Abiogenic Fischer–Tropsch synthesis of hydrocarbons in alkaline igneous rocks; fluid inclusion, textural and isotopic evidence from the Lovozero complex, N.W. Russia

A detailed fluid inclusion study has been carried out on the hydrocarbon-bearing fluids found in the peralkaline complex, Lovozero. Petrographic, microthermometric, laser Raman and bulk gas data are presented and discussed in context with previously published data from Lovozero and similar hydrocarbon-bearing alkaline complexes in order to further understand the processes which have generated these hydrocarbons. CH₄-dominated inclusions have been identified in all Lovozero samples. They occur predominantly as secondary inclusions trapped along cleavage planes and healed fractures together with rare H₂O-dominant inclusions. They are consistently observed in close association with either arfvedsonite crystals, partially replaced by aegirine, aegirine crystals or areas of zeolitization. The majority of inclusions consist of a low-density fluid with CH₄ homogenisation temperatures between −25 and −120 °C. Those in near-surface hand specimens contain CH₄+H₂ (up to 40 mol%)±higher hydrocarbons. However, inclusions in borehole samples contain CH₄+higher hydrocarbons±H₂ indicating that, at depth, higher hydrocarbons are more likely to form. Estimated entrapment temperatures and pressures for these inclusions are 350 °C and 0.2–0.7 kbar. A population of high-density, liquid, CH₄-dominant inclusions have also been recorded, mainly in the borehole samples, homogenising between −78 and −99 °C. These consist of pure CH₄, trapped between 1.2 and 2.1 kbar and may represent an early CH₄-bearing fluid overprinted by the low-density population. The microthermometric and laser Raman data are in agreement with bulk gas data, which have recorded significant concentrations of H₂ and higher hydrocarbons up to C₆H₁₂ in these samples. These data, combined with published isotopic data for the gases CH₄, C₂H₆, H₂, He and Ar indicate that these hydrocarbons have an abiogenic, crustal origin and were generated during postmagmatic, low temperature, alteration reactions of the mineral assemblage. This would suggest that these data favour a model for formation of hydrocarbons through Fischer–Tropsch type reactions involving an early CO₂-rich fluid and H₂ derived from alteration reactions. This is in contrast to the late-magmatic model suggested for the formation of hydrocarbons in the similar peralkaline intrusion, Ilímaussaq, at temperatures between 400 and 500 °C.     

Thermicité et déformation de la marge continentale dans le Sud de la Tasmanie (Australie) : résultats préliminaires d'une analyse par traces de fission et d'une étude microstructuraleFission track reconnaissance of the thermal and tectonic settings of the South Tasman rise

We present and discuss a few fission track data, and microstructural observations, from rock samples dredged along the western and southwestern continental margin of Tasmania. The results allow assessing the thermal and tectonic regimes that were active prior to and during the margin creation. The different ages, as provided by fission tracks, and deformational styles, as evidenced from microstructures, are then tentatively correlated with the two main rifting episodes, in Late Jurassic–Cretaceous times and Eocene–Oligocene respectively, deduced from kinematical reconstructions, that have led to the present- day southern margin of Tasmania. To cite this article: M. Sélo et al., C. R. Geoscience 334 (2002) 59–66     

Decrepitometry of fluid inclusions in quartz from the guadalcazar granite of Mexico; principles and application to mineral exploration

A simple acoustic decrepitometric method, with which samples of granite quartz are heated to about 600°C while the number of decrepitations are counted, has been developed to study rock samples derived from the mineralized guadalcazar granite in Mexico. Decrepitation temperatures for individual inclusions have also been determined by observing the point at which they rupture upon heating using a microscope heating stage. Decrepitation temperatures of individual fluid inclusions in granite quartz are influenced by a variety of factors notably size, shape, composition, homogenization temperature and proximity to the surface. There is a positive correlation between total decrepitation activity and fluid inclusion abundances (determined optically using point counting methods). Decrepitographs show a period of low intensity decrepitation activity below 390°C followed by a period of intensive decrepitation up to 570°C. The onset of massive decrepitation at around 390°C is constant for all samples, but variations in decrepitation activity often occur between mineralized and barren samples. These variations reflect complex differences in the fluid inclusion populations, but illustrate the potential for applying simple audio-decrepitometry as an aid to mineral exploration in granite terrains.     

A magnetotelluric study in the western Canadian sedimentary basin

Summary The MT method is applied to the study of a buried graben in the western Canadian sedimentary basin. Tensor analysis and two dimensional numerical modeling techniques were applied as an aid to the interpretation. The results obtained indicate the presence of a buried geoelectric structure and the need of interpreting MT apparent resistivity curves in terms of the regional geology and major structural features which are not necessarily one dimensional.     

