Discussion de l'article de M. G. Michard: Signification du potentiel redox dans les eaux naturelles

Sans résumé     

Hydrothermal dolomite marbles associated with charnockitic magmatism in the Proterozoic Bamble Shear Belt,south Norway

Deformed and metamorphosed dolomite marbles in the Kragerø area of the Bamble Shear Belt occur as lenses within metasupracrustal sequences, as matrix in bodies resembling magmatic breccias, and as veins/dykes cutting amphibolites and metagabbros. A common origin is not evident from the field relationships, but is nevertherless probable due to great geochemical similarities between dolomite from the different occurrences. They are characterized by higher REE, Ni, Co, Cr and Sc, and lower Ba and Sr contents relative to metasedimentary marbles occurring nearby. Sm-Nd isotope data shows that the dolomites are of Sveconorwegian age (1175±37 Ma). The dolomite marbles are very weakly LREE-enriched and display in most cases positive Eu anomalies. Their stable isotope compositions are uniform (¹⁸O=+9.6 to +10.7; ¹³C=-8.5 to-6.2), their initial ⁸⁷Sr/⁸⁶Sr ratios are high (0.706–0.709), whereas their age corrected _Nd varies from +0.7 to-1.5. Geochemically the dolomitic marbles differ considerably from sedimentary or metasedimentary marbles as well as from carbonatites. The Kragerø dolomite marbles represent deformed and metamorphosed hydrothermal veins or vein-complexes deposited in tensional fractures in the deep crust. Although the dolomitic marbles regionally are of minor volume, the dolomite deposition represents a specific and important event in the geological evolution of the Bamble Shear Belt. Their geochemical and isotopic homogeneity on a regional scale suggest that the hydrothermal solutions were supplied from a very large, homogeneous reservoir. Trace elements, stable and radiogenic isotopes, and field and isotopic age relationships are consistent with a deposition from hydrothermal solutions which were exsolved from crystallizing charnockitic intrusions and subsequently interacted with the crust. The parental magma to these intrusions underplated the crust and raised the geotherm of a carbonated ultramafic uppermost mantle, and imposed decarbonation and fluid release. These fluids were channelled into a large degassing zone now found as a deformed, regional zone with hydrothermal dolomite deposits, albitites, apatite-veins and widespread scapolitization. Whether the CO₂-rich fluids, which precipitated the Kragerø dolomites, pervasively infiltrated crustal rocks at a deeper level and caused granulitization is ambiguous, but possible.     

Hydrological budget,carbon sources and biogeochemical processes in Lac Pavin (France): Constraints from δO of water and δC of dissolved inorganic carbon

Lac Pavin (French Massif Central) is a permanently stratified lake: the upper water layers (mixolimnion, from 0 to 60 m depth) are affected by seasonal overturns, whereas the bottom water layers (monimolimnion, from 60 to 90 m depth) remain isolated and are never mixed. Hence, they are capable of storing important quantities of dissolved gases, mainly CO₂. With the aim of better constraining the water balance and of gaining new insights into the carbon cycle of Lac Pavin, an isotopic approach is used. The δ¹⁸OH₂O$\delta {}^{18}{\text{O}}_{{\text{H}}_2\text{O}}$ profiles lead the authors to give a new evaluation of the evaporation flow rate (8 L s⁻¹), and to propose and characterize two sub-surface springs. The sub-surface spring located at the bottom of the lake can be deduced from the 1% isotopic difference between the upper water layers (mean δ¹⁸OH₂O$\delta {}^{18}{\text{O}}_{{\text{H}}_2\text{O}}$ value: −7.3‰) and the bottom water layers (δ¹⁸OH₂O=-8.4‰$\delta {}^{18}{\text{O}}_{{\text{H}}_2\text{O}}=-8.4‰$). It is argued that this sub-surface spring has isotopic and chemical characteristics similar to those of the magmatic CO₂-rich spring (i.e. Fontaine Goyon, δ¹⁸OH₂O=-9.4‰$\delta {}^{18}{\text{O}}_{{\text{H}}_2\text{O}}=-9.4‰$), and we calculate its flow rate of 1.6 L s⁻¹. The second sub-surface spring is located around 45 m depth, with a composition close to those of the water surface streams (δ¹⁸OH₂O<-7.6‰$\delta {}^{18}{\text{O}}_{{\text{H}}_2\text{O}}<-7.6‰$).     

