Fluid inclusions in high-pressure granulites of the Pan-African belt in Tanzania (Uluguru Mts): a record of prograde to retrograde fluid evolution

The high-pressure granulites of the Uluguru Mountains are part of the Pan-African belt of Tanzania, the metamorphic evolution of which is characterized by an anticlockwise P-T path. Mineral assemblages that represent distinct metamorphic stages are selected for fluid inclusion studies in order to deduce the fluid evolution in metapelites and pyroxene granulites from the prograde to the retrograde stage. Fluid inclusion data improve the petrologically derived P-T path and confirm the anticlockwise evolution. Fluid inclusions in quartz enclosed in garnet porphyroblasts in metapelites preserve prograde fluids of CO₂–N₂ composition and later-trapped pure CO₂. During isochoric heating at temperatures near the peak of metamorphism, deformation and recrystallization led to fluid homogenization yielding N₂-poor CO₂ composition in the metapelites. Near-peak CO₂–N₂ fluid inclusions in quartz of metapelites and CO₂ inclusions in garnet-pyroxene granulites are characterized by perfect negative crystal shape. Garnet formed in veins and as coronas around orthopyroxene represent the near-isochoric/isobaric cooling stage which is characterized by high-density CO₂-rich fluid inclusions. Up to 15 mol% N₂ in some primary CO₂ inclusions in corona garnet indicate small-scale fluid heterogeneity during the static garnet growth. The fact that high-density fluid inclusions are preserved, suggests a shallow dP/dT slope of the uplift path. Nevertheless, some fluid inclusions decrepitated or re-equilibrated and low-density CO₂ inclusions were trapped in the garnet-pyroxene granulite while N₂–CH₄ inclusions formed in the metapelites. Different fluid compositions in metapelite and metabasite argue for an internal control of the fluid composition by phase equilibria. In shear zones where the pyroxene granulite was transformed into scapolite-biotite schist, CO₂–N₂ and low-density N₂–CH₄ fluid inclusions indicate several stages of tectonic activity and suggest fluid influx from the nearby metapelites. High- and low-salinity aqueous inclusions observed beside CO₂ inclusions in garnet-pyroxene granulites, in vein quartz and shear zones could be of high-grade origin but are mainly re-equilibrated or re-trapped along healed microfractures during lower-grade stages. Received: 21 May 1997 / Accepted: 6 October 1997     

Renaturalization of streams and rivers — the special importance of integrated ecological methods in measurement of success. An example from Saxony-Anhalt (Germany)

Since hydromorphology in about 80% of German streams and rivers is degraded to a high degree, increased efforts in hydromorphological renaturalization are necessary. A measurement of the success of the first realized projects shows that improvement in stream morphology has a remarkably positive influence on aquatic ecology. An example of a restored stretch of a lowland stream in Saxony-Anhalt is used to describe the possibilities of success measurement programs for improvement of poor renaturalization. Therefore, a combined morphological and hydrobiological approach was developed. An integrated ecological assessment is possible by using the multimetric index EQI_M (Ecological Quality Index using benthic Macroinvertebrates) and the GFI (German Fauna Index). The latter represents a tolerance measure to evaluate the hydromorphological status of a site by using certain taxa that indicate either positive or negative physical attributes. To consider the special characteristics of the stream in its landscape unit, specific reference conditions (‘Leitbild’) were defined for macroinvertebrate communities by sampling comparable but undisturbed streams in the same landscape unit. Only the combination of biological indices, hydromorphological mapping and comparison to the reference status allows for an expressive evaluation of renaturalization measures and precise conclusions for their improvement.     

