Origine et composition du plateau océanique Caraïbe

Mémoires de Géosciences Rennes, n° 97, 358 p. ISBN: 2-905532-96-3.The Caribbean plateau is believed to have formed in the Pacific through melting in a mantle plume. Parts of the plateau accreted around the Caribbean and on the Pacific coast of Colombia and Equador; other parts form the floor of the Caribbean Sea. We studied samples from Curaçao Island and the Beata Ridge in the central Caribbean, and from Gorgona Island in the Pacific. The samples include volcanic (basalts, picrites and komatiites) and intrusive rocks (gabbros, dolerites, dunites and wehrlites). The methods employed were petrographic and electron microprobe studies, and the analysis of major and trace elements and Sr, Nd and O isotope compositions.The Beata Ridge consists mainly of a gabbro-dolerite sill or dike complex. Rocks from Beata, and from Curaçao Island, were derived from basaltic parental liquids and have chondrite-like flat rare-earth-element (REE) patterns. Their source was isotopically depleted. 40Ar/39Ar ages indicate a long magmatic history for this part of the Caribbean plateau including magmatic episodes at 90 Ma, 76 Ma and 55 Ma. All plutonic rocks from Gorgona Island crystallized from basaltic liquids with flat to moderately depleted LREE patterns. The picrites and komatiites formed from ultra-magnesian liquids with moderate to extreme LREE depletion. Their Nd isotope compositions indicate two distinct mantle sources; one strongly depleted (epsilon Nd = +10), the other less depleted (epsilon Nd = +6). Similarities between trace element and isotopic compositions suggest that magmas parental to the intrusive rocks differentiated from komatiitic liquids in deep magma chambers. They were later emplaced into shallow-level sills. The main conclusions of this work are:(a) the plumbing system of the oceanic plateau is highly complex.(b) parental liquids in the plateau are heterogeneous - ultra-magnesian to basaltic, with highly variable depletion of incompatible elements. The mantle source contained enriched and depleted components.(c) partial melting in the plume head resulted in strong chemical zoning. High- temperature, ultramagnesian liquids formed in the plume core through a critical melting process that resulted in highly depleted liquids. Lower temperature melting at the rims preferentially tapped the enriched source component and produced basaltic liquids. These liquids pooled after separation from the source to yield magmas with uniform, flat REE patterns.Le plateau Caraïbe s'est formé, sans doute dans le Pacifique, par la fusion de la tête d'un panache mantellique. Il s'est ensuite accrété autour des Caraïbes et sur la côte ouest de l'Amérique du sud. Nous avons étudié des échantillons de l'île de Curaçao et de la ride de Béata, dans les Caraïbes, et de l'île de Gorgona dans le Pacifique. Ces échantillons sont des roches volcaniques (basaltes, komatiites et picrites) ou plutoniques (gabbros, dolérites, dunites et wehrlites). Nos méthodes sont celles de la pétrologie et de la géochimie élémentaire et isotopique (Nd, Sr, O).La ride de Béata est formée de gabbros et de dolérites. Ces roches, comme celles de Curaçao, dérivent de liquides basaltiques et montrent des spectres de terres rares (TR) plats. Leur source est isotopiquement appauvrie. L'âge des roches de Béata suggère une longue histoire magmatique pour la formation du plateau en trois épisodes (90 Ma, 76 Ma et 55 Ma).Toutes les roches plutoniques de Gorgona (gabbros, dunites et wehrlites) dérivent de liquides basaltiques montrant des spectres de TR plats ou appauvris en TR légères. Les picrites et les komatiites dérivent de liquides ultra-magnésiens appauvris ou très appauvris en TR légères. Les compositions isotopiques de Nd indiquent deux sources mantelliques : une appauvrie et une plus enrichie (epsilon Nd +10 et +6). La similitude des rapports d'éléments incompatibles et des compositions isotopiques suggère que certaines roches plutoniques dérivent par différenciation des liquides komatiitiques. Ces magmas se mettent ensuite en place dans des sills à faible profondeur.Les conclusions principales sont :(a) le système d'alimentation du plateau est complexe(b) les liquides parents sont hétérogènes : ultra-magnésiens ou basaltiques, appauvris ou non en éléments incompatibles. La source comprend un composant enrichi et un composant appauvri.(c) la tête du panache est zonée. Ceci est lié au processus de fusion partielle. Au cœur, de haute température, les liquides ultra-magnésiens se forment. Un processus de fusion fractionnée avancé permet de former des liquides très appauvris et l'échantillonnage du composant appauvri. Dans les bordures, plus froides, les liquides basaltiques se forment par des taux de fusion fractionnée moindres, ce qui favorise l'échantillonnage du composant enrichi. Ces liquides sont ensuite homogénéisés et acquièrent des signatures géochimiques uniformes

Improvements in Digestion Protocols for Trace Element and Isotope Determinations in Stream and Lake Sediment Reference Materials (JSd-1, JSd-2, JSd-3, JLk-1 and LKSD-1)

