Influence of anthropogenic activity on the lead isotope signature of Thau Lake sediments (southern France): origin and temporal evolution

Lead concentrations and isotopic compositions were determined on both bulk sediments deposited in the Thau lake in southern France during the last 200 years, and leachates derived from a series of sequential leachings of the sediments, making it possible to identify the sources, natural (i.e. indigenous lithologic) or anthropogenic, and to quantify the different inputs of Pb.Two distinct inputs of Pb could be distinguished. One of these corresponds to the terrigenous material entering the basin, representative of the local natural Pb ‘background’. Its supply remained steady most of the time with ²⁰⁶Pb/²⁰⁷Pb ratios of 1.200±0.003, except at the time of heavy storms producing voluminous and sudden depositions, such as that of September 1875. This Pb supply is mainly hosted by the detrital silicate fraction of the sediments. The second Pb input is a direct consequence of anthropogenic activities of various industrial and domestic emissions in the region, particularly due to the city of Sète and, to a lesser extent, to the villages in the watershed. The ²⁰⁶Pb/²⁰⁷Pb ratios of this input are of 1.142–1.162. The Pb added to gasoline could also be identified in the uppermost sediments, because of its specific ²⁰⁶Pb/²⁰⁷Pb ratios of 1.069–1.094. The leaching experiments also showed that the anthropogenic Pb is mainly hosted by the oxi-hydroxides of the sediments and to a lesser extent by the carbonates. It may also be adsorbed on particle surfaces, while only limited amounts are bound to organic matter.     

Paleomagnetic evidence for non-apenninic origin of the sila nappes (Calabria)

The magnetization of Lower Permian rocks from Sila has a mean direction D = 56.5°, I= +20.4° with ₉₅ = 9.1° after correction for Upper Neogene tilting. A further correction for the attitude of the nappes after their Middle Miocene emplacement establishes paleolatitudes consistent with those from the Lower Permian Tethys. The remarkable internal consistency of the data has not supported the distinction of units with opposite vergences within the Sila crystalline nappes. The declination indicates that the Sila massif has rotated counter-clockwise by about 90° relative to the Apennines, Sardinia and the Southern Alps and therefore the well-known Apenninic rotation alone does not account for the total change of direction in tectonic transport. Accordingly, the structural trends of tectonic phases older than the emplacement time of the Calabrian nappes should no longer be referred to present-day geographic coordinates. The post-Late Cretaceous motion relative to the north Calabrian Apennines enhances the geotectonic role of the northern boundary of the Calabrian—Peloritan arc, since its sinistral-shear character permits both tectonic transport from the west and counter-clockwise motion during tectonic transport.     

Geogenic and anthropogenic lead isotope signatures in the urban environment of Natal (NE-Brazil)

In this study the effect of anthropogenic emissions on the lead isotopic composition of sediments from the Potengi-Jundiai river system near the fast growing city of Natal, NE-Brazil, is investigated. The lead isotope signatures of sediments from the region of Natal were measured by ICP QMS and can be discussed in terms of three different end members of lead. Two geogenic lead endmembers can be distinguished and also be spatially separated, as higher lead isotope ratios occur in the vicinity of the town of Macaiba while the lead isotope ratios decrease towards the city of Natal and the mouth of the estuary. Proterozoic rocks of different age are potential lead sources as Paleoproterozoic rocks occur in the catchment of Jundiai river and younger, Neoproterozoic rocks predominate towards the mouth of the river. The lead isotope signatures of the anthropogenically affected samples deviate from the signatures of the unaffected samples indicating the existence of a third, anthropogenic source of lead. This source represents the lead isotope signature of anthropogenic emitters like waste- and coal-combustion which is also revealed by other geochemical studies conducted in Brazil.     

