Patrick Dudoignon

Alteration processes of a thick basaltic lava flow of the Paraná Basin (Brazil): petrographic and mineralogical studies

Abstract Petrographic and mineralogical studies of a 45 m thick basaltic lava flow in southern Parana Basin (Rio Grande do Sul, Brazil) enables a detailed description of three structural levels: the lower vesicular zone (LVZ), inner massive zone (IMZ), and upper vesicular zone (UVZ). The three levels, inherited from cooling stages, are characterized by vertical zonation of the petrographic features and associated secondary mineral assemblages, such as clay minerals and zeolites. Zeolite crystallization is limited to the vesicle infilling and partial replacement of albitized plagioclases. The clay mineral sequence observed in vesicle infilling is a celadonite, saponite, chlorite/saponite mixed layer. The mesostasis of the three levels, which constitutes reduced sites of clay mineral crystallization from the peripheral levels (top and base) to the inner, massive, and vesicle-free part of the flow, presents a saponite to C/S mixed layer sequence. Petrographic and chemical observations support three steps for the alteration mechanisms. The earliest alteration stages are related to postmagmatic mechanisms. They are marked by earliest celadonite precipitation in the oxidative condition of the highly permeable UVZ, saponite with homogeneous compositions in reducing conditions, and C/S mixed layer conversion in the inner part of the flow, where temperature gradients have been preserved during the final stages of cooling. The albitization of plagioclase associated with zeolite crystallization and the compositional changes in clay mineralogy should be attributed to high water/basalt alterations during the low-grade burial metamorphic conditions in more permeable vesicular levels of the flow.

Hydrothermal alteration at Mururoa Atoll (French Polynesia)

Abstract Petrological study of altered aerial basaltic flows, submarine pillow lavas and oceanitic basalts of a rift zone sampled in two drill-cores in the Mururoa Atoll (Viviane, 1000-m depth; Fuchsia, 550-m depth) shows that during hydrothermal alteration processes the secondary clay minerals evolve the following three different trends controlled by permeability of the rocks and by rock-fluid interaction: Mg-saponite, Al-saponite and Al-montmorillonite. In the basaltic pillow lavas, first “celadonite”-saponite assemblage crystallizes in fresh-glass-rich rocks. In altered and brecciated rocks, the latter mineral assemblage is followed by Fe-saponite and Mg-saponite. In breccias, the glass is replaced by Fe-saponite; the residual voids are sealed by zeolites (mainly phillipsite), apophyllite or gypsum, indicating an upper temperature limit of 65°C. In the aerial basaltic flows, massive rocks are affected by celadonite and random mixed-layer chlorite-saponite crystallizations. In the strongly altered rocks (veined or brecciated), the primary minerals and glass are partly replaced by Al-saponites and locally by chabazite. The same Al-saponite fills up the veins and constitutes the major part of the cement. As rock-fluid interaction increases the Si, Al and Ca content in the solution increases, inducing the late precipitation of aluminous smectite and chabazite. In the oceanitic basalts sampled in the rift zone, massive rocks are affected by crystallization of secondary clay minerals which is governed by local equilibrium: FeMg clay minerals (talc, Mg-saponite, celadonite) partly replace the olivine phenocrysts and Al-saponite replaces the glass. In veined and brecciated rocks the paragenesis is dioctahedral Al-smectite + silica (amorphous) for a temperature range 2+ content in the solution.

Biotite chloritization process in hydrothermally altered granites

Abstract Chloritization of biotite in a hydrothermal environment was studied in two granitic rocks of the Massif Central (France). Four types of transformation were observed: (1) Mg-chlorite growth upon (001) biotite planes in contact with quartz; (2) biotite pseudomorphosed by a corrensite—residual biotite assemblage; (3) corrensite—residual biotite pseudomorphosed by a ferromagnesian chlorite; and (4) biotite pseudomorphosed by chlorite. These transformations induce an expansion of the site of the ferromagnesian minerals which can be observed by the corrosion of neighbouring minerals and fracturation of the rock around the new chlorites (open fractures or chloritic veinlets). If Al is considered as a strictly inert component, the reaction biotite → chlorite produces a decrease of the ferromagnesian mineral volume of ∼ 13% (Ferry, 1979). This is not in agreement with petrographic observations of the rocks studied here, therefore writing of a chloritization reaction must take into account a volume increase. In this case Al does not behave as an inert component but as a mobile one at the scale of the microsystem biotite—neighbouring crystals. Albitization of plagioclase which occurs simultaneously with the chloritization process is the Al furnisher. In addition, Fe and Mg must be brought into the microsystem during the transformation of biotite into chlorite, this explains why the Mg ( Mg+Fe ) ratio is not the same between biotite, chlorite growths and chlorite pseudomorphs. Because the bulk-rock composition is enriched in these components, the hydrothermal fluids must contain Fe and Mg before they flow into the rock. Chloritization in propylitized granites is not an isochemical process.