International audienceTotal dissolution is a critical step in geochemical analysis. Despite the number of published protocols, this issue still draws attention for sediment samples, which are particularly difficult to dissolve due to the common occurrence and high abundance of refractory phases such as zircon. We present tests of different chemical digestion procedures carried out on reference materials (RM) of stream (JSd-1, JSd-2 and JSd-3) and lake (JLk-1 and LKSD-1) sediments from the Geological Survey of Japan (GSJ) and the Canadian Certified Reference Material Programme (CCRPM). We demonstrate that the fusion technique is not appropriate for our studies as not all elements of interest were recovered and blank levels were too high to permit further Sr and Pb isotopic composition measurements. Similarly, conventional HF+HNO3 dissolution methods were not efficient enough for detrital samples. Our preferred method involved using high pressure Teflon® vessel bombs in association with HClO4. This protocol ensured a complete dissolution of the powder, as well as a complete recovery of trace elements. Moreover, blank levels were sufficiently low that Sr or Pb isotope compositions could be measured from the same mother solution. We also tested the homogeneity of RM powders by performing tests on various amount of powder

Distinct control mechanism of fine-grained sediments from Yellow River and Kyushu supply in the northern Okinawa Trough since the last glacial

© 2017. American Geophysical Union. All Rights Reserved. High-resolution multiproxy records, including clay minerals and Sr-Nd-Pb isotopes of the clay-sized silicate fraction of sediments from IODP Site U1429 in the northern Okinawa Trough, provide reliable evidence for distinct control mechanism on fine-grained sediments input from the Yellow River and the southern Japanese Islands to the northern Okinawa Trough since 34 ka BP. Provenance analysis indicates that the sediments were mainly derived from the Yellow River and the island of Kyushu. Since the last glacial, clay-sized sediments transported from the Yellow River to the study site were strongly influenced by sea-level fluctuation. During low sea-level stage (∼34–14 ka BP), the paleo-Yellow River mouth was positioned closer to the northern Okinawa Trough, favoring large fluvial discharge or even direct input of detrital sediments, which resulted about four times more flux of clay-sized sediments supply to the study area as during the relatively high sea-level stage (∼14–0 ka BP). The input of Kyushu-derived clay-sized sediments to the study site was mainly controlled by the Kuroshio Current and Tsushima Warm Current intensity, with increased input in phase with weakened Kuroshio Current/Tsushima Warm Current. Our study suggests that the Kuroshio Current was very likely flowed into the Okinawa Trough and thus influenced the fine-grained sediment transport in the area throughout the last glacial and deglacial. During ∼34–11 ka BP, the Kyushu clay-sized sediment input was mainly controlled by the Kuroshio Current. Since ∼11 ka BP, the occurrence of Tsushima Warm Current became important in influencing the Kyushu fine-grained sediment input to the northern Okinawa Trough

Petrogenesis and Alteration of Young Pacific Ocean Crust Formed at a Fast-Spreading Ridge: Results From ODP Leg 203, Hole 1243B

ODP Leg 203 established a new legacy hole for a long-term Dynamics of Earth and Ocean Systems/Ocean Seismic Network observatory and sampled basement in a young fast-spreading environment at Site 1243. The age of basement at this site, based on a full spreading rate of 141 mm/yr, the East Pacific Rise subsidence curve, and the lower most sediments is $\sim$11 Ma. Basement was drilled and cored to a total depth of 195.3 mbsf, sampling 87.1 m of oceanic crust with an average recovery of 25.5\%. Although Leg 203 did not quite achieve the proposed crustal penetration of 100 m, the core returned from Hole 1243B is significant given the sparse sampling of deep basement rocks from young Pacific seafloor. All the recovered volcanic rocks are aphyric or plagioclase$\pm$olivine-phyric basalts. On a SiO2 vs total alkali diagram, two alkaline units are distinguished from the five remaining tholeiitic units. Samples from Units 1 and 3 display the most primitive compositions while the alkaline units are more evolved. The tholeiitic units display REE patterns characteristics of N-MORB and the alkaline units are typical of E-MORB. The basement sampled at Hole 1243B is relatively fresh with Loss On Ignition (LOI) varying between 0.10\% and 3.95\%. The alteration paragenesis is dominated by iron-oxyhydroxide (Fe(O,OH)x), brown clay minerals, Ca-carbonate and zeolites and restricted to narrow rims of pillows. The Sr isotope compositions of the basement samples are close to unaltered basalt isotope compositions. Most 87Sr/86Sr ratios vary from 0.7029 to 0.7035 although three samples yielded higher 87Sr/86Sr ratios (from 0.7042 to 0.7047). All measured O isotope compositions are above mantle values ranging between 6.5 and 8.5 within the tholeiitic units and from 6.6 to 9.1\permil within the alkaline units. There is a clear relationship between both the O and the Sr isotope compositions of the basalts and their LOI. These geochemical data are indicative of low temperature interactions between rocks and seawater-derived fluids and typical of the low temperature seafloor weathering of the ocean crust. The characteristics of basement recovered at Site 1243 are distinctive from those previously sampled in young Pacific oceanic crust in the well studied Holes 504B or 896A. The main differences are (a) the occurrence of more enriched units composed of alkali basalts and typical of E-MORB and (b) the overall low degree of alteration of the oceanic crust

