à savoir: le soulèvement et le renversement venant compléter ceux précités; ceci est illustré par le développement d'une procédure d'analyse complète d'un bloc monolithique découpé par deux fissures.

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With 13 Figures     

Uraniferous diagenetic xenotime in northern Australia and its relationship to unconformity-associated uranium mineralisation

Stratabound, uraniferous diagenetic xenotime cements provide a minimum depositional age of 1,632±3 Ma for the sedimentary Birrindudu Group in the Killi Killi Hills, Tanami Region in northern Australia. The age of xenotime formation is broadly coeval with that recently proposed (1,650–1,600 Ma) for uranium mineralisation in the unconformity-associated deposits of the Pine Creek Inlier, northern Australia, and Athabasca Basin, Canada. The geological setting and formation model for the uraniferous xenotime crystals are similar to those widely proposed for unconformity-associated uranium deposits, suggesting a genetic link between the two. However, xenotime formation in the Birrindudu Group occurred during an apparently earlier stage of diagenesis, compared to late diagenetic formation of unconformity-associated uranium deposits. This could be explained by variations in the thickness of sediment cover and diachronous diagenesis across the basin, at the time of the basin-wide uranium mobilisation event, herein dated at ca. 1,630 Ma. In such a scenario, stratabound uraniferous xenotime cements could represent the remote distal zones of a more deeply buried, uranium mineralising system. Alternatively, the xenotime layer represents a precursor to, or a source for, later unconformity-associated ore deposition. In this case, the presence of diagenetic uraniferous xenotime in an area prospective for unconformity-associated uranium mineralisation would be an indication of, and still provide an approximate age for, uranium mobilisation within the cover sequence. Xenotime is a far more robust mineral than uraninite for U–Pb geochronology and can potentially provide a more reliable and precise timeframe for uranium mineralisation and subsequent recrystallisation events if present in the immediate uranium-ore environment.     

Early diagenetic behaviour of selenium in freshwater sediments

The vertical distributions of dissolved Se species [Se(IV), Se(VI) and organic Se] and diagenetic constituents [Fe(II) and Mn(II)] were obtained in porewater samples of two Sudbury area lakes (Clearwater and McFarlane). The sedimentary concentration profiles of total Se, Se species bound to Fe–Mn oxyhydroxides and to organic matter, and of elemental Se were also determined along with the concentrations of Fe, Mn and S in different extractable fractions. Results indicated that the concentrations of total dissolved Se in porewater samples were very low, varying from around 2.0 nM to a maximum level of 6.5 nM, while the concentrations of total Se species in the solid phase varied between 2 and 150 nmol/g on a dry weight basis. The two lakes showed striking differences in the presence of Se(IV) and Se(VI) at the sediment–water interface (SWI). In Clearwater Lake, Se(VI) was present at this interface and Se(IV) was not detectable, whereas the opposite was found in McFarlane Lake. This suggests that reducing conditions might have existed near the SWI of McFarlane Lake at the sampling time; this hypothesis was confirmed by several other measured chemical parameters. The profiles of total dissolved Se of both lakes suggest upward and downward diffusion of dissolved Se species along the concentration gradients. Assuming that no precipitation occurred at the SWI, the fluxes of dissolved Se species across the SWI in Clearwater and McFarlane lakes were estimated to be 0.108 and 0.034 nmol cm⁻² a⁻¹, respectively. These values do not include the possible losses of volatile Se species due to microbial methylation. In the reducing sediments of both lakes, the formation of elemental Se and pyritic Se were found to be important mechanisms for controlling the solubility of Se in this environment. The main geochemical processes involving Se identified in this study are: the adsorption of Se onto Fe–Mn oxyhydroxides at or near the SWI, the release of adsorbed Se by the reduction of the same oxyhydroxides and the mineralization of organic matter, and the removal of Se from porewaters to form elemental Se and a S mineral phase such as Se–pyrite or pure ferroselite.     

Calculation of equilibrium distributions of chemical species in aqueous solutions by means of monotone sequences

A numerical method for the calculation of equilibrium distributions of chemical species in aqueous solutions of electrolytes is presented. This method is constructed by transforming the problem of determining the set of unknown concentrations satisfying the mass balance and mass action equations into the equivalent problem of finding the limits of certain well-behaved mathematical sequences in a multivariable direct iteration scheme. The total (analytical) concentrations are taken as the starting estimates, and the recursive equations are constructed from the starting equations. It is shown that the sequences so constructed are both monotonic and bounded, hence convergent in an orderly fashion by a mathematical axiom. Each unknown free ion concentration (that is, each limit) is approached simultaneously from above and from below, being effectively sandwiched in an ever tightening manner. Strict error bounds therefore are easily constructed. The method has been found to be exceedingly efficient in practice, with the error bound (maximum fractional error in taking the upper estimate as the final answer) decreasing in an approximately exponential fashion with respect to iteration number, commonly 0.5–1.5 orders of magnitude per iteration. Correction for nonideality is presented, and the possibility of this giving rise to more than one solution is discussed in connection with three examples of natural waters of low-, medium-, and high-ionic strengths.     