X-rays spectroheliograms in lines of Mg xi and Mg xii

Spectroheliograms in the L Mg xii line and in the Mg xi resonance (R) 1s ²¹ S ₀-1s2p ¹ P ₁ line, intercombination (I) 1s ²¹ S ₀-1s2p ³ P _1,2, line, and the forbidden (F) 1s ²¹ S ₀-1s2s ³ S ₁ line, have been obtained.Two Bragg crystal spectrometers were used mounted with mechanical collimators to obtain a spatial resolution of 1 × 3. The apparatus was launched on a sounding rocket on July 2nd, 1971. A particularly thorough study was made of the brightest active region (MC 11402).Variations in the F to I Mg xi line intensity ratio from one point to another in the active region did not reveal the presence of high electron densities.The observed intensities of the Mg xi R line, Mg xii L line and Mg x 1s ²2s ² S _1/2-1s2p ¹ P 2s × × ² P1_{1/2, 3/2} S line are not well explained by an isothermal model. Good agreement between computed and observed intensities is obtained using the non-isothermal model proposed here.     

The Pb-Sr-Nd isotope geochemistry of some recent circum-Mediterranean granites

Pb, Sr and Nd isotopic compositions have been analyzed in recent granites from Northern Africa, Northern Italy and Greece. Lead isotope compositions of K-feldspars are rather homogeneous, and cluster close to the modern lead of Stacey and Kramers (1975) but with slightly higher²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratios. The Cyclades samples, however, have higher²⁰⁶Pb/²⁰⁴Pb ratios. Addition of mantle-derived lead was probably very limited, which supports a quasi-closed system evolution of this element in the continental crust. The Sr, Nd data fall in the enriched part of the¹⁴³Nd/¹⁴⁴Nd vs.⁸⁷Sr/⁸⁶Sr diagram and define a smooth hyperbolic mixing curve. Over a wide area, straddling different orogens, most granites may be accounted for by a binary mixture between a recycled crustal component and a depleted mantle-like component. No correlation is observed between either Pb and Sr or Nd isotopic ratios, or any isotopic ratio and major element contents. Quantitative modelling suggests that two cases fit the Sr and Nd characteristics of these granites: they both require anatexis of the crust on a scale large enough to average the isotopic properties of heterogeneous terranes. In the first case, the mantle-derived component may be represented by differentiated Island Arc-type magmas, and the granites result from mixing these magmas with anatectic melts. In the second case, mantle-derived igneous rocks, such as obducted ophiolites, are part of the crustal source and their variable involvement in the anatectic process causes isotopic variations.CRPG Contribution n 630.     

Chemical mass balance of calcrete genesis on the Toledo granite (Spain)

The chemical mass balance of calcrete genesis is studied on a typical sequence developed in granite, in the Toledo mountains, Central Spain.

Field evidence and petrographic observations indicate that the texture and the bulk volume of the parent rock are strictly preserved all along the studied calcrete profile.

Microscopic observations indicate that the calcitization process starts within the saprolite, superimposed on the usual mechanisms of granite weathering: the fresh rock is first weathered to secondary clays, mainly smectites, which are then pseudomorphically replaced by calcite. Based on this evidence, chemical mass transfers are calculated, assuming iso-volume transformation from the parent rock to the calcrete.

The mass balance results show the increasing loss of matter due to weathering of the primary phases, from the saprolite towards the calcrete layers higher in the sequence. Zr, Ti or Th, which are classically considered as immobile during weathering, are also depleted along the profile, especially in the calcrete layer. This results from the prevailing highly alkaline conditions, which could account for the simultaneous precipitation of CaCO₃ and silicate dissolution.

The calculated budget suggests that the elements exported from the weathering profile are provided dominantly by the weathering of plagioclase and biotite. We calculate that 8–42% of the original Ca remains in granitic relics, while only 15% of the authigenic Ca released by weathering is reincorporated in the calcite. This suggests that 373 kg/m² of calcium (i.e., three times the original amount) is imported into the calcrete from allochtonous sources, probably due to aeolian transport from distant limestone formations.