The Elbe Fault System in North Central Europe—a basement controlled zone of crustal weakness

The Elbe Fault System (EFS) is a WNW-striking zone extending from the southeastern North Sea to southwestern Poland along the present southern margin of the North German Basin and the northern margin of the Sudetes Mountains. Although details are still under debate, geological and geophysical data reveal that upper crustal deformation along the Elbe Fault System has taken place repeatedly since Late Carboniferous times with changing kinematic activity in response to variation in the stress regime. In Late Carboniferous to early Permian times, the Elbe Fault System was part of a post-Variscan wrench fault system and acted as the southern boundary fault during the formation of the Permian Basins along the Trans-European Suture Zone (sensu [Geol. Mag. 134 (5) (1997) 585]). The Teisseyre–Tornquist Zone (TTZ) most probably provided the northern counterpart in a pull-apart scenario at that time. Further strain localisation took place during late Mesozoic transtension, when local shear within the Elbe Fault System caused subsidence and basin formation along and parallel to the fault system. The most intense deformation took place along the system during late Cretaceous–early Cenozoic time, when the Elbe Fault System responded to regional compression with up to 4 km of uplift and formation of internal flexural highs. Compressional deformation continued during early Cenozoic time and actually may be ongoing. The upper crust of the Elbe Fault System, which itself reacted in a more or less ductile fashion, is underlain by a lower crust characterised by low P-wave velocities, low densities and a weak rheology. Structural, seismic and gravimetric data as well as rheology models support the assumption that a weak, stress-sensitive zone in the lower crust is the reason for the high mobility of the area and repeated strain localisation along the Elbe Fault System.     

Die Faziesentwicklung der Reichenhaller Schichten und die Tektonik im Süden des Achensees,Tirol

Zusammenfassung Im östlichsten Karwendel setzen sich die oberostalpinen, unteranisischen Reichenhaller Schichten aus einer mächtigen Folge mariner dolomitischer Kalke, sandiger Mergelkalke sowie sedimentärer Brekzien zusammen, die mehr oder weniger tektonisch überprägt sind.Die Brekzien treten in mehrfachem rhythmischem Wechsel zwischen Mergelkalken auf und erreichen erst im obersten Drittel der Schichtserie, die das gesamte Hydasp umfaßt, größere Mächtigkeiten. Ihr polymikter Komponentenbestand wird sowohl aus skythischen Gesteinen als auch aus Aufarbeitungsmaterial der besprochenen Serie gebildet. Ein rein tektonischer Charakter der Brekzien scheidet aus.Relativ rascher, z. T. periodischer Fazieswechsel deutet veränderliche paläogeographische Verhältnisse an.Der tektonische Bau erwies sich einfacher, als früher angenommen wurde.An die Hauptstörungszonen (Inntaleinheit über Lechtaleinheit) sind besondere tektonische Körper gebunden.

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In the extreme eastern Karwendel Range the subanisian Reichenhall Layers (Oberostalpin) are composed of an imposing sequence of initially marine dolomitic limes, sabulous marlaceous limes and sedimentary breccias more or less tectonically marked. The breccias appear in multiple rhythmic alternation between marlaceous limes. A purely tectonic character of the breccias is ruled out.

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Résumé Dans la partie la plus à l'est du Karwendel les couches anisiennes inférieures de Reichenhall (Oberostalpin) se composent d'une énorme masse d'abord de calcaires marins des Dolomites, puis de calcaires de marnes sablonneuses ainsi que de brèches sédimentaires qui sont plus ou moins structurées. Les brèches apparaissent par périodes régulières entre les calcaires de marne. Il est exclu que les brèches aient un caractère purement tectonique.

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Reichenhaller Achensee (). , , .

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Erweitertes Manuskript eines Vortrags, gehalten auf der 56. Jahrestagung der Geologischen Vereinigung in Wien am 26. Februar 1966.     

Geosynklinaler Vulkanismus in den oberen penninischen Decken Graubündens (Schweiz)