Stability and chemical equilibrium of amphibole in calc-alkaline magmas: an overview, new thermobarometric formulations and application to subduction-related volcanoes

This work focuses on a rigorous analysis of the physical–chemical, compositional and textural relationships of amphibole stability and the development of new thermobarometric formulations for amphibole-bearing calc-alkaline products of subduction-related systems. Literature experimental results (550–1,120°C, <1,200 MPa, −1 ≤ ΔNNO ≤ +5), H₂O–CO₂ solubility models, a multitude of amphibole-bearing calc-alkaline products (whole-rocks and glasses, representing 38 volcanoes worldwide), crustal and high-P (1–3 GPa) mantle amphibole compositions have been used. Calcic amphiboles of basalt-rhyolite volcanic products display tschermakitic pargasite (37%), magnesiohastingsite (32%) and magnesiohornblende (31%) compositions with aluminium number (i.e. Al# = ^[6]Al/Al_T) ≤ 0.21. A few volcanic amphiboles (~1%) show high Al# (>0.21) and are inferred to represent xenocrysts of crustal or mantle materials. Most experimental results on calc-alkaline suites have been found to be unsuitable for using in thermobarometric calibrations due to the high Al# (>0.21) of amphiboles and high Al₂O₃/SiO₂ ratios of the coexisting melts. The pre-eruptive crystallization of consistent amphiboles is confined to relatively narrow physical–chemical ranges, next to their dehydration curves. The widespread occurrence of amphiboles with dehydration (breakdown) rims made of anhydrous phases and/or glass, related to sub-volcanic processes such as magma mixing and/or slow ascent during extrusion, confirms that crystal destabilization occurs with relatively low T–P shifts. At the stability curves, the variance of the system decreases so that amphibole composition and physical–chemical conditions are strictly linked to each other. This allowed us to retrieve some empirical thermobarometric formulations which work independently with different compositional components (i.e. Si^*, Al_T, Mg^*, ^[6]Al^*) of a single phase (amphibole), and are therefore easily applicable to all types of calc-alkaline volcanic products (including hybrid andesites). The Si^*-sensitive thermometer and the fO₂–Mg^* equation account for accuracies of ±22°C (σ_est) and 0.4 log units (maximum error), respectively. The uncertainties of the Al_T-sensitive barometer increase with pressure and decrease with temperature. Near the P–T stability curve, the error is <11% whereas for crystal-rich (porphyritic index i.e. PI > 35%) and lower-T magmas, the uncertainty increases up to 24%, consistent with depth uncertainties of 0.4 km, at 90 MPa (~3.4 km), and 7.9 km, at 800 MPa (~30 km), respectively. For magnesiohornblendes, the ^[6]Al^*-sensitive hygrometer has an accuracy of 0.4 wt% (σ_est) whereas for magnesiohastingsite and tschermakitic pargasite species, H₂O_melt uncertainties can be as high as 15% relative. The thermobarometric results obtained with the application of these equations to calc-alkaline amphibole-bearing products were finally, and successfully, crosschecked on several subduction-related volcanoes, through complementary methodologies such as pre-eruptive seismicity (volcano-tectonic earthquake locations and frequency), seismic tomography, Fe–Ti oxides, amphibole–plagioclase, plagioclase–liquid equilibria thermobarometry and melt inclusion studies. A user-friendly spreadsheet (i.e. AMP-TB.xls) to calculate the physical–chemical conditions of amphibole crystallization is also provided.     

A chemical equilibrium algorithm for highly non-ideal multiphase systems: Free energy minimization

A method is presented for calculating equilibrium phase assemblages in very nonideal systems. It may be applied to any system for which a thermodynamically consistent model of the free energy which satisfies the usual Maxwell relations and convexity criterion is available. The algorithm minimizes the Gibbs free energy by independently choosing stable reaction directions. The procedure is described in detail and various numerical problems encountered and strategies for dealing with them are discussed. It will be shown that the necessary and sufficient conditions for solution phase selection may be derived from the values of the Lagrange multipliers corresponding to constraints on phases that are not present in the system. The method for evaluating the solution phase Lagrangian multipliers and choosing the optimum composition with which to bring the new solution phase into the system involves a separate constrained minimization problem. This method is sufficiently general so that the correct phase assemblage is chosen free from external control. Special procedures for adding and removing phases including solution phases are also described.     

