Podiform chromitite formation in a low-Cr/high-Al system: An example from the Southwest Indian Ridge (SWIR)

Recent reassessment of abyssal peridotites obtained during the dredging of the oblique supersegment and the easternmost subsection of the Southwest Indian Ridge by the R/V Knorr Cruise 162 and the R/V Yokosuka YK98-07 revealed the occurrence of dunites containing podiform chromitites and dunites with variable chromite concentration closely associated with lherzolite and harzburgite. The size of the chromitite pods varies from a few mm to 2 cm in width. Chromites in the podifom chromitites have very low Cr# (=0.22–0.23) and low TiO₂ (<0.17 wt%). They are almost free of silicate inclusions except for a few euhedral sulfide grains which occur far from cracks and lamellae and are considered primary in origin. The lherzolite which possibly represents the wallrock hosting the dunites with podiform chromitites also show low spinel Cr#(=0.16) and low Cr# in the clinopyroxenes (=0.09–0.10) and orthopyroxenes (=0.07–0.09). The small size of the SWIR podiform chromitites is strongly controlled by the low Cr/Al available in the wallrock and the invading melt. The presence of sulfide inclusions and the absence of PGEs further attest to the low Cr/Al (i.e. low refractoriness) in the system involved in the genesis of the SWIR podiform chromitites. Lastly, the discovery of podiform chromitites in the SWIR implies that the formation of podiform chromitite at mid-oceanic ridges, regardless of its spreading rate, is highly possible.     

Cumulate gabbros from the Southwest Indian Ridge, 54°S-7° 16′ E: implications for magmatic processes at a slow spreading ridge

A diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the Southwest Indian Ridge (54°S, 7°16 E) between the Islas Orcadas and Shaka Fracture Zones. The cumulus nature of the gabbroic rocks in the suite is indicated by phase, modal and cryptic layering, igneous lamination, and low incompatible element abundances. We present a mass-balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk-rock major element compositions. The model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid. Calculated proportions of trapped liquid range from 3%–15%; values that are characteristic of adcumulates to mesocumulates. Models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids, thus explaining the high TiO₂ contents observed in magnesian clinopyroxenes. Cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers, but disequilibrium plagioclase compositions suggest some crystallization at greater depth. Furthermore, basalt compositions projected onto the olivine-clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures (from less than 10 to greater than 20 kb) as well as polybaric fractional crystallization. We suggest that the Southwest Indian Ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros. The adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large, but ephemeral magma chambers below segments of the Southwest Indian Ridge.     

Three-dimensional model and late stage warping of the Plattengneis Shear Zone in the Eastern Alps

The Plattengneis shear zone is a 250–600 m thick, flat lying, Cretaceous, eclogite facies, mylonitic shear zone, with north-over-south transport direction, that is exposed over almost 1000 km² in the Koralpe region along the eastern margin of the Alps. Although the shear zone is one of the largest in the Alps, its role in the Eoalpine metamorphic evolution and the subsequent exhumation of the region, remain enigmatic and its large-scale geometry is not well understood. The outcrop pattern suggests that the shear zone is made up of a single sheet that is folded into a series of open syn- and antiforms with wavelengths of about 10 km. Eclogite bodies occur above, within and below the shear zone and there is no metamorphic grade change across the shear zone. In the south, the fold axes strike east–west and plunge shallowly to the east. In the north, the fold axes are oriented in north–south direction and form a dome shaped structure of the shear zone. Total shortening during this late stage warping event was of the order of 5%. Indirect evidence constrains this folding event to have occurred between 80 and 50 Ma and the fold geometry implies that the final exhumation in the Koralpe occurred somewhat later than further north. Interestingly, the shear zone appears to strike out of the topography in the south and dip into the topography in the north, so that north of the shear zone only hanging-wall rocks are exposed and south of it only foot-wall rocks. Possibilities for the geometric relationship of the Plattengneis shear zone with the surrounding south dipping detachments are discussed.     