Zusammenfassung Die oberpennimsche Platta-Decke im südöstlichen Graubünden (Schweiz) wird im wesentlichen aus Ophiolithen aufgebaut. Daneben finden sich die zugehörigen Sedimente, d. h. Radiolarite, Kalk- bis Kieselschiefer sowie flyschartige Mergelund Tonschiefer, deren Alter zwischen oberem Jura und Kreide liegt. Unter Ophiolithen verstehen wir Gesteine, die als basische bis ultrabasische Magmen, an Bruchund Schwächezonen gebunden, meist in die wenig mächtigen unverfestigten Tiefseesedimente eines eugeosynklinalen Ablagerungsraumes eindrangen.So handelt es sich bei diesen alpinen Ophiolithen um ehemalige Basalte, Peridotite, Pyroxenite und Gabbros. Für diese Vorkommen trifft auch die TrilogieSteinmanns (1926) zu, jedoch konnten keine Primärkontakte zwischen ultrabasischen und basischen Ophiolithen erkannt werden. Mögliche Kontakte wurden durch eine intensive Schuppentektonik, deren Abscherungshorizont meist mit Serpentinmassen ausgefüllt sind, vollkommen verwischt. Ferner wurden diese Ophiolithe während der alpinen Gebirgsbildung schwach metamorph überprägt.Bedingt durch die Vielfalt dieser Probleme, wollen wir die ultrabasischen Glieder weglassen. Vergleiche mit einigen gut bearbeiteten Beispielen submariner Basalte (Monti Iblei, Sizilien und Quadra Island, Britisch Kolumbien) ließen innerhalb der einzelnen Platta-Schuppen Abfolgen submariner basischer Extrusionen und Intrusionen erkennen: Basal-Breccien, Pillow-Laven, Pillow-Breccien (jetzt Meta-Hyaloklastit-Breccien) und Meta-Hyaloklastite s. str.Petrographisch handelt es sich bei diesen Vulkaniten heute natürlich um Diabase, die sowohl einen basaltischen als auch spilitischen Chemismus besitzen.Vom Hangenden zum Liegenden läßt sich innerhalb der Platta-Decke in den Gesteinen eine fortschreitende metamorphe Überprägung vom anchimetamorphen zum epimetamorphen Bereich verfolgen.Die guten Aufschlußverhältnisse erlaubten, je nach dem Grad der Metamorphose, Veränderungen der ursprünglichen Vulkanitstrukturen zu erkennen. So konnten Pillow-Laven, Breccien und Meta-Hyaloklastite durch den Grünschiefer-Fazies-Bereich hindurch bis in den Bereich der Prasinitisierung verfolgt werden. Dies brachte eine gewisse Klärung in die Problematik der alpinen Grünschiefer.Mit Hilfe der Vulkanitstrukturen, der vulkanischen Abfolgen und der Sedimente, konnte die Schuppentektonik innerhalb der Platta-Decke weitgehend geklärt werden.

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The upper-penninic Platta-Decke located in the south-east part of Grisons (Switzerland) consists essentially of ophiolites. These rocks are associated with upper-jurassic radiolarites, cherts and limestones as well as lower-cretaceous flysch-like calcareous shales and schists.Ophiolites are defined as ultrabasic and basic magmas that intruded thin and unconsolidated deep-sea sediments of a eugeosyncline. They were originally basalts, periodotites, pyroxenites and gabbros. These volcanic sequences conform to the trilogy proposed bySteinmann (1926), although we could not find primary contacts between ultrabasic and basic ophiolites. The contacts are masqued by the effect of imbricate thrust-sheets where the tectonic contact zones are now filled with serpentinites.In addition all the ophiolites were transformed into true greenschists during the alpine metamorphism.Only the basic submarine volcanics will be discussed and described in this work. Comparisons with some well studied examples of submarine basalts (Monti Iblei, Sicily and Quadra Island, British Columbia) gave us the key to recognize in the Platta-Decke equivalent suites of sills, flows, pillow-lavas, pillow-breccias and meta-hyaloclasites.Petrographically all these rocks are now diabases with a basaltic or spilitic chemical composition.Within the Platta-Decke we can recognize from the top to the botton a sequence which starts with anchimetamorphic and continues into epimetamorphic rocks.The clear outcrops allowed us to distinguish the original volcanic structures where the intensity of metamorphism had not destroyed them. Thus we could follow the gradual transition from pillow-lavas and meta-hyaloclastites s. l. through the greenschist-facies into the facies of prasinitisation. These observations contributed significantly to the understanding of the alpine greenschists. By thorough investigations of structures and sequences of the eugeosynclinal volcanic and associated sediments, it was possible to demonstrate the structural relationships within the Platta-Decke.