On the widespread occurence of oxidized chlorites in the pennine zone of the western Italian Alps

Oxidized chlorites, having optical properties quite similar to those of biotite, stilpnomelane or iron-rich vermiculite, are described from the schistes lustrés and other similar metamorphites of the Pennine Zone of the western Italian Alps. Optical, x-ray, chemical, DTA and infrared spectroscopic data are furnished. From the results of the chemical study it seems reasonable to conclude that the chlorite has undergone an internal oxidation. Possible petrogenetic implications of its occurenee are discussed in some details.     

Grain‐scale pressure variations and chemical equilibrium in high‐grade metamorphic rocks

In the classical view of metamorphic microstructures, fast viscous relaxation (and so constant pressure) is assumed, with diffusion being the limiting factor in equilibration. This contribution is focused on the only other possible scenario – fast diffusion and slow viscous relaxation – and brings an alternative interpretation of microstructures typical of high‐grade metamorphic rocks. In contrast to the pressure vessel mechanical model applied to pressure variation associated with coesite inclusions in various host minerals, a multi‐anvil mechanical model is proposed in which strong single crystals and weak grain boundaries can maintain pressure variation at geological time‐scales in a polycrystalline material. In such a mechanical context, exsolution lamellae in feldspar are used to show that feldspar can sustain large differential stresses (>10 kbar) at geological time‐scales. Furthermore, it is argued that the existence of grain‐scale pressure gradients combined with diffusional equilibrium may explain chemical zoning preserved in reaction rims. Assuming zero net flux across the microstructure, an equilibrium thermodynamic method is introduced for inferring pressure variation corresponding to the chemical zoning. This new barometric method is applied to plagioclase rims around kyanite in felsic granulite (Bohemian Massif, Czech Republic), yielding a grain‐scale pressure variation of 8 kbar. In this approach, kinetic factors are not invoked to account for mineral composition zoning preserved in rocks metamorphosed at high grade.     

Crystal chemistry of diagenetic zeolites in volcanoclastic deposits of Italy

Zeolites from the most important volcanoclastic deposits of Italy include: (1) phillipsite and heulandite from the cinerite of the central northern Apennines; (2) chabazite and phillipsite from the phonolitic tephritic ignimbrite with black pumices; (3) phillipsite from the “tufo lionato” of Vulcano Laziale; (4) chabazite and phillipsite from the Campanian ignimbrite; (5) phillipsite from the Neapolitan yellow tuff; and (6) chabazite and phillipsite from the pyroclastics of Monte Vulture. Compared with sedimentary phillipsites and chabazites described in the literature, the chabazites and phillipsites studied here have lower Si/Al ratios and higher K contents. These chemical peculiarities are correlated with both the K-rich vesuvitic-leucititic, latitic-phonolitic, and potassic alkali-trachytic chemistry of the ash from which they were derived and, very likely, with the character of the hydrologically open system environment in which they formed. The zeolite of the heulandite-clinoptilolite group from the cinerite of the central northern Apennines is classified as a true heulandite on the basis of its chemical composition and thermal behavior.     

Identification of perylene in sediments: Occurrence and diagenetic evolution

Perylene and penta-aromatic hydrocarbons were determined in sediments as part of a study that was dedicated to the aquatic ecosystem of Elelenwo Creek (Southern Nigeria) in order to carry out a critical corroboration of occurrence and diagenetic evolution of perylene in the sediments of the creek. The results show that the annual mean levels of Benzo [g, h, i] Perylene ranged from 209.00–245.28 ?g/kg dry weight at the various stations sampled. Meanwhile, Station 3 recorded the highest mean level of 245.28 ?g/kg dry weight. The observed values for total penta-aromatic hydrocarbons were high (787.00–1154.36?g/kg dry weight) in all the stations sampled. In addition, the highest mean value of 1154.36?g/kg dry weight was again recorded at station 3 for the penta-aromatic hydrocarbons. One origin index or concentration ratio of Ip/Ip+BghiP was also used to evaluate the suitability of the penta-aromatic hydrocarbons as a tracer to distinguish between contaminations arising from different sources. The values for the sampling stations therefore ranged from 0.41 to 0.43. A critical appraisal of the PAH index, consequently, suggested that petroleum combustion is the major penta-aromatic hydrocarbon source in sediments of the creek. The PAH group profile shows that perylene was high in the sediments and would pose apparent effects in fauna. The high concentration of perylene in the sediments was also indicative of an in situ biogenic derivation. Furthermore, a concentration of perylene > 10 % of total penta-aromatic hydrocarbons established a credible diagenetic origin.