Stable isotope evidence for the origin of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel

The oxygen isotope compositions of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel, have been used to identify porewater types during diagenesis. Changes in porewater composition can be related to major geological events within southern Israel. In particular, saline brines played an important role in late (Pliocene-Pleistocene) dolomitization of these rocks. Diagenetic carbonates included early siderite (δ¹⁸O_SMOW=+24.4 to +26.5‰δ¹³C_PDB=?1.1 to +0.8‰), late dolomite, ferroan dolomite and ankerite (δ¹⁸O_SMOW=+18.4 to +25.8‰; δ¹³C_PDB=?2.1 to +0.2‰), and calcite (δ¹⁸O_SMOW=+21.3 to +32.6‰; δ¹³C_PDB=?4.2 to + 3.2‰). The petrographic and isotopic results suggest that siderite formed early in the diagenetic history at shallow depths. The dolomitic phases formed at greater depths late in diagenesis. Crystallization of secondary calcite spans early to late diagenesis, consistent with its large range in isotopic values. A strong negative correlation exists between burial depth (temperature) and the oxygen isotopic compositions of the dolomitic cements. In addition, the δ¹⁸O values of the dolomitic phases in the northern Negev and Judea Mountains are in isotopic equilibrium with present formation waters. This behaviour suggests that formation of secondary dolomite post-dates the tectonic activity responsible for the present relief of southern Israel (Upper Miocene to Pliocene) and that the dolomite crystallized from present formation waters. Such is not the case in the Central Negev. In that locality, present formation waters have much lower salinities and δ¹⁸O values, indicating invasion of freshwater, and are out of isotopic equilibrium with secondary dolomite. Recharge of the Inmar Formation by meteoric water in the Central Negev occurred in the Pleistocene, and halted formation of dolomite.     

Oxygen isotope evidence for the origin of discrete granitoid plutons from the Archean western Wabigoon Belt,northwestern Ontario

The whole-rock δ ¹⁸O values of samples from twelve discrete Archean plutons intruding the western Wabigoon granite-greenstone belt, northwestern Ontario, range from 6.8 to 9.5‰. Most samples with δ ${}^{18}\text{O}\text{> 8.7‰}$, however, come from portions of the Burditt Lake stock, or the Esox Lake area, that have been affected by deuteric or metasomatic activity. The distribution of δ ¹⁸O values for the remaining samples is very similar to that known for the large batholithic complexes and gneissic terrains that dominate this portion of northwestern Ontario. The generally low δ ¹⁸O values of the discrete granitoid plutons suggest that ¹⁸O-rich supracrustal rocks were unimportant in their genesis. Like the granitoids of the batholithic complexes and gneissic terrains, the discrete granitoid plutons represent new additions to the Archean sialic crust.     

Geochemical evidence for the origin of moldavites

Forty-eight moldavites and samples of rocks from the impact crater of Ries were analyzed using non-destructive neutron activation analysis. The following elements have been determined: La, Ce, Sm, Eu, Lu, Sc, Co, Cs, Hf and Th; and Rb and Cr in two moldavites. The darker moldavites, more common in Moravia, show higher contents of trace elements than those lighter in colour, from Southern Bohemia.Terrestrial igneous rocks cannot be regarded as a suitable source material, but terrestrial sandy to silty claystones show strong similarities in major and trace element abundances, and exhibit analogous inter-element variations to moldavites. The Tertiary claystones and sandstones which probably covered the Ries area before the impact, are a possible source rocks for moldavites.     

Stable isotope evidence for the atmospheric origin of CO2 involved in carbonation of MSWI bottom ash

Stable isotopes were used to constrain the origin of CO₂ involved in the ageing process of municipal solid waste incineration (MSWI) bottom ash under open-air conditions. The δ¹³C and δ¹⁸O values of CaCO₃ occurring in MSWI bottom ash samples of variable age and the δ¹³C of the residual organic matter content were measured, and laboratory assessments made of the isotopic fractionation accompanying CaCO₃ neo-formation during accelerated carbonation experiments of bottom ash or pure lime with atmospheric or industrial CO₂. The results indicate that stable isotopic compositions exhibited by fresh and aged bottom ash samples reflect non-equilibrium processes resembling those described in the carbonation of concrete and mortar. They also lead to conclusions on the prevalent involvement of atmospheric CO₂ in the open-air carbonation of MSWI bottom ash.     

