Mireille Polvé

Major and trace elements associated with colloids in organic-rich river waters: ultrafiltration of natural and spiked solutions

Abstract This study presents the results of ultrafiltration experiments (0.20 μm–300,000 Da–5000 Da–1000 Da) performed on natural rich-organic waters (30–40 mg l−1 of dissolved organic carbon) sampled in wetland area of Cameroon (Nsimi-Zoetele site). A very strong decrease in all cation concentrations (major and trace elements) except Si was observed after filtration. Speciation calculations using data available in the literature for metal–humic substance complexation observations suggest that ultrafiltration performed on this water source at a pH of 4.74 induces a very strong retention of cations only weakly bound to humic substances in the aqueous solution [Viers, J., Dupre, B., Polve, M., Schott, J., Dandurand, J.L., Braun, J.J., 1997. Chemical weathering in the drainage basin of a tropical watershed (Nsimi-Zoetele site, Cameroon): comparison between organic-poor and organic-rich waters. Chem. Geol., 140, 181–206]. To minimize this artifact, ultrafiltration must be performed at low pH (=3) or with the addition of high concentrations of a complexing metal (e.g., Lanthanum). The goal of these ultrafiltration experiments is to anticipate the affinity of several major and trace elements to form organo-metallic complexes with humic substances. Experiments using Sr and Ba isotopes at fixed ratios were entered in order to determine the exchangeable fraction of base cation. Natural Sr isotopic ratio ( 87 Sr / 86 Sr ) in different filtrates and retentates as well as isotopic ratios of Ba and Sr ( 86 Sr / 84 Sr and 138 Ba / 135 Ba ) in spiked and filtered samples appear to remain constant. These results suggest that there is only one source of Sr in these natural waters and that it is present in an exchangeable form (i.e., free ion and/or complexed with organic matter). By analogy, we suppose that elements such as Ca or Mg are not present in organic or mineral colloids but in an exchangeable position. This is in agreement with the hypothesis of the filtration artifact. In the filtration experiment, performed at pH 3, more than 95% of Al, Ga, Fe, U, Th, Y and REEs, ≅50% of Cr and V, 25% of Cu, 10% of Co, and ≤5% of Ca, Mg, Na, K, Mn, Ba, Rb and Sr are complexed with organic material. According to the data gathered in this study and the results of speciation calculations, the order and the overall constants (log K) for the formation of metal–humate complexes are the following: Al, Ga, Fe, Th, U, Y, REEs (more than 7)≫Cr (5.5)≫Co (3)>Rb, Ba, Sr, Mn, Mg (≈2). These data are obtained for both low ionic strength and low metal concentrations. Using the filtration experiment performed with an addition of La, we observe that REEs appear to be complexed with humic substances via two types of site. The first site has a strong affinity for the REEs but is not abundant. So the complexation by this site will be important when the REEs concentration is low. The second type of site is much more abundant but has a much smaller affinity for the complexation of REEs. This site will dominate when the REEs concentration is high. The first site remains unassigned but the second should be related to the carboxylic functional groups.

Multiple plume events in the genesis of the peri‐Caribbean Cretaceous oceanic plateau province

The oceanic crust fragments exposed in central America, in north-western South America, and in the Caribbean islands have been considered to represent accreted remnants of the Caribbean-Colombian Oceanic Plateau (CCOP). On the basis of trace element and Nd, Sr, and Pb isotopic compositions we infer that cumulate rocks, basalts, and diabases from coastal Ecuador have a different source than the basalts from the Dominican Republic. The latter suite includes the 86 Ma basalts of the Duarte Complex which are light rare earth element (REE) -enriched and display (relative to normal mid-ocean ridge basalts, NMORB) moderate enrichments in large ion lithophile elements, together with high Nb, Ta, Pb, and low Th contents. Moreover, they exhibit a rather restricted range of Nd and Pb isotopic ratios consistent with their derivation from an ocean island-type mantle source, the composition of which includes the HIMU (high 238U/204Pb) component characteristic of the Galapagos hotspot. In contrast, the 123 Ma Ecuadorian oceanic rocks have flat REE patterns and (relative to NMORB) are depleted in Zr, Hf, Th, and U. Moreover, they show a wide range of Nd and Pb isotopic ratios intermediate between those of ocean island basalts and NMORB. It is unlikely, on geochemical grounds, that the plume source of the Ecuadorian fragments was similar to that of the Galapagos. In addition, because of the NNE motion of the Farallon plate during the Early Cretaceous, the Ecuadorian oceanic plateau fragments could not have been derived from the Galapagos hotspot but were likely formed at a ridge-centered or near-ridge hotspot somewhere in the SE Pacific.

The remelting of hydrothermally altered peridotite at mid-ocean ridgesby intruding mantle diapirs

Most gabbroic cumulates found at ocean spreading centres are thought to have been generated by the fractional crystallization of melts with the composition of mid-ocean ridge basalt (MORB). There are exceptions, however, including some cumulates which appear to have come from melts that contain more silica than MORB and are much more depleted in the incompatible elements (those elements that do not readily substitute into the main mineral phases). These unusual rocks bear witness to relatively deep petrological processes that are not accessible through the study of melts erupted on the sea floor, and their origin is still debated. Fortunately, the same lithologies can be studied in detail in ophiolites (sections of oceanic crust accreted to a continent). In a fossil mantle diapir of the Oman ophiolite, we have observed the same dichotomy between a suite of ‘normal’, MORB-type, cumulates (‘N-cumulates’) and a suite of cumulates issued from silica-enriched but incompatible-element-depleted melts (‘D-cumulates’). While the N-cumulates crystallized inside the diapir, the D-cumulates occur essentially as intrusions surrounding the diapir. The combination of silica enrichment, extreme depletion in incompatible elements, and seawater isotopic signature indicates that the D-cumulates were formed by the remelting at low pressure of hydrated residual peridotites left after MORB extraction at the ridge axis. The distribution of the D-cumulates relative to the N-cumulates suggests that such depleted melts are produced episodically at ridge axes when the lithospheric mantle is reheated by a new diapiric pulse.

