209 Fraccionação das razões Zr/Hf, Nb/Ta e Ti/Eu em rochas basálticas de ilhas oceânicas: a ilha do Fogo (Cabo Verde) como exemplo Zr/Hf, Nb/Ta and Ti/Eu fractionation in ocean island basaltic rocks: an example from Fogo Island (Cape Verde Archipelago) J. Munhá 1,2 ; J. Mata 1,2 ; S. Martins 1,2 ; J. Madeira 1,3,4 ; A. Brum da Silveira 1,3,4 1 Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa; 2 Centro de Geologia da Universidade de Lisboa; 3 LATTEX – Laboratório de Tectonofísica e Tectónica Experimental; 4 IDL – Instituto D. Luís jmunha@fc.ul.pt ; jmata@fc.ul.pt ; smmartins@fc.ul.pt ; jmadeira@fc.ul.pt ; antonio.brum@fc.ul.pt; SUMÁRIO A diferenciação de magmas basaniticos/melanefeliníticos na Ilha do Fogo produziu fonotefritos e tefrifonolitos que apresentam intensa fraccionação em Zr/Hf, Nb/Ta e Ti/Eu, consistente com dados experimentais (D Hf >D Zr , D Ta >D Nb ) aplicáveis à diferenciação de magmas alcalinos. Tal aconselha precaução no uso dessas razões na caracterização de processos geoquímicos no manto. Valores condríticos para as razões Ti/Eu e Zr/Hf em rochas primitivas não suportam metasomatismo carbonatado nas fontes magmáticas do Fogo. Palavras-chave: Cristalização fraccionada; Zr/Hf; Nb/Ta; Ti/Eu; Fogo; Cabo Verde SUMMARY Crystal fractionation of basanitic/melanephelinitic Fogo Island parental magmas produced phonotephrites/tephriphonolites, which display severe Zr/Hf, Nb/Ta, Ti/Eu fractionation, supporting experimental partitioning data (D Hf >D Zr , D Ta >D Nb ) for differentiation of alkaline magmas. This suggests caution when using element ratios to study upper mantle geochemical processes. Chondritic Ti/Eu and Zr/Hf ratios in primitive samples disfavour carbonate metasomatism in Fogo mantle source(s). Key-words: Crystal fractionation; Zr/Hf; Nb/Ta; Ti/Eu; Fogo; Cape Verde Introduction The high-field-elements (HFSE) Ti, Zr, Hf, Nb and Ta have long been used as a tool to study geochemical processes in the upper mantle and the genesis of continental crust. Selective enrichment and depletion of HFSE relative to large-ion lithophile elements (LILE) has become a valuable tool to characterise the source evolution of mantle- derived magmas and to assess their tectonic settings, particularly in subduction zone environments [e.g., 1; 2; 3]. In contrast to other element ratios among elements with similar incompatibility, but different chemical properties (e.g., La/Nb, Ti/Eu), the ratios of isovalent element pairs Zr-Hf and Nb-Ta with almost identical ionic radii have long been regarded as constant and close to chondritic (Zr/H ~ 34-37; Nb/Ta ~ 14-18) in the major mantle reservoirs. Superchondritic Zr/Hf and Nb/Ta in basalts and mantle rocks are rarely observed [see 4] and appear to be restricted to carbonate metasomatised mantle sources [5; 6], which are also characterised by low Ti/Eu (< 7500, sub-chondritic) ratios [7]. However, experimental studies revealed significant different partition coefficients for Zr-Hf and Nb-Ta between major silicate rock forming minerals (e.g. clinopyroxene (cpx), amphibole) and silicate melts [8; 9; 10]. In cpx mafic silicate melt systems D Hf /D Zr is predicted to be about 2 and D Ta /D Nb ~ 2-3, whereas D Ti /D Eu seems to increase from ~ 1 in cpx- silicate melts systems [11] to ~ 3 in cpx-carbonate melt systems [7]. Hence, Zr/Hf and Nb/Ta should also fractionate during either cpx-controlled magma crystallisation or partial melting of mantle lherzolites. Such cpx-melt controlled systems should produce high Zr/Hf and Nb/Ta ratios during