Low-pressure,water-assisted anatexis of basic dykes in a contact metamorphic aureole,Fuerteventura (Canary Islands): oxygen isotope evidence for a meteoric fluid origin

Migmatites produced by low-pressure anatexis of basic dykes are found in a contact metamorphic aureole around a pyroxenite–gabbro intrusion (PX2), on Fuerteventura. Dykes outside and inside the aureole record interaction with meteoric water, with low or negative δ¹⁸O whole-rock values (+0.2 to −3.4‰), decreasing towards the contact. Recrystallised plagioclase, diopside, biotite and oxides, from within the aureole, show a similar evolution with lowest δ¹⁸O values (−2.8, −4.2, −4.4 and −7.6‰, respectively) in the migmatite zone, close to the intrusion. Relict clinopyroxene phenocrysts preserved in all dykes, retain typically magmatic δ¹⁸O values up to the anatectic zone, where the values are lower and more heterogeneous. Low δ¹⁸O values, decreasing towards the intrusion, can be ascribed to the advection of meteoric water during magma emplacement, with increasing fluid/rock ratios (higher dyke intensities towards the intrusion acting as fluid-pathways) and higher temperatures promoting increasing exchange during recrystallisation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Hydrogen isotope evidence for the origin and evolution of the carbonaceous chondrites

We present new hydrogen isotope data for separated matrix, hydrated chondrules, and other hydrated coarse silicate fragments from nine carbonaceous chondrites. These data were generated using a micro-analytical method involving stepped combustion of tens to hundreds of micrograms of hydrous solids. We also re-evaluate hydrogen isotope data from previous conventional stepped combustion experiments on these and other carbonaceous chondrites.Hydrogen isotope compositions of matrix and whole-rock samples of CM chondrites are correlated with oxygen isotope indices, major and minor-element abundances, and abundance and isotope ratios of other highly volatile elements. These correlations include a monotonic decrease in δD with increasing extent of aqueous alteration and decreasing abundances of highly volatile elements (including C, N and Ar), between extremes of ∼0‰ (least altered, most volatile rich) and −200‰ (most altered, least volatile rich). In plots involving only abundances and/or isotope ratios of highly volatile elements, CI chondrites fall on the high-δD, volatile rich end of the trends defined by CM chondrites; i.e., CI chondrites resemble the least altered CM chondrites in these respects. These trends suggest the protoliths of the CM chondrites (i.e., before aqueous alteration) contained an assemblage of volatiles having many things in common with those in the CI chondrites. If so, then the volatile-element inventory of the CI chondrites was a more widespread component of early solar system objects than suggested by the scarcity of recognized CI meteorites. Differences in volatile-element chemistry between the CI and average CM chondrites can be attributed to aqueous alteration of the latter.Previous models of carbonaceous chondrite aqueous alteration have suggested: (1) the protoliths of the CM chondrites are volatile poor objects like the CO or CV chondrites; and (2) the CI chondrites are more altered products of the same process producing the CM chondrites. Both suggestions appear to be inconsistent with hydrogen isotope data and other aspects of the volatile-element geochemistry of these rocks. We present a model for aqueous alteration of the CM chondrites that reconciles these inconsistencies and suggests revised relationships among the major subtypes of carbonaceous chondrites. Our model requires, among other things, that the water infiltrating CM chondrites had a δD value of ∼−158‰, consistent with initial accretion of CM parent bodies at ∼4 AU.     

Carbon isotope evidence for CAM photosynthesis in the Mesozoic

Crassulacean acid metabolism (CAM) photosynthesis, a flexible adaptation to water‐stress, has not been shown to occur before the Pleistocene, despite suggestions that CAM first evolved in the Mesozoic or earlier. Here we report on multiple (≥7) positive organic carbon isotope excursions (δ¹³C_org > ?20 ‰ ) in Late Triassic and Early Jurassic (c. 200 Ma) sedimentary rocks from an arid terrestrial ecosystem in South Africa. These excursions are interpreted as evidence of the episodic dominance of CAM plants in response to heightened aridity in the heartland of Gondwana, and hint at climatic instability in a greenhouse world.     

