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GEOCHEMICAL STUDIES OF THE AMAZONIAN BASIN USING TRANSITION ELEMENTS AND REE
GERALDO R. BOAVENTURA 1
PARTRICK SEYLER 2
1Instituto de Geociências, Universidade de Brasília, 70.000 Brasília, D.F.
2IRD, França.
Anais da Academia Brasileira de Ciências(2000), 72(4): (Resumo de comunicações)
ABSTRACT
The Amazon River with an extension of 6.1 106km2 and more than 1000 tributaries has an average discharge volume of 209000 m3/s, suppling up to 15% of all the river water discharged to the ocean (Salati and Vose,1984). The Amazon River results of the mixing of two different types of waters: the blacks waters and the whites ones, chemically very well differentiated. The information given by the chemical studies of transition and REE elements, in addition with that given by the environmental context allows us to discriminate the origin and the quality of the waters and sediments encountered in the Amazonian rivers and to highlight the behaviour of a large number of trace elements. This work was part of the HiBAm program (Hidrologia e Geoquímica da Bacia Amazônica) which is supported by the CNPq/UnB/ANEEL/IRD partnership research program. The water samples were filtrated on 0.22
m Millipore membrane filters, in the laboratory of the research vessel, trace elements V, Cr, Mn, Co, Cu, Zn, As, Rb, Sr, Mo, Cd, Sb, Cs, Ba, U and REE analysis were performed by Inductively Coupled Plasma-Mass Spectrometry(ICP/MS) using a VG-Elemental PQ2 instrument at the Laboratoire de Géochimie (Montepellier University, France). REE analysis were performed by ICP-Atomic Emission Spectrometry using a Spectroflame FVM03 instrument in Geochemistry laboratory (Brasília University, Brazil). Dissolved REE results vary by more than a factor 10. The highest concentration is found in the Negro river (200 ppt for La, 500 ppt for Ce, 2 ppt for Lu) and the lowest in the Tapajós river (La: 21 ppt, Ce: 27ppt, Lu: < 0.1ppt). The REE behaviour in the Amazon rivers differs among the studied fractions. In the dissolved fraction, the REE concentration depends upon the source as well as the physical chemistry conditions (pH) and the amount of organic carbon. (DOC). In the particulate fraction (suspension material and bed load), REE behaves in a differentiated way. The correlation between pH and V, Cu, As, Sr, Cr, Mn, Co, Ni, Ba and U suggest that these elements have a common carbonate and/or evaporite source. However, Cr and Co closely correlate with Dissolved Organic Carbon. The correlation between Zn and Pb reflects a control by source rocks, probably sulfide mineralization where Zn and Pb are commonly associated. The geochemical studies of the Amazonian Basin have provided important information about the trace elements and REE behaviour in a such type of riverine environment.