Mg isotopic composition of carbonate: insight from speleothem formation

Simultaneous high-precision measurement of ²⁴Mg, ²⁵Mg and ²⁶Mg isotopic compositions were made by multiple collector inductively coupled mass spectrometry (MC-ICP-MS) relative to the international standard SRM980. Data are presented on low-Mg calcite speleothems and their associated host rocks and waters from four caves, one in the French Alps and three in Israel, covering various climate conditions. In addition, data are presented on three dolostones and three limestones from the Himalaya. The overall variation is 4.13‰ and 2.14‰ in δ²⁶Mg and δ²⁵Mg, respectively. This is 35 times the uncertainty of the measurements and clearly demonstrates that the terrestrial isotopic composition of Mg is not unique. Each speleothem shows a characteristic range of δ²⁶Mg values that are attributed to the isotopic composition of the local water. Differences between the isotopic composition of Mg in the water dripping from stalactites and that of the modern speleothem are interpreted as being due to Mg isotopic fractionation during carbonate precipitation in the temperature range of 4-18°C. The low-Mg calcite is enriched in light isotopes by 1.35‰/AMU and the dependence on temperature has been found to be less than 0.02‰/AMU/°C. Despite various geological settings, the δ²⁶Mg of the studied dolostones is 2.0±1.2‰ higher than the δ²⁶Mg of the limestones. All together, these results suggest a strong mineralogical control and a weak temperature effect on the Mg isotopic composition of carbonate.     

Geowissenschaftliche Probleme bei der Endlagerung radioaktiver Substanzen in Salzdiapiren Norddeutschlands

In the Federal Republic of Germany disposal of low-level radioactive waste is practiced in rock salt of the salt dome Asse since years. Projects also exist for the disposal of high-level waste from nuclear reactors in salt domes. The feasibility of the “evaporite concept? has to be founded on some basic principles of the origin and metamorphism of marine salt deposits. The German Zechstein salt deposits (Permian) were formed about 230 million years ago. Since that time the salt formations have had a chequered history: 1. Solution metamorphism and thermal metamorphism have altered the primary chemical and mineralogical composition of salt rocks in the presence of unsaturated solutions and/or temperatures up to 100? C and more. 2. Mechanical deformation and dynamic metamorphism processes have preserved the chemical and mineralogical composition of the evaporites. Solution metamorphism: The minerals halite, sylvite, and carnallite are very sensitive to subsequent penetrating unsaturated solutions. Very important in this context is the origin of specific equilibrium solutions and mineral associations. Primary salt rocks (e. g. carnallite rock) have been altered to secondary mineral associations (e. g. Hartsalze and sylvite rocks of the potash seams Thüringen, Hessen, Stassfurt). Most of the solutions which infiltrated the salt deposits from outside are later squeezed out into the adjacent rocks. Geochemical and physicochemical processes of the solution metamorphism are not limited to the geologic past. They are also efficient at the present time as a result of deformation processes in partially extracted salt deposits, for example. Solution metamorphism processes are also possible in the future during the period of about one million years which is necessary for the disposal of high-level radioactive waste in salt domes. A concept which is based upon observed properties of salt formations and with secure scientific foundation must guarantee no radioactive contamination of salt solutions in the case of long time contacts between radionuclides and brines. An excellent method for the immobilisation of radioactive waste seems to be production of a synthetic igneous rock system SYNROC (Ringwood, 1978). In such a rock the radionuclides are fixed in the lattices of minerals. Additional safety barriers which prevent the entry of radionuclides from the geologic barrier into the biosphere are the metallic alloy Ni₃Fe, for waste containers and layers of basalt and corundum. Borosilicate glasses are not suitable for the safe immobilisation of radionuclides because the glasses readly devitrify in contact with solutions and steam at elevated pressures and temperatures. Thermal metamorphism: The heat-generation of high-level waste produces zones of different temperatures in the evaporites of the salt domes. The salt hydrates kainite and carnallite react at relative low temperatures (> 072? C and > 80? C) and form solid phases with less hydrated compounds and equilibrium solutions. For example, effects of thermal metamorphism are to be expected in the case of influencing higher temperatures upon carnallite rocks which were squeezed into parts of rock salt during the geologic past. Increasing temperatures generated by radionuclides must be limited in carnallite rocks to 30 %–50 % of the recent rock temperatures in salt domes. A difference of 10? C should be guaranteed between the temperature zones around the high-level waste and the beginning of carnallite dehydratation at 80? C–85? C under normal pressure. Dynamic metamorphism: Dynamic metamorphism is concerned mainly with isochemical and isophase recrystallizations. The original thickness and composition of evaporites can be altered by the property of plasticity and by creep deformations of salt minerals and rocks. Solutions (e. g. pore solution) and increasing temperatures lead to the effects of dynamic metamorphism. A concept which is based upon observed properties of salt formations and with secure scientific foundation must guarantee temperatures ≦ 100? C in the vicinity around the high-level waste containers. Only for temperatures up to 100? C are relevant geologic data about the plastic deformation of rock salt available in the German salt deposits. The effects of salt rock deformation with increasing temperatures > 100? C could be examined only by laboratory experiments. It seems to be a considerable uncertainty and a risk to develop experimental deformation models for salt rocks valid for a time of 10²–10³ years. The present program in the Federal Republic of Germany for a geologic disposal of radionuclides in evaporites is sometimes characterized as the best concept of the world (e. g. v.Weizsäcker, 1978). For such a general statement no geoscientific foundation is available today. Alternative geologic disposal programs seem to be more acceptable in view of their geoscientific and technical base (e. g.Ringwood, 1978).     