CLAY MINERALS IN BASALT-HAWAIITE ROCKS FROM MURUROA ATOLL (FRENCH POLYNESIA). II. PETROGRAPHY AND GEOCHEMISTRY

The clay minerals formed in chilled margins and massive crystallized inner parts of three basalt-hawaiite bodies of Mururoa Atoll (French Polynesia) exhibit contrasting textures. Glass alteration textures are observed around fractures crosscutting the quenched margins: Fe-rich clays grow inward into the glass (retreating surface) while Mg-rich clays grow outward (open space). The textures of clay deposits filling the diktytaxitic voids (mesostasis) in the massive inner parts of the three volcanic bodies are different: unoriented clay matrix with embedded euhedral apatite and pyroxene microcrysts (submarine flow); pallisadic clays coating the void walls and the crystal surfaces of apatite and K-feldspar microcrysts (subaerial flow); and clay muffs covering all the apatite needles, with the central part of the void remaining empty (dike). The unoriented texture could result from the alteration of a glass precursor concomitant with the olivine phenocrysts (clay pseudomorphs). However, such an alteration implies important chemical transfers which are not observed. The pallisadic and muff textures form through heterogeneous nucleation on the solid surfaces and crystal growth from a saline solution. No glass precursor existed. As the center of the diktytaxitic voids in the dike is empty, the residual liquid was probably boiling. The amounts of light rare earth elements (LREE), Sr, and the most incompatible elements are greater in clays from diktytaxitic voids relative to the amounts formed in the altered glass of the chilled margins. Thus, diktytaxitic clays formed from a residual liquid which gave either an evolved glass or a saline solution after cooling (fractionation process). The δ18O variation vs. loss on ignition (LOI) indicates that sea water was involved either in rock alteration or magma contamination. This is confirmed by the 87Rb/86Sr ratio of bulk rocks and clay fractions from the quenched and massive inner parts of the three volcanic bodies which do not fit with the 11.5 Ma isochron indicating that the Rb-Sr system was not closed at any stage during the magmatic history.

CLAY MINERALS IN BASALT-HAWAIITE ROCKS FROM MURUROA ATOLL (FRENCH POLYNESIA). I. MINERALOGY

Clay minerals in chilled or brecciated margins (altered glass) and massive inner crystalline parts (mesostasis) of three basalt-hawaiite bodies from Mururoa Atoll (French Polynesia) have been studied in order to compare their chemical and mineralogical compositions. Polyphase assemblages comprise di- and trioctahedral phases, both of which consist of non-expandable layers (chlorite, celadonite) and two types of expandable layers (saponite and Fe-rich smectite or ‘nontronite-like’ material). The presence of the Fe-rich clays is supported by the presence of the X-ray diffraction 060 peak at 1.51–1.52 Å and of the infrared absorption bands at 875 and 822 cm−1 (Fe3+-Al-OH and Fe3+-Fe3+-OH groups, respectively). The chemical composition of the Fe-rich smectites does not fit with the theoretical nontronite field. The layer charge averages 1 per Si4O10 making these Fe-rich smectites close to ‘celadonite-type’ clays. This could explain the presence of mixed-layer celadonite-smectite. Plotted in an M+/4Si vs. Fe/sum octahedral cations diagram, the chemical compositions of clay minerals in the mesostasis form a continuous field limited by the celadonite-high-charge nontronite-like smectite and chlorite end-members. The clay assemblages are different from those formed in hydrothermal systems or low-grade metamorphic conditions which are characterized by the sequence: saponite → randomly ordered chlorite-smectite mixed-layered minerals (MLMs) → corrensite → chlorite. The systematic presence of Fe-rich clays either in the altered chilled margins or in the massive inner parts of the basalt-hawaiite bodies (high-charge nontronite-like smectite and mixed-layer nontronite-celadonite) makes the Mururoa sea-mount a potential terrestrial analogue for Mars surface exploration.

