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

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 Ce ~ 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. The rare earth elements' (REE) behaviour in surface weathering environments has been the subject of numerous works especially in the last two decades, contributing to our understanding of how these elements behave under different conditions of weathering. However, the results obtained so far are sometimes varied and a large number of studies was undertaken to confirm the use of REE for the original parent rock, based only on the REE

Comparative Study of the Mobility of Major and Trace Elements During Alteration of an Andesite and a Rhyolite to Bentonite, in the Islands of Milos and Kimolos, Aegean, Greece

“…The results obtained suggest that Th and the REE appear to enter in phosphates, although neoformed phyllosilicates might also be additional hosts as suggested in other studies (Spears and Kanaris-Sotiriou 1979;Elliott 1993). Similar observations have been made in weathering profiles and diagenetic environments (Banfield and Eggleton 1989;Milodowski and Hurst 1989;Milodowski and Zalasiewitz 1991), while Prudencio et al (1995) reported only partial incorporation of REE in clay minerals during weathering of basalts. The HREE were initially in the glass and were subsequently leached during alteration.…”

“…thoroughly, the mobility of trace elements including lanthanides, with few exceptions (Zielinski 1982;E1-liott 1993), have been examined thoroughly only in weathering profiles (Duddy 1980;Banfield and Eggleton 1989;Prudencio et al 1995). This contribution examines 2 alteration profiles in the volcanic islands of Milos and Kimolos, Aegean, Greece, in which an andesite and a rhyolitic ignimbrite have been converted to bentonite in a submarine environment.…”

Formation des latérites nickélifères et mode de distribution des éléments du groupe du platine dans les profils latéritiques du complexe de Musongati, Burundi /

“…Les résultats de plusieurs études de ces vingt dernières années sur le comportement des REE dans les environnements supergènes aboutissent à quatre principales constatations, souvent contradictoires: 1) les REE restent immobiles, 2) les REE sont légèrement remobilisés, 3) les REE sont remobilisés sans montrer aucune différenciation, 4) les REE sont à la fois remobilisés et différenciés (Nesbitt 1979;Duddy 1980;Topp et al 1985;Bonnot-Courtois and Flicoteaux 1989;Melfi et al 1990;Braun et al 1990;Prudêncio et al 1995;Sturesson 1995).…”

“…En effet, les propriétés chimiques des REE changent graduellement avec l'intensité de la latéritisation et ce, dépendamment des facteurs suivants: 1) les conditions Eh-pH dans les horizons latéritiques (Brookins 1983;Cantrell et Byrne 1987), 2) la présence des ligands organiques et inorganiques, incluant les argiles (Braun et al 1993;Prudêncio et al 1995), 3) la présence et la distribution des minéraux des REE dans la roche-mère.…”

“…Les rapports LREE/HREE n c varient de 4 à 9 pour le SL, la CU et la FE, de 7 à 15 pour les niveaux supérieurs de la saprolite, et de 50 à 160 à la base de la saprolite, contre une moyenne de 3.5 dans la roche-mère. (Prudêncio et al 1995;Angelica et Costa 1993;Braun et al 1990). Il existe donc une tendance généralisée pour le Ce à demeurer et s'enrichir dans les niveaux supérieurs du profil latéritique, en provoquant ainsi une anomalie positive à ce niveau par rapport aux REE de la roche totale, et une anomalie négative dans les niveaux latéritiques saprolitiques.…”