Isotopic evidence for the origin of sulfur in the Herrin (no. 6) coal member of Illinois

The sulfur isotopic composition of the Herrin (No. 6) Coal from several localities in the Illinois Basin was measured. The sediments immediately overlying these coal beds range from marine shales and limestones to non-marine shales. Organic sulfur, disseminated pyrite, and massive pyrite were extracted from hand samples taken in vertical sections.The $\text{δ}{}^{34}\text{S}$ values from low-sulfur coals (< 0.8% organic sulfur) underlying nonmarine shale were +3.4 to +7.3%₀ for organic sulfur, +1.8 to +16.8%₀ for massive pyrite, and +3.9 to +23.8%₀ for disseminated pyrite. In contrast, the $\text{δ}{}^{34}\text{S}\text{values from high-sulfur coals}$ (> 0.8% organic sulfur) underlying marine sediments were more variable: organic sulfur, ?7.7 to +0.5%₀, pyrites, ?17.8 to +28.5%₀. In both types of coal, organic sulfur is typically enriched in ³⁴S relative to pyritic sulfur.In general, δ ³⁴S values increased from the top to the base of the bed. Vertical and lateral variations in δ ³⁴S are small for organic sulfur but are large for pyritic sulfur. The sulfur content is relatively constant throughout the bed, with organic sulfur content greater than disseminated pyrite content. The results indicate that most of the organic sulfur in high-sulfur coals is derived from post-depositional reactions with a ³⁴S-depleted source. This source is probably related to bacterial reduction of dissolved sulfate in Carboniferous seawater during a marine transgression after peat deposition. The data suggest that sulfate reduction occurred in an open system initially, and then continued in a closed system as sea water penetrated the bed.Organic sulfur in the low-sulfur coals appears to reflect the original plant sulfur, although diagenetic changes in content and isotopic composition of this fraction cannot be ruled out. The wide variability of the δ ³⁴S in pyrite fractions suggests a complex origin involving varying extents of microbial H₂S production from sulfate reservoirs of different isotopic compositions. The precipitation of pyrite may have begun soon after deposition and continued throughout the coalification process.     

Lead-isotope evidence for a Hercynian origin of the Salsigne gold deposit (Southern Massif Central,France)

The Salsigne gold deposit contains a complex association of sulphide layers, gold-rich disseminations, quartz-bearing veins and flat reefs, which are hosted by folded and slightly metamorphosed Paleozoic sediments on both sides of a major thrust zone. It is demonstrated that these various Au-As ore types have similar lead-isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.22–18.56), which are characteristic of Hercynian ore deposits in the southern Massif Central. The Lower Cambrian to Devonian host rocks and associated Pb-Zn-Ba occurrences display distinctly less radiogenic corrected isotopic signatures (²⁰⁶Pb/²⁰⁴Pb = 17.83–17.98), which are characteristic of Cambrian lead in the Montagne-Noire. Concerning the controversial origin of the Salsigne gold mineralization, these results disagree with the former syngenetic hypothesis and support a new model of Hercynian syntectonic gold concentration.This work was supported by BRGM'S scientific program: Le gisement de Salsigne: caractérisation du modèle et évaluation du potentiel aurifère du district     

Stable isotope and fluid inclusion evidence for the origin of the Brandberg West area Sn–W vein deposits, NW Namibia

The Brandberg West region of NW Namibia is dominated by poly-deformed turbidites and carbonate rocks of the Neoproterozoic Damara Supergoup, which have been regionally metamorphosed to greenschist facies and thermally metamorphosed up to mid-amphibolite facies by Neoproterozoic granite plutons. The meta-sedimentary rocks host Damaran-age hydrothermal quartz vein-hosted Sn–W mineralization at Brandberg West and numerous nearby smaller deposits. Fluid inclusion microthermometric studies of the vein quartz suggests that the ore-forming fluids at the Brandberg West mine were CO₂-bearing aqueous fluids represented by the NaCl–CaCl₂–H₂O–CO₂ system with moderate salinity (mean=8.6 wt% NaCl_equivalent).Temperatures determined using oxygen isotope thermometry are 415–521°C (quartz–muscovite), 392–447°C (quartz–cassiterite), and 444–490°C (quartz–hematite). At Brandberg West, the oxygen isotope ratios of quartz veins and siliciclastic host rocks in the mineralized area are lower than those in the rocks and veins of the surrounding areas suggesting that pervasive fluid–rock interaction occurred during mineralization. The O- and H-isotope data of quartz–muscovite veins and fluid inclusions indicate that the ore fluids were dominantly of magmatic origin, implying that mineralization occurred above a shallow granite pluton. Simple mass balance calculations suggest water/rock ratios of 1.88 (closed system) and 1.01 (open system). The CO₂ component of the fluid inclusions had similar δ ¹³C to the carbonate rocks intercalated with the turbidites. It is most likely that mineralization at Brandberg West was caused by a combination of an impermeable marble barrier and interaction of the fluids with the marble. The minor deposits in the area have quartz veins with higher δ ¹⁸O values, which is consistent with these deposits being similar geological environments exposed at higher erosion levels.     