Die Bedeutung geochemisch-stratigraphischer Bezugshorizonte für die Altersstellung der Antimonitlagerstätte von Schlaining im Burgenland, Österreich

Zusammenfassung Es wird die Frage gestellt, ob es bei der Entwicklung unseres Planeten typische, an weltweite Lineamente gebundene Metallzufuhren in die Erdkruste gab, die ihren Erzinhalt so schicht- und zeitgebunden absetzten, daß er als Zeitmarke gewertet und zu stratigraphischen Einstufungen und Parallelisierungen auch fossilleerer Gesteine verwendet werden kann. Die Existenz derartiger geochemischer Leitschichten mit typischen Leitparagenesen und diejenige geochemisch-stratigraphischer Bezugshorizonte wird bejaht. Sie wird an Hand von Antimonitlagerstätten mit Scheelitführung bzw. an reinen schichtgebundenen Scheelitvorkommen (mit zum Teil Molybdän-, Beryllium- oder Goldführung) diskutiert. Hierbei wird näher auf die Altersstellung und Genese der Antimonitvorkommen von Schlaining im österreichischen Burgenland (in der Rechnitzer Schieferserie) eingegangen. Diese liegen räumlich isoliert zwischen den schichtgebundenen, altpaläozoischen Antimonit/Zinnober-Scheelit-Lagerstätten der Ostalpen und denen der Westkarpaten, die gemeinsam als ein geochemisch-stratigraphischer Bezugshorizont gewertet werden. Daraus wird für die Schlaininger Antimonitvorkommen das bereits in neueren, ungarischen Arbeiten für die Rechnitzer Schieferserie angegebene, altpaläozoische Alter bestätigt.

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Attention has been given to the question, whether during the evolution of our planet typical metals were conveyed along worldwide lineaments to the earth's crust, whereby ores have been deposited so stratabound and timebound that the ore content itself may be considered as a time indicator and may be used in stratigraphical positioning and correlating of rocks, even if these are not fossiliferous. The existence of such geochemical index horizons, containing typical index parageneses and of geochemical-stratigraphical marker horizons has been affirmed. The question has been discussed in connection with stibnite deposits containing scheelite and with stratabound scheelite occurrences free of stibnite (partially containing molybdenum, beryllium or gold). The age and genesis of the stibnite occurrences near Schlaining in the Burgenland, Austria (within the Rechnitz Schist Series) has been considered closely. These isolated occurrences are located between the stratabound, Early Paleozoic stibnite/cinnabar-scheelite deposits of the Eastern Alps and those of the Western Carpathians, which together are regarded as one common geochemical stratigraphical marker horizon. Thereby, not only the Early Paleozoic age of the Rechnitz Schist Series is confirmed, as recent Hungarian publications propose, but also the Early Paleozoic age of the Schlaining stibnite occurrences.

