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CONCLUSÕES
O zircão estudado constitui um material de referência adequado para análises de amostras de zircão via LA-ICP-MS. Com base em uma série de análises de alta qualidade, incluindo ID-TIMS e LA-ICP-MS, fomos capazes de demonstrar que este zircão é um padrão apropriado para normalização e controle de qualidade de análises dos sistemas isotópicos U- Pb e Lu-Hf. Vários cristais com qualidade de gema foram testados e mostraram-se altamente homogêneos e com idades U-Pb concordantes. Os cristais são marcados por um nível muito baixo de Pb comum e por altas concentrações de U e Pb, o que permitiu altas taxas de contagem e boas contagens estatísticas. As análises ID-TIMS deram uma idade 206Pb/238U de 562.58 ± 0.26 Ma. No entanto, uma das gemas deu uma idade um pouco mais jovem, de 556.25 ± 0.70 Ma, o que implica que futuras análises TIMS serão necessárias para melhor avaliar as idades U-Pb nos diferentes fragmentos do zircão estudado.
As análises isotópicas de Hf por LA-MC-ICP-MS produziram baixas razões Lu/Hf (média de 0.00008) e Yb/Hf (média de 0.000005), o que influencia a escolha do procedimento de correção das interferências isobáricas no valor e na incerteza das razões 176Hf/177Hf corrigidas. O valor 176Hf/177Hf de 0.281674 ± 0.000018 (2 S.D.) é considerado como a melhor estimativa da composição isotópica de Hf no zircão estudado e é válido para qualquer fragmento que venha a ser utilizado como amostra de referência.
As amostras apresentam um baixo grau de heterogeneidade interna, indicado pelas imagens de catodoluminescência, que, por sua vez, não mostraram um zoneamento interno significativo. Variações na composição química determinadas por LA-ICP-MS não excederam as incertezas experimentais, no entanto, futuras análises serão necessárias para demonstrar que o zircão estudado pode ser utilizado como material de referência para análises in situ de Elementos Terras Raras. As análises do estado estrutural (difratometria de raios-X e espectroscopia Raman) não indicaram heterogeneidade significativa nem dentro dos grãos individuais, nem entre os vários fragmentos do zircão estudado. O grau de acumulação de danos por radiação corresponde ao de outras amostras de zircão provenientes do Sri Lanka, o que indica que o zircão estudado não teve seu sistema isotópico perturbado desde sua cristalização e, portanto, é considerado estruturalmente adequado como um material de referência.
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