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Résumé La nappe pennique supérieure du Platta dans les Grisons (Suisse) est formée essentiellement d'ophiolithes. Ces roches s'associent étroitement à des sédiments, tels que radiolarites, cherts, schistes calcaires (calcschistes) du Jurassique supérieur, schistes marneux et argileux et quelques microbrèches, qui semblent représenter les avant-coureurs du flysch crétacé. Par ophiolithes nous entendons les produits de magmas basiques et ultrabasiques liés à des zones de fracture et de faiblesse et qui la plupart du temps se sont insérés dans les sédiments géosynclinaux peu épais et encore meubles. Il s'agit donc d'anciens basaltes, péridotites, pyroxénites et gabbros. La trilogie deSteinmann (1926) y est aussi vérifiée, mais des contacts primaires entre ophiolithes basiques et ultrabasiques n'ont pu être reconnus. De possibles contacts ont été détruits par un écaillage intense, dont les horizons de décollement sont remplis de serpentinites. De plus, ces ophiolithes ont été légèrement métamorpbisées lors de l'orogénèse alpine. La multiplicité et la complexité de ces problèmes étant si grandes, nous écarterons les membres ultrabasiques. Des comparaisons avec des exemples soigneusement étudiés de basaltes sous-marins (Monti Iblei, Sicile et Quadra Island, Colombie britannique) ont permis de reconnaître à l'intérieur des écailles du Platta des cycles d'extrusions et d'intrusions basiques sous-marines: brèches de base, lave en coussins, brèches en coussins (maintenant des brèches méta-hyaloclastiques) et des méta-hyaloclastites s. l. Du point de vue pétrographique, ces volcanites sont naturellement des diabases possédant un chimisme basaltique aussi bien que spilitique. La sollicitation métamorphique dans la nappe passe progressivement du degré anchimétarnorphique au toit à un degré épimétamorphique vers la base. Les conditions favorables d'affleurement ont permis de suivre, correspondant à leur degré de métamorphisme, les changements opérés dans les structures volcaniques primitives. Ainsi les laves en coussins, les brèches et les méta-hyaloclastites traversent le « domaine faciel des schistes verts » pour atteindre celui de la « prasinitisation ».Ceci a jeté quelque lumière sur le problème des « schistes verts alpins ». L'étude des structures volcaniques, des cycles et des sédiments, a permis de débrouiller en grande partie l'écaillage tectonique à l'intérieur de cette nappe du Platta.

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- . . . , . . , , - . , , , , . , , , . (Mt. Iblei, Sizilien; Quadra, Island; Kolumbien). , ( , Pillow-Lava, Pillow-Breccien, Meta-Hyaloklastit)

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Die vorliegende Arbeit, die einen Teil meiner Dissertation darstellt, wurde auf Anregung meiner verehrten Lehrer Herrn Prof. Dr. A.Gansser und Herrn Prof. Dr. R.Trümpy unternommen, denen ich. für viele Diskussionen, Hinweise und wertvolle Kritik im Gelände als auch im Institut sehr danke.

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Ebenso gilt mein Dank Herrn Prof. Dr. F. deQuervain, Zürich, Herrn Prof. Dr. P.Bearth, Basel, Herrn Prof. Dr. M.Vuagnat, Genf und Herrn PD. Dr. Tj.Peters, Bern, die mit ihren reichen Kenntnissen in alpinen Ophiolithen viele mineralogische und petrographische Anregungen, Ratschläge sowie fruchtbare Diskussion vermittelten.

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Herrn Prof. Dr. M.Weibel, Zürich, danke ich bestens für die Anfertigung der chemischen Analysen.

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Für die Altersbestimmung sei Herrn Dr. F. W.McDowell und Herrn Prof. Dr. P.Signer, Zürich, sehr gedankt.

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Den Mitarbeitern des Institutes für Kristallographie und Petrographie der ETH, besonders Herrn E.Schähli und Herrn R.Gubser möchte ich für ihre technische Hilfe in der Dünnschliffanfertigung, Photographie und Röntgenographie danken.     