MAGMATIC EVOLUTION OF SULAWESI (INDONESIA) : CONSTRAINTS ON THE CENOZOIC GEODYNAMIC HISTORY OF THE SUNDALAND ACTIVE MARGIN

Abstract Tertiary and Quaternary magmatic rocks from West Sulawesi record the complex history of part of the Sundaland margin where subduction and collision have been and are still active. The present study, based on petrographic data, major- and trace-element chemistry and 40 K 40 Ar dating aims to document the age and chemical characteristics of the magmatic formations from West Sulawesi and to determine the corresponding constraints on the geodynamic evolution of the Sundaland border. The West Sulawesi magmatic province includes the South Arm of Sulawesi (Ujung Pandang area), the western part of Central Sulawesi with the Toraja and Palu areas, and finally, the North Arm, extending from Palu to Manado, which includes the Tolitoli and Manado areas. Paleocene magmatic activity seems to be restricted to an episode of calc-alkaline magmatism in the Ujung Pandang area (61-59 Ma). The major Eocene (50-40 Ma) magmatic event is tholeiitic and is documented in all areas except in Ujung Pandang. It led to the emplacement of tholeiitic pillow-lavas and basaltic dykes of back-arc basin (BAB) affinity. These rocks are potential equivalents to the Celebes Sea basaltic basement. From Oligocene to Miocene, magmatic eruptions produced successively island-arc tholeiitic (IAT) and calc-alkaline (CA) rock series. The youngest IAT activity occurred around 18 Ma in the central part (Palu area) and around 14 Ma in the North Arm (Tolitoli area) while CA magmas were emplaced in the North Arm at ca. 18 Ma (Tolitoli and Manado areas). Typical calc-alkaline activity resumed only in the North Arm (Tolitoli and Manado areas) during the Late Miocene (9 Ma) and is still active in the Manado region. In other areas (Palu, Toraja and Ujung Pandang areas) an important and widespread magmatic event occurred between 13 and 10 Ma and emplaced K-rich magmas, either silica-undersaturated alkali-potassic basalts (AK), ultrapotassic basanites (UK) or shoshonites (SH). K-rich activity continued in the south until the Pleistocene (0.77 Ma) with alkali-potassic, ultrapotassic and shoshonitic magmas. In Central Sulawesi (Toraja and Palu areas) the most recent magmatic event occurred between 6.5 and 0.6 Ma. The corresponding products are granitic rocks and widely distributed rhyolitic pyroclastic flow deposits. All these rocks are acidic in character (SiO 2 > 60%), with trace-element and isotopic signatures (SrNdPb) typical of a strong continental imprint. The most striking tectonic implication of this magmatic evolution is that West Sulawesi can no longer be considered as a typical magmatic arc as previously assumed. With the exception of the Manado area beneath which subduction is still active, calc-alkaline and island-arc tholeiitic lavas and plutonics are volumetrically minor with respect to K-rich magmas. Their occurrence through time is also fairly restricted, mostly to the period between 30 and 15 Ma. Another important feature is the occurrence of island-arc tholeiitic and calc-alkaline magmas crosscutting an older terrane of BAB affinity, the Tinombo Formation (Manado, Tolitoli and Palu areas). As this formation is being regarded as an equivalent to the Celebes Sea floor, the most likely explanation for this feature is the hypothesis of tectonic erosion linked to the NW-dipping subduction beneath the North Arm. The Late Miocene high-K magmatic activity in Central and South Sulawesi reflects the prevalence of a post-collisional tectonic regime following the docking of microcontinents of Australian origin to Central Sulawesi during Neogene times. The incompatible element-enriched character of these high-K rocks might reflect their derivation from a mantle source enriched through metasomatism related to a previous subduction event. Such a model cannot account for the Plio-Pleistocene CAK magmatism of Central Sulawesi, the acidic composition of which does not support a derivation from an ultrabasic source. The trace-element patterns of the CAK rocks are very similar to those of the high-grade metamorphics of Central Sulawesi, suggesting that the latter might represent their possible source. Such an anatectic model implies a collisional to post-collisional tectonic regime limited to Central Sulawesi, while a post-subduction regime prevailed in the south.

Late Jurassic Oceanic Crust and Upper Cretaceous Caribbean Plateau Picritic Basalts Exposed in the Duarte Igneous Complex, Hispaniola

Four distinct rock units have been recognized near El Aguacate, in the Janico–Juncalito–La Vega area of the Duarte complex (Dominican Republic): (1) serpentinites crosscut by numerous diabasic dikes, (2) basalts interbedded with Late Jurassic ribbon cherts, (3) picrites and ankaramites relatively enriched in incompatible trace elements, and (4) amphibolites and gneissic amphibolites chemically similar to Oceanic Plateau Basalts. Similar Ar‐Ar ages of late magmatic amphibole from a picrite, and hornblende from an amphibolite ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Tertiary and quaternary magmatism in Central Sulawesi: Chronological and petrological constraints

86.1\pm 1.3

Influence of source distribution and geochemical composition of aerosols on children exposure in the large polymetallic mining region of the Bolivian Altiplano.

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