The onset of anthropogenic activity recorded in lake sediments in the vicinity of the Horne smelter in Quebec,Canada: Sulfur isotope evidence

Contents and δ³⁴S values of several S compounds, enumerations of S-reducing bacteria (SRB) and Fe-reducing bacteria (IRB), and Fe, Pb and In concentrations were determined for ²¹⁰Pb-dated sediment cores from two lakes in Quebec, Canada. Both lakes are located approximately 70 km downwind of the Horne smelter and refinery in Rouyn-Noranda. Increases in Fe, Pb and In concentrations and a decrease in the δ³⁴S values of total S in both lake sediment cores coincide with the start-up of the smelter in 1927. The shift towards more negative δ³⁴S values was primarily caused by an increase in the extent of S isotope fractionation during bacterial (dissimilatory) SO₄ reduction due to SO₄ loading of the lakes after smelting began. Consequently, an enhanced accumulation of ³²S-enriched reduced inorganic S compounds is evident in the sediments. δ³⁴S values of organic S in the sediments decreased only slightly due to the smelter emissions between 1930 and 1980. Hence, due to the sulfide depositing mechanisms, S isotope ratios constitute a useful tracer recording the onset of S pollution in sediments of the two previously SO₄-limited lakes investigated. In contrast, total S concentrations alone are not reliable indicators for anthropogenic S loading in lake sediment records.     

Saline groundwaters in the hercynian granites (Chardon Mine,France): geochemical evidence for the salinity origin

In this study, the chemical evolution of high Cl⁻ Chardon mine groundwaters is modelled as a mixing between an oxidising recharge and an old marine component on which the water–rock interaction is superimposed. Chemical and isotopic similarities with saline Carnmenellis mine groundwaters are emphasised and a general comparison with other brines is discussed.The cation content of deep granitic groundwaters is indicative of the water–rock interaction. In the case of Chardon and Carnmenellis groundwaters, the high Na/Cl ratio can still be related to the contribution of a brine of sedimentary origin to the water salinity. The differences in the hydrochemistry related to their geological context only appears at the trace metals level. On the contrary, brines in plutonic rocks which exhibit a low Na/Cl ratio represent groundwaters having a residence time in the host rock, long enough to equilibrate with secondary aluminosilicates. In that case, the brine origin is difficult to assess if only based on the water cation content.     

Sr isotopic evidence for a multi-source origin of the potassic magmas in the neapolitan area (S. Italy)

New Sr isotopic data on lavas and xenoliths from Somma-Vesuvius and other nearby volcanic areas (Phlegrean Fields and Ischia) are presented. Chemical and isotopic evidences show that not all the Phlegrean Fields rocks belong to the low K series, but some of them may be interpreted as low pressure differentiates of Somma magmas, i.e. as a part of the high K series. Two rock groups are defined in the Ischia low K series, which are well identified both in time and in chemical and isotopic features, and cannot be derived from the same magma source. The low K series in the studied area generally has lower Sr isotopic values than the high K series.Historical Vesuvian lavas show two distinct linear trends with negative slopes when⁸⁷Sr/⁸⁶Sr ratios are plotted against their ages of eruption. Such trends are interpreted to result from mixing of magmas in two separate reservoirs. Evidence from the Vesuvian ejecta shows that Somma-Vesuvius magmas underwent high or low pressure fractionation, in connection with different events of the Vesuvian activity. Distinct magma reservoirs developed episodically at different depths. Isotopic and geochemical evidences do not favour large scale assimilation of crustal materials by Somma-Vesuvius magmas, but instead appear to reflect mantle characteristics.A minimum of three different (inhomogeneous) source regions is necessary to account for the isotopic features of the studied rocks.     

