Geoid anomalies over Gorringe Ridge,North Atlantic Ocean

Gorringe Ridge is a strong uplifted block of oceanic crust and upper mantle lying at the eastern end of the Azores-Gibraltar plate boundary. The geoid over this structure derived from Seasat altimeter data exhibits a 9-m height anomaly with a north-south lateral extension smaller than 200 km. An attempt is made to interpret this geoid together with the gravity anomalies and with the seismicity, which has been compiled as a function of depth.It is first shown that the flexure of the oceanic lithosphere due to the ridge loading does not provide a good fit of the geoid anomalies and probably should be discarded, as it assumes a continuous unfractured elastic plate.Models involving local heterogeneities are then tested. The comparison of the observed geoid anomalies with the anomalies due to the uncompensated relief indicates that the topographic high has no shallow compensation.Uncompensated models, previously proposed to explain the gravity anomalies, are tested using the geoid. One model (Purdy and Bonnin, in Bonnin [11]), which involves an uplift of upper mantle material at depth, generates too strong geoid anomalies and must be discarded. Another model, which represents a nascent subduction zone (Le Pichon et al. [25]), fits both the gravity and geoid anomalies, but leads to difficulties in explaining the deep seismicity north of Gorringe Ridge.A model in isostatic equilibrium is also able to fit both gravity and geoid anomalies. This model involves a deep root of density 3.0 g cm^?3, as has been previously proposed for many oceanic ridges and plateaus. This model is compatible with the deep seismicity, but the origin of this low-density material at great depth is up to now an unresolved question.More likely, dynamical models taking into account the forces induced by the convection related to the slow plate convergence in this area will have to be considered.     

Chemistry of solutions from the 13°N East Pacific Rise hydrothermal site

Ten samples were recovered by the submersible “Cyana” submersible from two groups of hydrothermal vents located 2600 m deep along the East Pacific Rise at 13°N. The maximum measured temperature was 317°C and minimum pH 3.8. A systematic determination of major and trace elements has been carried out and mixing lines between a high-temperature component (HTC) and seawater are observed. The water chemistry of the HTC slightly differs for several elements at the two sites. This HTC is deprived of SO₄ and Mg and is greatly enriched in most other species. Maximum concentrations are (in units per kg):Cl = 0.72mol; Br = 1.1mmol; Na = 0.55mol; K = 29mmol; Rb = 14 μmol; Ca = 52mmol; Sr = 170 μmol; Mn = 750 μmol; Fe = 1mmol; Al = 15 μmol; Si = 21mmol. For many elements, the magnitude of the anomaly relative to seawater does not compare with the results obtained from the Galapagos or East Pacific Rise 21°N. The enrichment of cations relative to seawater is likely related to the huge Cl excess through charge balance. TheBr/Cl ratio is close to that for seawater. However, it is not clear whether the Cl excess is due to gas release or basalt hydration (formation of amphibole chlorite or epidote).P-T dependence of SiO₂ solubility suggests that water-rock interaction last occurred at a depth in excess of 1 km below the sea floor. A mixing line of⁸⁷Sr/⁸⁶Sr vs. Mg/Sr demonstrates that the HTCs have a nearly identical⁸⁷Sr/⁸⁶Sr ratio of 0.7041 for both sites. A water/rock ratio of about 5 is inferred, which differs from the 1.5 value obtained at 21°N.     

Constantes de formation des complexes hydroxydés de l'aluminium en solution aqueuse de 20 a 70°C

Stability constants of hydroxocomplexes of Al(III):Al(OH)²⁺ and A1(OH)₄^? have been measured in the 20–70°C temperature range by reactions involving only dissolved species. The stability constant ${}^1\text{K}{}_1$ of the first complex ion is studied by measuring pH of solutions of aluminium salts at several concentrations. ${}^1\text{β}{}_4$ of aluminate ion is deduced from equilibrium constants of the reaction between the trioxalato aluminium (III) complex ion and Al³⁺ in acid medium, and between the same complex ion and A1(OH)₄^? in alkaline medium. The K values and the associated ΔH are ${}^1\text{K}{}_1\text{= 10}{}^{\mathrm{?5.00}}$ and ΔH₁ = 11.8 Kcal; ${}^1\text{β}{}_4\text{= 10}{}^{\mathrm{?22.20}}$ and ΔH₄ = 42.45 Kcal. These last results are not in agreement with the values of recent tables for $\text{ΔG}{}^0\text{?}$ and $\text{ΔH}{}^0\text{?}$ of Al³⁺ and Al(OH)₄^?. We suggest a consistent set of data for dissolved and solid Al species and for some aluminosilicates.     