     

Die Klassifikation der Eruptivgesteine

Zusammenfassung Verf. berichtet über die Ergebnisse einer Umfrage, die verschiedene Fragen abklären sollte, die sich bei einer Klassifikation der Eruptivgesteine stellen. Auf Grund der Antworten und anschließender Diskussionen schlägt Verf. für Gesteine mit weniger als 90% dunkeln Gemengteilen eine Klassifikation vor, die auf dem Doppeldreieck Quarz-Alkalifeldspat-Plagioklas-Foid beruht. Verf. bespricht sodann die Abgrenzungen Andesit/Basalt, Diorit/Gabbro, Tephrit/Basanit und erörtert den Gebrauch verschiedener Gesteinsnamen (Granit, Dacit und Rhyodacit, Monzodiorit und Monzogabbro, Foyait und Plagifoyait, Trachyt und Phonolith, Essexit und Theralith, Pikrit und Alkalipikrit), femer die Verwendung besonderer Namen für paläovulkanische Ergußgesteine und die Nomenklatur im Bereich basaltischer Gesteine.

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The author reports the results of an enquiry which he addressed to those interested in petrography, with the aim to establish a suitable classification of igneous rocks. For rocks with less than 90 per cent of mafic minerals, the author proposes a classification on the base of the double triangle quartz-alkali feldsparplagioclase feldspathoid. He discusses the delimitations andesite/basalt, — diorite/gabbro, and tephrite/basanite, and the use of several rock names (as e. g. granite, dacite and rhyodacite, monzodiorite and monzogabbro, foyaite and plagifoyaite, trachyte and phonolite, essexite and theralite, picrite and alkalipicrite), as well as the use of special names for paleo-volcanic rocks and the nomenclature in the group of basaltic rocks.-- .

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Résumé L'auteur indique les résultats d'une enquête adressée aux pétrographes intéressés, dans le but d'établir une classification rationelle des roches éruptives. Pour les roches contenant moins de 90 pour cent d'éléments foncés, l'auteur propose une classification basée sur le double triangle quartz-feldspath alcalin- plagioclase-feldspathoïde. Il discute ensuite diverses questions de nomenclature, à savoir les délimitations andésite/basalte, diorite/gabbro, téphrite/basanite, ainsi que l'emploi de certains noms (granite, dacite et rhyodacite, monzodiorite et monzogabbro, foyaïte et plagifoyaïte, trachyte et phonolite, essexite et théralite, picrite et alcali-picrite), l'emploi de noms spéciaux pour les roches paléovolcaniques et la nomenclature dans le groupe des roches basaltiques.

     

Die Verteilung des Yttriums und der Lanthaniden in einem Olivin-Alkali-Basalt mit Peridotit-Einschlüssen

Zusammenfassung In zwei Peridotit-Einschlüssen und dem sie umgebenden Olivin-Alkali-Basalt des Hirzsteins (Nordhessen) wurde die Verteilung des Yttriums und der Lanthaniden quantitativ bestimmt. Die Summe der Lanthaniden (La-Lu, einschließlich Y) beträgt für die Peridotit-Einschlüsse 10–20 ppm, für den Olivin-Alkali-Basalt 280–360 ppm. Die relative Verteilung der Lanthaniden in den Peridotit-Knollen, bezogen auf Chondrite, gibt keinen Hinweis auf große Unterschiede zwischen den leichteren (La-Sm) und schwereren (Eu-Lu) Elementen. Dagegen sind in dem Olivin-Alkali-Basalt die Lanthaniden La-Sm deutlich stärker angereichert gegenüber den Elementen Eu-Lu. Dieser Befund steht in Einklang mit der Vorstellung, daß die chemisch stärker fraktionierten Olivin-Alkali-Basalte eventuell aus weniger stark differenzierten primären Gesteinen (Eklogit, Granat-Peridotit, tholeiitischer Basalt) entstanden sind. In Übereinstimmung mit anderen Beobachtungen handelt es sich bei den Peridotit-Einschlüssen des untersuchten Vorkommens wahrscheinlich um Gesteinsstücke, die in keiner unmittelbaren genetischen Beziehung zu dem umgebenden Basalt stehen.