O,H,and Sr isotope evidence for origin and mixing processes of the Gudui geothermal system,Himalayas,China

Tho Gudui geothermal field records the highest temperature at equivalent borehole depths among the lainland hydrothermal systems in mainland China.Located about 150 km southeast of Lhasa City,the capital of Tibet,the Gudui geothermal field belongs to the Sangri-Cuona rift belt,also known as the Sangri-Cuona geothermal belt,and is representative of the non-volcanic geothermal systems in the Himalayas.In this study,oxygen-18 and deuterium isotope compositions as well as ~(87)Sr/~(86)Sr ratios of water samples collected from the Gudui geothermal field were characterized to understand the origin and mixing processes of the geothermal fluids at Gudui.Hydrogen and oxygen isotope plots show both,deep and shallow reservoirs in the Gudui geothermal field.Deep geothermal fluids are the mixing product of magmatic and infiltrating snow-melt water.Calculations show that the magma fluid component of the deep geothermal fluids account for about 21.10%-24.04%;magma fluids lay also be a contributing source of lithium.The linear relationship of the ~(87)Sr/~(86)Sr isotopic ratio versus the 1/Sr plot indicates that shallow geothermal fluids form from the mixing of deep geothermal fluids with cold groundwater.Using a binary mixing model with deep geothermal fluid and cold groundwater as two end-members,the nixing ratios of the latter in most surface hot springs samples were calculated to be between 5% and 10%.Combined with basic geological characteristics,hydrogen and oxygen isotope characteristics,strontium concentration,~(87)Sr/(86)Sr ratios,and the binary mixing model,we infer the 6 th-Class Reservoirs Evolution Conceptual Model(6-CRECM) for the Gudui geothermal system.This model represents an idealized summary of the characteristics of the Gudui geothermal field based on our comprehensive understanding of the origin and mixing processes of the geothermal fluid in Gudui.This study may aid in identifying the geothermal and geochemical origin of the Gudui high-temperature hydrothermal systems in remote Tibet of China,whose potential for geothermal development and utilization is enormous and untapped.     

Fluid inclusion, stable isotope and Ar-Ar evidence for the age and origin of gold-bearing quartz veins at Mont Chemin, Switzerland

Summary A new Swiss gold occurrence at Mont Chemin, comprising gold-bearing quartz veins, displays many characteristics that are typical of mesothermal gold deposits within the Alps and globally. The most notable of these features are: i) the presence of NaCl-H₂O-CO₂-bearing fluid with an XCO₂ of approximately 0.016 and NaCl equivalents in the range 4.6 to 10.6 weight percent, ii) greenschist formational temperatures and pressures in the range 265-285 °C and 700-1400 bars; and iii) the proximity of the occurrence to the Rhone-Simplon Line, a deep crustal structure in the Swiss Alps.Corrected Ar-Ar data for hydrothermal adularia, considered to be contemporaneous with mineral deposition from the gold-bearing fluid, yields an age of 9.9 ±1.0 Ma. Geothermal gradients and uplift rates derived from the Ar-Ar age data and the geothermometry are in agreement with existing data for this region, and indicate that the hydrothermal activity at the Mont Chemin gold occurrence records one of the last Alpine metamorphic events in the northeastern Mont Blanc massif.Temperature estimates from fluid-muscovite-quartz-feldspar equilibrium and oxygen isotope thermometry of coexisting adularia and quartz are combined with the fluid inclusion isochores to derive depositional pressures. These data yield geothermal gradients on the order of 50 °C/km and uplift rates of 0.44 mm/a for the NE portion of the Mont Blanc massif.

