Determination of Nb, Ta, Zr and Hf in Micro-Phases at Low Concentrations by EPMA |
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Authors: | Florence Kalfoun Claude Merlet Dmitri Ionov |
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Institution: | (1) ISTEEM, CC 056, CNRS, Université de Montpellier II, Sciences et Techniques du Languedoc, – Pl. E. Bataillon 34095 Montpellier Cedex 5-France, FR;(2) Department of Earth and Environmental Sciences-CP 160/02 Université Libre de Bruxelles-50 av. F.D. Roosevelt-B-1050 Brussels-Belgium, BE |
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Abstract: | Depletion of high field strength elements (HFSE: Nb, Ta, Zr, Hf ) relative to other lithophile trace elements in arc magmas
and variations of Nb/Ta and Zr/Hf ratios in mantle-derived rocks can be addressed through studies of minerals, which concentrate
and fractionate these elements. The presence of rutile, a common accessory Ti-oxide phase in various mantle rocks, has often
been invoked to explain the Nb and Ta depletion in arc lavas because it has the highest HFSE abundances among the known mantle
minerals. In this study, we measure the concentrations of Nb, Ta, Zr and Hf (at > 200 ppm) in rutile of two metasomatized
mantle lherzolites using a Cameca SX-100 electron microprobe and obtain Nb/Ta ratios with an accuracy of about ± 5%. Mass
balance calculations indicate that ≤ 1−5% of Nb and Ta in the rocks reside in major minerals and that the balance is hosted
by accessory Ti-oxides. The Nb/Ta ratios vary significantly in nearby rutile grains in both peridotites (17–33, average 23;
12–37, average 21). Therefore, individual rutile grains may not be representative of the total grain population. However,
Nb/Ta ratios measured in the bulk rock lherzolites by solution ICP-MS (21 ± 0.3) are within the analytical error of the average
Nb/Ta values calculated for 5–7 rutile grains in both samples. These results emphasise that a representative grain selection
must be analysed in order to determine trace elements contents of bulk rocks from data on accessory phases. |
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Keywords: | : Trace analysis EPMA geochemistry high field strength elements |
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