Evolution of fluids responsible for iron skarn mineralisation: An example from the Vyhne-Klokoc deposit, Western Carpathians, Slovakia

Summary Vyhne-Klokoc, the largest Fe-Skarnn deposit in the Western Carpathians, is related to the emplacement of a large granodiorite pluton in the central zone of a Neogene stratovolcano. Skarn mineralisation is developed in places where apophyses of the pluton intruded basement carbonates. Granodiorite in the apophyses grades into rocks of granitic composition, involving the replacement of mafic minerals and a concomitant decrease in Fe-content. Ca-magnetite exoskarns (not accompanied by endoskarns) developed in three paragenetic stages. Fluid inclusion (Fl) data for quartz in granodiorite suggest the existence of aqueous fluid immiscibility during the early hydrothermal stages. Three end-members of Fis were recognised, with a continuum between all three types. High salinity, liquid-rich, probably secondary Fls (29-68 wt % NaCl eq., Th 450 to 570'C, composed of NaCl+FeCl₂+KCl) coexist with vapour-rich Fls with low but variable salt contents (+CO₂). Probably late secondary Fls (1-25 wt % NaCl eq., composed mainly of NaCl+CaCl₂, Th 188–283°C) form the other end-member type of Fls trapped in granodiorite quartz. Fls from skarn garnets show a large variation in salinity (4-23 wt % NaCl eq., composed of NaCl±FeCl₂+CaCl₂+ KCl+MgCl₂) and Th (220–370°C), independent of the garnet types, probably reflecting variable amounts of magmatic fluids and low salinity meteoric waters. Fls in retrograde quartz, calcite and sphalerite show progressively more dilute (0-4 wt % NaCl eq, Th 215–380°C), probably dominantly meteoric fluids with evidence for boiling at shallow depth. Chlorite crystallisation temperatures, calculated from the chlorite geothermometer, are in good agreement with the Th data for Fls in associated skarn minerals. Compositional changes in the granodiorite apophyses are the result of subsolidus autometasomatic reactions of accumulated saline magmatic fluid inside the apophyses with pre-existing mafic mineral phases. Reactions add the iron to the fluid -the potential source for magnetite skarn. Later mixing with dilute, cooler probably meteoric waters had the effect of decreasing the salinity and density of the equilibrated magmatic fluid, making it more buoyant and capable of moving out from the apophyses into the country rocks, causing metasomatic reactions and precipitating magnetite. An overlap exists between the FI microthermometry data from primary Fls in garnets and late secondary Fls in the granodiorite quartz indicating the same sources of the hydrothermal fluids - probably mixtures of magmatic and meteoric waters. Based on fluid inclusion, geological, petrological and mineralogical data, an integrated fluid evolution model involving magmatic and meteoric fluids is developed to explain the geological and fluid controls on Fe-skarn mineralization associated with granodiorite intrusions. Die Evolution von Fluiden bei Fe-skarn Mineralisation: Ein Beispiel von der Lagerstätte Vyhne-Klokoc, West-Karpathen, Slowakei.

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Die Evolution von Fluiden bei Fe-Skum Mineralisation: Ein Beispel von der Lagerstátte Vyhne-Klokoc, West-Karpathen, Slowakei.

Zusammenfassung Vyhne-Klokoc ist die größte Fe-skarn Lagerstätte in den Westkarpathen. Sie steht in Beziehung zur Platznahme eines großen Granodiorit-Plutons in der Zentralzone eines neogenen Stratovulkans. Skarn-Vererzung ist dort zu finden, wo Apophysen des Plutons Karbonate des Basements intrudieren. In den Apophysen geht Granodiorit in Gesteine granitischer Zusammensetzung über, wobei mafische Minerale verdrängt werden und der Fe-Gehalt abnimmt. Ca-Magnetit-Exoskarne (nicht von Endoskarnen begleitet) entstanden in drei paragenetischen Stadien. Flüssigkeits-Einschluß-Daten (F1) für Quarz in Granodiorit weisen auf Unmischbarkeit von Fluiden während der frühen hydrothermalen Stadien hin. Drei Endglieder von FI liegen vor, die durch Kontinuum miteinander verbunden sind. Hochsalinare, wahrscheinlich sekundäre FI mit hohem Anteil fluider Phase (29-68 wt % NaCl eq., Th 450-570°C, bestehend aus NaCl+FeCl₂+KCl) koexistieren mit Gas-reichen FI mit niedrigem aber variablem Salzgehalt (+C02). Sekundäre, wahrscheinlich spät gebildete FI (1-25 wt% NaCl eq., Hauptbestandteile NaCl+CaCl₂, Th 188–283°C) bilden das andere Endglied von FI in Granodiorit-Quarz.FI aus Skarn-Granaten zeigen größere Variationen der Salinität (4-23 wt % NaCl eq., Hauptkomponenten NaCl±FeCl₂+CaCl₂+KCl+M₉Cl₂) und Th (220–370°C). Diese Zusammensetzungen sind unabhängig von der Art der Granate und dürften das Ergebnis von Mischung variabler Mengen magmatischer Fluide und meteorischer Wässer niedriger Salinität sein. FI in retrogradem Quarz, Calcit und Sphalerit zeigen zunehmend mehr verdünnte (0-4 wt% NaCI eq., Th 215–380°C), wahrscheinlich großteils meteorische Fluide mit Hinweisen auf Kochen in geringer Tiefe. Temperaturen für die Kristallisation von Chlorit wurden mit dem Chlorit-Geothermometer ermittelt; diese stimmen gut mit Th-Werten für FI in assoziierten Skarn-Mineralen überein. Änderungen der Zusammensetzung der Granit-Apophysen sind das Ergebnis von autometasomatischen Subsolidus-Reaktionen magmatischer Fluide, die sich in den Apophysen angesammelt haben, mit präexistierenden mafischen Mineralen. Solche Reaktionen erhöhen den Fe-Gehalt in den Fluiden - die potentielle Quelle für Magnetit-Skarne. Spätere Mischung mit verdünnten, kühleren Fluiden, wahrscheinlich meteorischer Herkunft, senkte Salinität und Dichte der magmatischen Fluide und erleichterte so ihren Aufstieg in die Nebengesteine. Dies führte zu metasomatischen Reaktionen und zur Ausfällung von Magnetit. Mikrothermometrische Daten von primären FI in Granat und von sekundären FI in Granodiorit-Quarz überlappen teilweise und weisen auf ähnliche Ausgangs-Fluide hin, wahrscheinlich Mischungen magmatischer und meteorischer Wässer. Geologische, petrologische, mineralogische und FI-Daten ermöglichen die Entwicklung eines integrierten Modells für die FluidEvolution bei der Bildung von Fe-Skarnen in Granodiorit-Intrusionen.

