Rare earth geochemistry of basalts from Norfolk Island, and implications for mantle inhomogeneity in the rare earth elements.

Abstract

The basalts of Norfolk Island range from tholeiites through transitional basalts to alkali olivine basalts, and plot in a unique field on a Ti-Zr-Y diagram . REE abundances in seven representative samples are similar to those in tholeiites and alkali olivine basalts from Hawaii , i.e. light REE enrichment relative to chondrites, and a slightly higher La/Yb value for the alkali olivine basalts relative to the tholeiites . Three tholeiitic samples with anomalously high Y (84-105ppm) show unique REE patterns for basaltic rocks, with a relative enrichment of the middle REE . Apart from Y and REE these samples are chemically identical to the `normal' Y-REE tholeiites from Norfolk Island, and it is suggested that the unique Y and REE contents are due to variable Y and REE concentrations in an accessory mantle phase such as apatite and/or the Y acceptor RE phosphate minerals such as xenotime or samarskite inhomogeneously distributed in the source region for the basalts.

Evolução metamórfica dos metassedimentos da Nappe Lima Duarte e rochas associadas do Complexo Mantiqueira, sul da Faixa Brasília (MG)

“…O granada-hornblenda granulito do grupo 1 apresenta uma anomalia negativa de Ce acentuada (Figura 93), o que sugere a atuação de processos hidrotermais na oxidação do Ce, resultando um padrões de ETR com anomalias negativas de Ce (Rollinson, 1993). Diversos autores reconheceram enriquecimento em ETR e anomalias negativas de Ce em rochas toleiíticas e em álcalibasaltos (Frey & Roy, 1978;Mc Donough & Sun, 1985;Green, 1978;Fodor et al, 1987 Tabela 11: Análises de elementos terras-raras normalizadas ao condrito (Normalização aos valores de Taylor & McLennan, 1985).…”