Origine des évaporites aptiennes de l'Atlantique Sud : contraintes géochimiques dans la partie proximale de la marge du Congo

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The provenance of sediments in the Gulf of Lions, western Mediterranean Sea

International audienceIn this study, we undertook a reconnaissance study of sediments provenance in the Gulf of Lions focusing over the last 16 ka. We used geochemical and isotopic tracers to determine the source of sediments and give insight into the weathering conditions prevailing. Sediments samples were selected both onshore and offshore from the western, eastern, and central part of the Gulf of Lions. We analyzed bulk sediments, coarse and fine silt, and clay fractions. Elemental and Nd isotope compositions appeared to differ from one grain size fraction to another one. These are interpreted in terms of zircon addition in the coarse silt fraction for the elemental concentrations and variable sources influences for the Nd isotope compositions. Our results indicate that sediments in the Gulf of Lions mainly originated from the Rhône River watershed although a contribution of Saharan dust is seen in one sample. Influence of Pyrenean small rivers is minor in these samples. Some Sr isotope compositions shifts are interpreted as reflecting variable amounts of chemical weathering that are consistent with published paleoclimatic reconstructions

Variations in weathering conditions related to sea level changes during the Albian-Santonian interval in the Western Australian margin as evidenced by clay minerals and Nd/Sr isotopes

International audienceThe Albian-Santonian interval (∼113–83 Ma) is a key interval for Cretaceous climatic and environmental changes. This interval is associated to a rise in sea level until the Late Cenomanian (∼ 94 Ma) and to a progressive increase in oceanic temperatures, leading to consider the Late Cenomanian-Santonian interval (∼ 94–83 Ma) as the warmest period of the last 200 Myr. While the Albian-Santonian interval has been well studied in the Northern Hemisphere, the climatic and environmental variations and their consequences on weathering conditions are less documented of the Southern Hemisphere, especially in mid and high latitudes. To better understanding the evolution of weathering conditions, associated to continental climate and sea level changes during this key period, an integrated study, based on a coupled mineralogical and geochemical approach, was carried out on the clay-sized (< 2 μm) fraction from sediments of International Ocean Discovery Program Site U1513 (Mentelle Basin, South-western Australia) and Oceanic Drilling Program Site 763 (Carnarvon Basin, North-western Australia). To determinate variations in weathering conditions, the mineral assemblages of the clay fraction was determined by X-ray diffraction and observed for selected samples by electron microscopy. To identify sources of clay minerals, the concentrations of major and selected trace elements, including rare earth elements, together with strontium and neodymium isotopic measurements were performed on the clay-sized fraction. The X-ray diffraction analyses reveal that clay fraction is dominated by R0-type illite/smectite mixed-layers (smectites), followed by variable proportions of illites, kaolinites for both sites and only on Site 763 of palygorskites. These clay minerals are associated with opal and clinoptilolites. Electron microscopy observations have highlighted the dominance of detrital fleecy smectites but also the occurrence of recrystallized lathed smectites and volcanogenic folded smectites for Site U1513, and authigenesis of clay minerals, associated to palygorskites especially on Turonian to Santonian deposits for Site 763. This authigenesis, confirmed by negative cerium anomaly, is also carried by other minerals in the clay-sized fraction (e.g., clinoptilolites, barite). These different authigenic minerals record a seawater-derived isotopic signature. Therefore, isotopic compositions of the clay-sized fraction reflect a mix between a continental contribution, carried by detrital clay minerals and marine one, supported by authigenic minerals, which complicates the identification of sediment sources. However, despite this, our study demonstrates the major influence of sea level variations on mineralogical variations from Albian to Cenomanian in Western margin of Australia. The decrease in kaolinite proportions at Site U1513 from the Albian to the end of the Cenomanian, is probably related to the increase of sea level, which cause a decrease in detrital inputs in the Mentelle Basin associated with a decrease of weathering and drainage conditions. The progressive sea level rise has also enhanced the effect of the differential settling process of clay minerals by the deposition of coarse clays (e.g., kaolinites and illites) on the continental shelf. Kaolinite-bearing clay deposits, derived from the weathering of the Western Australian Craton during the Albian-Cenomanian times, have been progressively substituted from the Turonian by a carbonate sedimentation, for which the terrigenous component consists only of rare detrital and volcanogenic smectites. In Site 763, the occurrence in kaolinite content during the Coniacian would indicate a decrease of differential settling process related to the weathering of the north part of the Western Australia Craton