Age and significance of the North Pyrenean metamorphism

³⁹Ar-⁴⁰Ar and ⁸⁷Rb-⁸⁷Sr studies of some metamorphic minerals from the North Pyrenean zone indicate that they crystallized about 92–104 m.y. ago on the east, 85 m.y. or older on the west. An amphibole from a lherzolite in the eastern area gives a plateau age at 103 m.y. The North Pyrenean metamorphism is shown to be a thermal effect of forcible lherzolite emplacement along the North Pyrenean zone. This latter process is related to the early breakup of the Europe-Iberia plate during the middle Cretaceous time.     

Etude de l''acquisition d''éléments dissous par les eaux de lessivage des roches granitiques sous climat tempéré

Using chemical potential diagrams and budget calculations, we show that the acquisition of solutes by surface water in granitic rocks under temperate climate corresponds with transformation of primary minerals into kaolinite or halloysite. This mineral controls aluminium solubility.     

North American Precambrian history recorded in a single sample: high-resolution UPb systematics of the Potsdam sandstone detrital zircons,New York State

Zircons separated from the Cambrian Potsdam sandstone of New York yield four distinct populations which can be defined by a number of analytical techniques. U-Pb isotopic analyses of small samples and monozircons of each population reveal a fine chronology not apparent in milligram-sized sample analysis, and define source area ages of 1180, 1320, 2100 and 2700 m.y. for the Cambrian detrital suite. These ages correspond to well-defined sources in the Superior and Grenville Provinces of the Canadian Shield (2700 and 2100 m.y.) and the well established Grenville age rocks of the Adirondack Mountains (1180 m.y.). The 1320-m.y. age appears to be derived from the Adirondacks, and suggests the existence of pre-Grenville basement in that massif. Our techniques allow the interpretation of the Precambrian history of a large portion of eastern North America from a single sample, and thus should be valuable in the definition of source areas in paleogeographic reconstruction, and in studies of continental crustal evolution.     

Attenuation of Rayleigh waves across the volcanic area of the Massif Central,France

Rayleigh wave attenuation is investigated for periods ranging from 20 to 90 s, along a 450 km-long profile following the Oligocene tensile zone of the French Massif Central. A model is deduced by inversion, assuming that the S-wave intrinsic quality factor Q_β is frequency-independent, and yields a mean value Q_β = 43 ± 10 for the first 100 km in the upper mantle. This value, far lower than the mean value obtained in Eurasia, is close to those obtained in other recent tensile areas, e.g., the western United States or mid-oceanic ridges.A velocity-depth model for S-waves, deduced in a previous study from surface-wave propagation, has been corrected for the attenuation effect. We find a discrepancy between the corrected S-model and P-wave residuals in the same area, implying that Q_β must be frequency-dependent. This can be a clue for partial melting in the upper mantle beneath this region.     

Relations entre les concentrations des ions alcalins dans les eaux thermales sulfurées sodiques du Midi de la France

Alkali ion concentrations were studied in hot sulphurated springs in the south of France. Strong correlations between elements (K-Rb, K-Cs and Na-Li) are found. The element concentrations are also closely related with deep temperature as estimated by classical geothermometers. A discussion of the origin of the interelement correlations is presented. Further studies on the behaviour of trace elements during mineral dissolution are needed and can yield interesting methods of investigation in geothermal exploration.

Résumé

L'étude des concentrations des ions alcalins dissous dans les eaux thermales sulfurées du Midi de la France montrent de très nettes corrélations entre éléments (K-Rb, K-Cs et Na-Li) ainsi que des corrélations entre concentrations d'éléments et température profonde estimée à l'aide des géothermomètres classiques. Plusieurs suggestions quant à l'explication des relations observées sont présentées. Une bonne compréhension du comportement des ions en trace lors de la dissolution des minéraux par l'eau aurait certainement des retombées intéressantes en prospection géothermique.     