U-Pb and Rb-Sr radiometric dates and their correlation with metamorphic events in the granulite-facies basement of the serre,Southern Calabria (Italy)

An approximately 7 km thick, continuous sequence of granulite-facies rocks from the lower crust, which contains a lower granulite-pyriclasite unit and an upper metapelite unit, occurs in the NW Serre of the Calabrian massif. The lower crustal section is overlain by a succession of plutonic rocks consisting of blastomylonitic quartz diorite, tonalite, and granite, and is underlain by phyllonitic schists and gneisses.Discordant apparent zircon ages, obtained from granulites and aluminous paragneisses, indicate a minimum age of about 1,900 m.y. for the oldest zircon populations. The lower intersection point of the discordia with the concordia at 296±2 m.y. is also marked by concordant monazites. Therefore, the age of 296±2 m.y. is interpreted as the minimum age of granulite-facies metamorphism.Concordant zircon ages were obtained from a metamorphic quartz monzogabbronorite sill (298±5 m.y.) and an unmetamorphosed tonalite (295±2 m.y.); they are interpreted as the intrusion ages.Discordant zircon ages from a blastomylonitic quartz diorite gneiss, situated between the lower crustal unit and the non-metamorphosed tonalite, reveal recent or geologically young lead loss by diffusion. The ²⁰⁷Pb/²⁰⁶Pb ages of the two analysed size-fractions point to an intrusion age similar to that of the overlying tonalite.Rb-Sr mineral ages are younger in the granulite-pyriclasite unit than in the overlying metapelite unit. Feldspars from the granulite-pyriclasite unit yield ages of about 145 m.y. and those from the metapelite unit 176±5 m.y. In the same way, the biotite cooling ages range between 108 and 114 m.y. in the granulitepyriclasite and between 132 and 135 m.y. in the metapelite unit and the tonalite. Some still younger biotite ages are explained by the influence of tectonic shearing on the Rb-Sr systems. A muscovite from a postmetamorphic aplite in the metapelite unit yields a cooling age of 203±4 m.y.The Rb-Sr isotopic analyses from migmatite bands do not lie on an isochron, perhaps due to limited isotopic exchange between the small scale layers during the long cooling period after the peak of metamorphism.In the phyllonitic gneisses and schists a Hercynian metamorphism is indicated by a muscovite age of 268±4 m.y., whereas the biotite age of 43±1 m.y. from the same sample can be correlated with an Alpine greenschist-facies metamorphism.On the basis of the radiometric dates and of the P-T path of the lower crustal section deduced petrologically, the following model is presented: the end of the Hercynian granulite-facies metamorphism was accompanied by an uplift of the lower crustal rocks into intermediate crustal levels and by synchronous plutonic intrusions into the lower crust and higher crustal levels, but essentially into the latter. Substantial further uplift did not occur until after cooling from the temperature of the granulite-facies metamorphism to the biotite closing temperature. This cooling lasted for about 185 m.y. in the lower part and for about 160 m.y. in the upper part of the lower crust section.A comparison between the geologic evolutions of the NW Serre of Calabria and the Ivrea Zone of the Alps demonstrates striking similarities. The activity of deep seated faults in both areas at least since late Hercynian time raises the possibility that a fault precursor of the boundary of the Adriatic microplate already existed at this time.     

Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting

During ten days of phreatomagmatic activity in early April 1977, two maars formed 13 km behind the Aleutian arc near Peulik volcano on the Alaska Peninsula. They have been named “Ukinrek Maars”, meaning “two holes in the ground” in Yupik Eskimo. The western maar formed at the northwestern end of a low ridge within the first three days and is up to 170 m in diameter and 35 m in depth. The eastern maar formed during the next seven days 600 m east of West Maar at a lower elevation in a shallow saddle on the same ridge and is more circular, up to 300 m in diameter and 70 m in depth. The maars formed in terrain that was heavily glaciated in Pleistocene times. The groundwater contained in the underlying till and silicic volcanics from nearby Peulik volcano controlled the dominantly phreatomagmatic course of the eruption.During the eruptions, steam and ash clouds reached maximum heights of about 6 km and a thin blanket of fine ash was deposited north and east of the vents up to a distance of at least 160 km. Magma started to pool on the floor of East Maar after four days of intense phreatomagmatic activity.The new melt is a weakly undersaturated alkali olivine basalt (Ne = 1.2%) showing some transitional character toward high-alumina basalts. The chemistry, an anomaly in the tholeitic basalt-andesite-dominated Aleutian arc, suggests that the new melt is primitive, generated at a depth of 80 km or greater by a low degree of partial melting of garnet peridotite mantle with little subsequent fractionization during transport.The Pacific plate subduction zone lies at a depth of 150 km beneath the maars. Their position appears to be tectonically controlled by a major regional fault, the Bruin Bay fault, and its intersection with cross-arc structural features. We favor a model for the emplacement of the Ukinrek Maars that does not link the Ukinrek conduit to the plumbing system of nearby Peulik volcano. The Ukinrek eruptions probably represent a genetically distinct magma pulse originating at asthenospheric depths beneath the continental lithosphere.     

Coexisting phengite,paragonite and margarite in metasediments of the mittlere Hohe Tauern,Austria

The assemblages phengite-paragonite, phengite-margarite and phengite-paragonitemargarite are very common in metasediments of a N-S profile in the middle sector of the Hohe Tauern. The Si⁴⁺-content of phengite shows no regular change with increasing temperature from north to south along the profile. The variations in the d ₀₀₂ basal spacings of phengite coexisting with paragonite are not only dependent on the Na⁺ content of phengite but also on the Mg²⁺+Fe²⁺ content of the micas. Neither the sodium content in phengite nor the potassium content in paragonite shows any dependence on temperature. Chemical analyses of coexisting phengite, paragonite and margarite give the extent of the three-phase-region which is characterized by a small amount of margarite in paragonite (4 Mol%), by a large quantity of Na⁺ in margarite (28 Mol% paragonite), and limited miscibility between phengite and paragonite.     

Computer modelling of Al/Si ordering in sillimanite

Computer simulation is used to investigate the effect of Al/Si disordering over the tetrahedral sites on the lattice energy and the lattice constants of the mineral sillimanite Al₂SiO₅. A methodology for an atomistic assessment of the energy of the reaction 2(Si-O-Al)→(Si-O-Si)+(Al-O-Al) and its various contributions is established. This ordering energy is 0.97 eV for nearest neighbour sites in the ab-plane and 0.56 eV for those separated in the c-direction. The large difference is due to a greater constraint on the atomic relaxation in the ab-plane and shows the structural dependence of the ordering energy. Its magnitude appears to be determined by a complicated balance between Coulomb and short-range repulsive energy involving strain over many bonds, both in the ordered and disordered structures. There is also a significant interaction between second neighbour sites whereas the contribution of more distant neighbours is negligible. The lattice energies of most of the 154 configurations studied show a linear behaviour as a function of short-range order, specified by the number of Al-Al pairs. The ordering temperature T_c, estimated on the basis of a statistical mechanical model of disordering, and the calculated ordering energies are in semi-quantitative agreement with experimental values.     

Organic Geochemical Microanalysis by Time‐of‐Flight Secondary Ion Mass Spectrometry (ToF‐SIMS)

Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) is a powerful method for the chemical analysis of solid surfaces. In this paper, the capabilities and limitations of this technique and the potential for its use in geochemical research are outlined. Using ToF‐SIMS, the chemical composition of sample structures down to 10–100 μm can be determined, without the need for pre‐selection or labelling of the analysed substances. In addition, the lateral distribution of organic and inorganic compounds can be mapped in geochemical samples at a resolution in the micrometre range. The capabilities of the technique in geochemistry are illustrated by two examples. In the first example, it is shown that ToF‐SIMS can be used to detect biomarkers in oil samples, making it a promising method for the analysis of biomarkers in fluid inclusions. In the second example, a number of specific lipid biomarkers were identified and mapped on the surface of a microbial mat cryosection surface. Post‐measurement optical microscopy correlated the localisation of the lipids with the presence of methanotrophic archaea in the microbial mat.