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REE distribution in presentday and ancient surface environments of basaltic rocks (Central Portugal)

Published online by Cambridge University Press:  09 July 2018

M. I. Prudêncio ,
M. A. Gouveia  and
M. A. Sequeira Braga
M. I. Prudêncio
Affiliation:
Departamento de Quírnica, ITN, Estrada Nacional 10, 2685 Sacavém, Portugal
M. A. Gouveia
Affiliation:
Departamento de Quírnica, ITN, Estrada Nacional 10, 2685 Sacavém, Portugal
M. A. Sequeira Braga
Affiliation:
Universidade do Minho, Ciências da Terra, 4719 Braga codex, Portugal
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Abstract

Rare earth elements are mobilized, fractionated and precipitated during weathering processes of basaltic rocks of the Lisbon Volcanic Complex. In general there is an increase in the REE contents in the whole samples with increasing weathering and fractionation between LREE and HREE. Cerium is partially oxidized and Ce4+ is retained resulting in negative Ce anomalies in the whole samples of the base and middle profile zones. In the clay fraction there is an increase in the REE contents from the top to the bottom of the profiles, except for Ce, giving rise to positive Ce anomalies in the top and negative anomalies of this element in the middle and bottom of the profiles. Significant positive Ce anomalies occur in the finer fractions of palaeoweathered (Upper Cretaceous/ Lower Eocene) basalts associated with low REE contents relative to the whole samples indicating more intense leaching conditions in the Upper Cretaceous/Lower Eocene than at the present day. However, in both cases no positive Ce anomalies were found in the whole samples. There appears to be a negative correlation between the (La/Yb)ch ratio variation and the clay mineralogy/drainage conditions.

Type
Research Article
Information
Clay Minerals , Volume 30 , Issue 3 , September 1995 , pp. 239 - 248
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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