De l'emploi des methodes chimiques et spectrographiques pour l'etude des poteries antiques

Résumé Le but de mon travail est de vérifier dans quelle mesure l'analyse chimique et spectrographique des poteries antiques peut permettre de différencier entre elles ces poteries, et apporter des élémentes susceptibles de résoudre certains problèmes archéologiques de localisation.J'ai analysé un certain nombre échantillons de poteries antiques — 300 échantillons environ — qui ont été mis à ma disposition par MonsieurVallet, professeur à la Faculté des Lettres de Clermont-Ferrand. Ces échantillons sont de provenance diverse: France, Italie, Sicile, Grèce propre et îles grecques, Asie Mineure, Afrique du Nord.L'analyse chimique a porté sur les éléments suivants: Fer, Titane et Manganèse. L'analyse spectrographique porte sur les oligoéléments métalliques suivants: Plomb, Etain, Argent, Cuivre, Chrome, Nickel et Cobalt.Les teneurs en Titane, Plomb, Etain et Argent ne montrent pas de différences significatives. Par contre, les teneurs en Fer, Manganèse d'une part, Cuivre, Chrome, Nickel et Cobalt d'autre part, apportent des éléments de différenciation.

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The aim of this study is to verify to what extent chemical and spectrochemical analysis can distinguish among sherds from various localities and to give new data to solve some of the archaeological problems about location.About 300 sherds have been analysed; I got them from D.Vallet, Professor at the Faculté des Lettres in Clermont-Ferrand, Auvergne — These samples come from various countries, namely from: France, Italy, Greece, Greek Islands, Asia Minor, North Africa.The chemical analysis was for: Iron, Titanium and Manganese. The determination of trace elements such as: Lead, Tin, Silver, Copper, Chronium, Nickel, Cobalt, is now being carried out by means of spectrochemical analysis.The amount of Titanium, Lead, Tin and Silver shows no significant differences — On the contrary the amount of Iron, Manganese, Copper, Chronium, Nickel and Cobalt, give a good criterion for comparing and contrasting sherds of different provenance.

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Zusammenfassung Das Ziel meiner Arbeit ist, zu untersuchen, in welchem Maße es die chemische und spektrographische Analyse von antiken Tonwaren erlaubt, zwischen diesen Tonwaren zu unterscheiden und Lösungen für archäologische Ortsprobleme zu finden.Ich habe eine bestimmte Anzahl von antiken Tonwaren — ungefähr 300 Proben — analysiert die mir M.Vallet, Professor an der Geisteswissenschaftlichen Fakultät von Clermont-Ferrand, zur Verfügung gestellt hat. Diese Proben sind verschiedener Herkunft: Frankreich, Italien, Sizilien, Griechisches Festland, Griechische Inseln, Kleinasien und Nordafrika.Die chemische Analyse betraf die folgenden Elemente: Eisen, Titan, Mangan. Durch die spektrographische Analyse wurden die folgenden Elemente untersucht: Blei, Zink, Silber, Kupfer, Chrom, Nickel und Kobalt.Der Gehalt an Titan, Blei, Zink und Silber der einzelnen Proben zeigte keine wesentlichen Differenzen. Im Gegensatz dazu schwankte der Gehalt an Eisen und Mangan einerseits und Kupfer, Chrom, Nickel und Kobalt anderseits.

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Apatite fission track analysis in the Argentera massif: evidence of contrasting denudation rates in the External Crystalline Massifs of the Western Alps

Apatite fission track dating from a central transect in the Argentera massif (southernmost External Crystalline Massif = ECM) yielded ages between 8.05 ± 0.6 and 2.4 ± 0.2 Myr, with a positive age/altitude correlation above 3 Ma, 1200 m. Recognising a thermal peak at c . 250°C, 33 Ma, based on stratigraphic, metamorphic and ³⁹Ar/⁴⁰Ar data, the present results suggest a slow cooling rate (8–5°C) for the Argentera massif during the Oligocene–early Pliocene. This rate compares with that from the Pelvoux massif, but contrasts with those observed in the northern ECM (Mont-Blanc and Aar: up to 14°C Myr⁻¹) for the same time interval. This can be related to the different location of the ECM within the collided European margin. At about 3–4 Ma, the denudation rate would have increased up to c . 1 mm yr⁻¹ in the Argentera massif, reaching the same value as in the Belledonne and northern ECM, likely a consequence of Penninic thrust inversion.