HYDROTHERMAL AND SUPERGENE ALTERATIONS IN THE GRANITIC CUPOLA OF MONTEBRAS, CREUSE, FRANCE

A mineralogical investigation of the highly kaolinized Chanon granite and albite-muscovite granite of the Montebras cupola, Creuse, France, indicates that the magmatic stage was followed by two hydrothermal events related to successive cooling stages and by late weathering. The hydrothermal alteration was accompanied first by greisen formation and then a broad kaolinization process, which pervasively affected the granitic bodies. In the Chanon granite, the greisens are characterized by a trilithionite-lepidolite-quartz-tourmaline assemblage and are surrounded by concentric alteration zones. From the greisen to the fresh granite three zones were distinguished: (1) a zone characterized by secondary brown biotite (<400°C), (2) a zone characterized by secondary green biotite and phengite (300–350°C), and (3) a zone characterized by the presence of corrensite (180°–200°C) located around greisen veinlets. In the albite-muscovite granite the greisen is composed of lepidolite and quartz. This mineral assemblage was followed locally by Li-tosudite crystallization. During the second hydrothermal event (<100°C) an assemblage of kaolinite, mixed-layer illite/smectite (I/S), and illite formed pervasively and in crack fillings; the smectite layers of the US are potassic. Weathering produced Fe oxide and kaolinite. This kind of alteration developed mainly in the overlying Chanon granite. Here, Ca-Mg-montmorillonite formed in subvertical cracks, which transect the two granitic bodies, and hydrothermal I/S was obliterated by Ca-Mg-montmorillonite.The hydrothermal parageneses were apparently controlled by magmatic albitization and the bulk chemistry of the two granitic bodies. The albitization, the formation of large micaceous greisens, and the successive recrystallizations of biotite (which was the most susceptible phase to alteration) provide information on the temperature range and chemical mobility during successive cooling stages. Si and Mg activities increased as the temperature of alteration decreased, and secondary Mg-biotite and Mg-phengite crystallized as long as the K activity was sufficient. The crystallizations of secondary biotite and phengite were followed by the crystallization of I/S during stages of low K activity. Secondary hydrothermal phases in the Chanon granite contain substantial Fe and Mg. Secondary hydrothermal phases in the albite-muscovite granite contain only small amounts of Fe and Mg, suggesting a lack of chemical exchange between the enclosing Chanon granite and the albite-muscovite granite, which is depleted in Fe-Mg-rich primary phases, such as biotite.

Polder Effects on Sediment-to-Soil Conversion: Water Table, Residual Available Water Capacity, and Salt Stress Interdependence

The French Atlantic marshlands, reclaimed since the Middle Age, have been successively used for extensive grazing and more recently for cereal cultivation from 1970. The soils have acquired specific properties which have been induced by the successive reclaiming and drainage works and by the response of the clay dominant primary sediments, that is, structure, moisture, and salinity profiles. Based on the whole survey of the Marais Poitevin and Marais de Rochefort and in order to explain the mechanisms of marsh soil behavior, the work focuses on two typical spots: an undrained grassland since at least 1964 and a drained cereal cultivated field. The structure-hydromechanical profiles relationships have been established thanks to the clay matrix shrinkage curve. They are confronted to the hydraulic functioning including the fresh-to-salt water transfers and to the recording of tensiometer profiles. The CE1/5 profiles supply the water geochemical and geophysical data by their better accuracy. Associated to the available water capacity calculation they allow the representation of the parallel evolution of the residual available water capacity profiles and salinity profiles according to the plant growing and rooting from the mesophile systems of grassland to the hygrophile systems of drained fields.

ALTERAÇÃO HIDROTERMAL DE PREENCHIMENTO DE UM VEIO DE ROCHA BASÁLTICA ASSOCIADA ÁS AMETISTAS - VILA SÃO GABRIEL/PLANALTO, RIO GRANDE DO SUL

Clay minerals, zeolites, calcite and silica (quartz, opal and chalcedony) are the most common hydrothermal alteration products of the basalts with veins in Sao Gabriel (Planalto) Region. The petrographic and geochemical studies led to the identification of the secondary phases which fill the veins. From the border to the centre, the filling of the veins comprises: a) brown clay mineral (ferri-montmorillonite), b) prismatic zeolite (heulandite) and/or fibro-radiated saponite/chlorite, and c) interstratiphied saponite/chlorite.