     

Rare-earth elements in iron-manganese concretions from some New Zealand soils

Iron-manganese concretions and their surrounding soil materials from six sites in New Zealand have been analysed for rare-earth elements (REE). The samples studied fall into two groups: (1) those in which Ce is the only REE enriched in the concretions relative to the surrounding soil materials; and (2) those in which all REE are enriched in the concretions relative to the surrounding soil materials and in which Ce and Tb show consistently higher degrees of enrichment than do other REE. Comparison of enrichment factors (i.e., concentration in concretions/concentration in surrounding soil materials) indicates that Ce and total REE enrichment is related to Mn rather than Fe enrichment in concretions. Other trace element results show that As is enriched in all the concretion samples.     

An asymptotic solution for the stellar case of the non-planar three-body problem

A simplified model of the Non-Planer Three-Body Problem is considered in which to particles, forming a close binary, orbit a distant point. A small parameter , related to the distance separating the binary and the remaining mass, is defined. The time is eliminated from the equations of motion and an angular variable is used instead. A three-variable expansion procedure is used to find an asymptotic solution of the problem. It is possible to obtain a solution up to the order six in without secular terms only if the mutual inclinationi ₀ of the unperturbed orbits is less than a critical inclinationi ₁ (i ₁39°).

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Resumé On considère un modèle simplifié du Problème Non-Plan des Trois Corps, dans lequel deux particules, formant une binaire proche, sont en orbite par rapport à un troisième point éloigné des deux autres. On définit un petit paramètre , lié à la distance séparant la binaire de la particule restante. On élimine le temps des équations du mouvement et on utilise une variable angulaire comme nouvelle variable indépendante. Une méthode de développement à trois échelles est utilisée permettant d'obtenir une solution asymptotique du problème. On montre qu'il est possible d'obtenir une solution jusqu'à l'ordre six en sans termes séculaires uniquement si l'inclinaison mutuellei ₀ des orbites non perturbés est inférieure à un angle d'inclinaison critiquei ₁ (i ₁39°).

     

Accumulation of dietary organochlorines and vitamins in Baltic seals

We studied the accumulation of polychlorinated biphenyls (PCBs), 1,1,1.trichloro-2,2-bis[p-chlorophenyl]ethane (DDT) and its metabolites, and vitamins A and E in the top levels of the Baltic Sea food web. The investigation focused on the transfer of contaminants and vitamins to the ringed seal (Phoca hispida baltica) and the grey seal (Halichoerus grypus) from their main prey species. The trophic level of the seals was investigated using stable isotopes of nitrogen and the results indicated that both species of Baltic seal feed at approximately the same level. PCBs accumulated to a greater extent in the grey seal than in the ringed seal. Biomagnification factors for DDT compounds were similar for both species of Baltic seal (85-140). Differences in observed DDT levels were due to different prey selection by the two species, while differences in PCB levels indicated a species-specific metabolic system. There was a clearly greater accumulation of DDT compounds than of PCBs in both species of seal. The supply of dietary vitamin A was normally above the recommended level in all the seal populations studied. Low levels of hepatic vitamin A in the Baltic seals could therefore indicate the toxic effects of a high level of persistent organic pollutants on vitamin A dynamics, at least in the ringed seal. In the grey seals, low hepatic vitamin A levels could also be explained by lower levels of dietary vitamin A, compared to the reference grey seals, as it is not known if seals can store unlimited amounts of vitamin A. The greater uptake of vitamin E by Baltic seals, compared to seals in the reference areas, demonstrated by elevated levels of vitamin E in the blubber, could be a response to oxidative stress caused by the high contaminant load. These results further support our previous hypothesis that the toxic effects of environmental contaminants could be causing the observed divergence in vitamin levels between the Baltic seals and the reference seal populations.