Meteoric fluid‐rock interaction in Variscan shear zones

Variscan shear zones in the Armorican Massif represent sites of strong fluid‐rock interaction. The hydrogen isotope composition of muscovite (δD_Ms) from syntectonic leucogranite allows to determine the source of fluids that infiltrated the footwall of three detachment zones and the South Armorican Shear Zone. Using temperatures of hydrogen isotope exchange estimated from microstructural data, we calculate the hydrogen isotope ratios of water (δD_water) present within the shear zones during high‐temperature deformation. A ~40‰ difference in δD_water values from deep to shallow crustal level reveals a mixing relationship between deep crustal fluids with higher δD values that range from ?34 to ?33‰, and meteoric fluids with δD values as low as ?74‰ in the upper part of detachment footwalls.     

Effet de l'agrégation sur le transport de la kaolinite recouverte d'acide humique à travers un sable de quartz d'origine naturelleAggregation effect on the transport of humic-coated kaolinite colloids through a natural quartz sand

To evaluate the risk of contaminant transport by mobile colloids, it is necessary to understand how colloids and associated pollutants behave during their migration through uncontaminated soil or groundwater. In this study, we investigated the influence of aggregation induced by Ca²⁺ and trace metals (Pb²⁺, Cu²⁺) concentrations on the transport of humic-coated kaolinite colloids through a natural quartz sand at pH=4. Adsorbed divalent cations reduce the colloids surface charge and thereby induce aggregation and deposition in porous media. To cite this article: R. Ait Akbour et al., C. R. Geoscience 334 (2002) 981–985.     

Fine sediment transport and accumulations at the mouth of the Seine estuary (France)

A comprehensive study of fine sediment transport in the macrotidal Seine estuary has been conducted, including observations of suspended particulate matter (SPM), surficial sediment, and bathymetric data, as well as use of a three dimensional mathematical model. Tide, river regime, wind, and wave forcings are accounted. The simulated turbidity maximum (TM) is described in terms of concentration and location according to tidal amplitude and the discharge of the Seine River. The TM is mainly generated by tidal pumping, but can be concentrated or stretched by the salinity front. The computed deposition patterns depend on the TM location and are seasonally dependent. The agreement with observations is reasonable, although resuspension by waves may be overestimated. Although wave resuspension is likely to increase the TM mass, it generally occurs simultaneously with westerly winds that induce a transverse circulation at the mouth of the estuary and then disperse the suspended material. The resulting effect is an output of material related to wind and wave events, more than to high river discharge. The mass of the computed TM remains stable over 6 months and independent of the river regime, depending mainly on the spring tide amplitude. Computed fluxes at different cross-sections of the lower estuary show the shift to the TM according to the river flow and point out the rapidity of the TM adjustment to any change of river discharge. The time for renewing the TM by riverine particles has been estimated to be one year.     

Seasonal dynamics of aggregates and their typical biocoenosis in the Elbe Estuary

In 1995, an extensive investigation was carried out in the Elbe Estuary in Germany. Special attention was paid to organisms, including bacteria, amoebae, ciliates, flagellates, rotifers, larvae ofDreissena polymorpha, and nematodes, dispersed in the water column and associated firmly or loosely with different types of aggregates. The abundance, size, and colonization by microorganisms of the aggregates varied during the seasons. The largest aggregates were found during spring and summer, when diatoms, rotifers, and crustaceans were present. The most aggregates were encountered in spring and from summer to autumn. Most of the species observed during the study were more common in pelagic habitats than in benthic ones. However, the presence of ciliates in the groups Hypotrichida and Sessilida as well as as flagellates in the groups Biosoecida, Cercomonadida, Choanoflagellida, Chrysomonadida, Kinetoplastida, and amoebae and some nematodes in the open water depends, upon the availability of surfaces, because they are sessile or poor swimnters, and some flagellates and amoebae need to attach themselves to an object to feed. Most organisms were much more abundant in or on aggregates than in the surrounding water during spring and summer, which indicates that aggregates enhance the habitat and promote the development of the organisms. From spring through autumn, the structure of the community associated with the aggregates is influenced by the pelagic environment. The presence of the species in the benthic community was detected only during summer.     

Effect of some co-dissolved inorganic groups on the photodecomposition of aqueous nitrates

Summary The rate of nitrite formation, in UV-irradiated aqueous solutions of alkali metals, is found to depend on co-dissolved chemical compounds and on their reactivities. The effect of some common and simple components of the atmospheric aerosol is investigated in the laboratory.