---

Evidenz aus Flüssigkeitseinschluß-, stabilen Isotopen- und Ar- Ar Daten fü r das Alter und den Ursprung Gold führender Quarzgänge am Mont Chemin, Schweiz

Zusammenfassung Ein neues Schweizer Goldvorkommen am Mont Chernin, es handelt sich um Goldführende Quarzgänge, zeigt viele Charakteristika, die für mesothermale Goldlagerstätten der Alpen und weltweit typisch sind: i) Die Anwesenheit von NaCl-H₂O-CO₂ Fluiden mit einem XCO₂ von ca. 0.016 und NaCl zwischen 4.6 und 10.6 Gew. % Äquiv.ii) Grünschieferfazielle Bildungstemperaturen und -drucke von 265-285°C bzw. 7001400bar. iii) Die Nähe der Vorkommen zur Rhone-Simplon Linie, einer tiefgreifenden Struktur in der Kruste der Schweizer Alpen.Korrigierte Ar-Ar Daten von hydrothermalem Adular, der als zeitgleich mit den Minerallagerstätten gebildet, angesehen wird, ergaben ein Alter von 9.9 ± 1.0 Ma. Die aus aus den Ar-Ar Daten bestimmten geothermalen Gradienten und Hebungsraten und die Ergebnisse der Geothermometrie stimmen mit bisher existierenden Daten aus dieser Region überein und zeigen, daß die hydrothermale Aktivität in den Goldvorkommen des Monte Chemin eines der letzten alpidischen metamorphen Ereignisse im nordöstlichen Mont Blanc Massiv darstellt.Temperaturabschätzungen aus Fluid-Muscovit-Quarz-Feldspat Gleichgewichten und Sauerstoffisotopen-Thermometrie an koexistierendem Quarz und Adular werden mit den Isochoren der Flüssigkeitseinschlüsse kombiniert, um die Bildungsdrucke abzuleiten. Diese Daten ergeben geothermische Gradienten in der Größenordnung von ca. 50 °C/km und Hebungsraten von 0.44 mm/Jahr für den Nordostteil des Mont Blanc Massives.

     

A mantle-derived bimodal suite in the Hercynian Belt: Nd isotope and trace element evidence for a subduction-related rift origin of the Late Devonian Brévenne metavolcanics, Massif Central (France)

In the Brévenne Series (NE Massif Central), a low-grade bimodal association of metabasalts and metarhyolites is exposed, together with intrusive trondhjemite bodies. Zircon U-Pb dating constrains their magmatic emplacement at 366 ± 5 Ma and 358 ± 1 Ma, respectively. The metabasalts are characterized by a distinct enrichment in incompatible elements (e.g. Th and LREE) and positive ɛNd_i (from +5 to +8). Combined isotope and trace element systematics rule out crustal contamination of mafic melts as a suitable cause of the LILE (large ion lithophile element)-enrichment. Rather, a mixing process between a component similar to mid ocean ridge basalts and an enriched end-member with ɛNd_i > +5 is suggested. An enriched-mantle source of ocean island basalt affinity is precluded by the relative depletion of high field strength elements, especially Nb which shows negative anomalies in chondrite-normalized patterns. On the contrary, a subduction-related origin for the LILE enrichment would be more consistent. It may be inferred that arc-like melts [enriched in Th and LREE (light rare earth elements) and depleted in Nb, with ɛNd_i > +5] were produced through partial melting of a depleted-mantle source, to which a small amount of crustally derived component had been added. The metarhyolites are enriched in LILE, and have a close genetic relationship with the metabasalts, as evidenced by their high ɛNd_i (from +4.7 to +6.8). Although the chemical evidence remains ambiguous, it is suggested that fractional crystallization, accompanied by subordinate assimilation, is the petrogenetic process most consistent with the data. The trondhjemites are isotopically distinct from the metarhyolites. Their ɛNd_i values (from −1.0 to +2.2) reflect an important contribution of continental crust to their genesis, and disprove their inferred cogenetism with the felsic volcanics. A review of modern environments in which such bimodal suites are exposed, shows that settings involving incipient rifting of a volcanic arc fringing a continental margin, or built upon young, thin continental crust might provide suitable analogues. Geodynamic reconstructions are complicated by subsequent tectonic events which disrupted the initial patterns, and by Mesozoic-Cenozoic sedimentary cover. However, this subduction-related magmatism enlarges the growing body of evidence for southward subduction processes until the Late Devonian during the evolution of the northern flank of the European Variscides. As a general implication, it is suggested that the combined use of the Sm-Nd system with incompatible elements relatively resistant during alteration and low-grade metamorphism (REE, Th, Zr, Nb) may provide diagnostic criteria for recognizing the tectonic setting of bimodal metaigneous suites in ancient orogenic belts. Received: 21 April 1997 / Accepted: 30 June 1997