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The patterns of the relative distributions of the lanthanides have been determined in two lherzolite inclusions and their surrounding alkaline olivine basalt from a locality in Northern Hessia (Hirzstein), Germany. The total contents of Y, La-Lu in the lherzolite inclusions are 10–20 ppm and in the alkaline olivine basalt 280–360 ppm. There is no great difference in the ratios ppm Y, La-Lu in rocks/ppm Y, La-Lu in chondrites if we compare the light (La-Sm) and the heavier lanthanides (Eu-Lu) in the lherzolite inclusions. In the case of the alkaline olivine basalt however the related ratios increase from Sm to La to a maximum of about 250. These results correspond to an assumption that the alkaline olivine basalts with their higher degree of chemical fractionation are formed from less differentiated primary rocks (eclogite, garnet-peridotite, tholeiitic basalt). The lherzolite inclusions of the investigated locality have probably no direct genetic relation to the surrounding basalt.

     

Le minerai de fer d'Hajigak (Afghanistan). Position stratigraphique,cadre géologique et type du gisement

Age and grade of metamorphism of the considerable iron deposit of Hajigak (Central Afghanistan) have been controversial until now. A recent field study on the eastern part of the district produced new data concerning the geological environment and paragenesis of the ore. The iron beds are situated in the Upper Kalu Formation (= Awband Suite) of lower Paleozoïc, may be Silurian to Lower Devonian age. Basic lavas and tuffs, converted into green-schists, always are present near the ore bodies; and straight relationship is evident between basic rocks and iron ore. We consider the succession of events at Hajigak to be as follows: — at the bottom of a Paleozoïc sedimentary basin, an andesitic submarine volcanism brought along tuffs, lavas, and sericitic and chloritic sediments of exhalative origin; in the same time, hematite and magnetite precipitated. — afterwards gabbro and dolerite sills were intruded; as a result, iron oxides were remobilized, filling fractures across gabbro boundaries, and replacing neighbouring dolomites. — at last, but before Upper Devonian, Lower Paleozoïc rocks of Hajigak and Turkman area were affected by two kinds of metamorphism: the first one, of green-schist facies, is contemporaneous with magmatic activity; the second one, sericitic dynamo-metamorphism, appeared when the folding took place, mainly in the eastern region (Khesh and Zerok areas). In conclusion, the Hajigak iron ore appears to be a submarine — exhalative deposit, synchronous with a Lower Paleozoïc basic volcanic activity (but non ophiolitic).     

Geochemistry of the Rare Earth elements in spilites from the oceanic and continental crust

Analytical data including major elements, the Rare Earths, Ni, Cu, Zn, Rb, and Sr are presented for twenty-three spilites from the Mid-Atlantic Ridge, the Hercynian part of the Variscan geosyncline in Germany, and several localities in Switzerland. Low grade metamorphism (up to approximately 400° C) and spilitization of basaltic rocks apparently do not alter the original Rare Earth element (REE) distributions. This fact permits comparison of the spilites and unaltered tholeiitic basalts from the Mid-Atlantic Ridge. The relative REE distributions thus appear suitable for delineating the original basalt types of spilites formed by metamorphism. The spilites from the Hercynian part of the Variscan geosyncline have a REE distribution pattern which is characteristic of continental tholeiites. It is thus probable that during the formation of this geosyncline the principal magma extruded was of tholeiitic composition and that these rocks were later converted to spilites by metamorphism.