Nb/Ta Fractionation in Cambrian Island Arc
and Back Arc Basalts

Carsten Münker Institut für Geologie und Dynamik der Lithosphäre / Geochemisches Institut,

Goldschmidtstr. 3, 37077 Göttingen, Germany


In geochemistry, Nb and Ta have long been regarded as behaving identical during geochemical fractionation processes, having the chondritic Nb/Ta ratio of 17.6. Recent work, however, suggests fractionation of Nb/Ta in island arc suites (Green, 1995; Stolz et al., 1995). This study presents evidence for systematic Nb/Ta variations in an Cambrian island arc system of the Early Paleozoic Takaka Terrane, New Zealand. These volcanics of a very narrow age range (20 ma) comprise an unusual variety of compositions ranging from back arc basin basalts through boninites and arc tholeiites to calc-alkaline basalts and andesites (Münker & Cooper, 1995). The volcanics have been analyzed by ICP-MS and display Nb contents between 0.15 and 30 ppm and Ta contents between 0.02 and 2 ppm. Accuracy of the analytical procedure has been ascertained by simultaneous analysis of Nb and Ta in 15 geostandards. HCl has been added to the sample solutions and measurements have been made within 24 hours of dissolution to avoid precipitation of both elements. The constancy of Nb/Ta in samples of single volcanic suites at different H2O and CO2 contents and at different Mg# and Ni/Cr contents shows, that neither alteration processes nor crystal fractionation affected the primary Nb/Ta ratios of the samples. Nb/Ta between the different volcanic suites is varying from 10 to 24. The more mafic samples (Mg# 50-75) show good correlation between Nb/Ta and La/Yb for all rock types. Nb/Ta also correlates well with TiO2 in samples with arc signature (Nb/La <0.9), whereas the back arc basins basalts are offset towards higher TiO2, the latter interpreted to reflect incipient tholeiitic fractionation or initially higher Ti-contents in the primitive melt. Higher Nb/Ta with higher TiO2 is very likely to reflect either the role of a residual Ti phase or of amphibole in mantle source. Brenan et al. (1994) and Ionov & Hofmann (1995) showed, that both, mantle amphibole and fluid equilibrated rutile have DNb/DTa>1 and high partition of Ti, which would cause an increase of Nb/Ta in the melt with increasing Ti in the melt. Contamination with continental crust with a poorly constrained Nb/Ta of 11 (Green, 1995) also might influence the Nb/Ta of the volcanics. Further isotopic studies on the volcanics and accurate measurements of Nb/Ta in the possible contaminants of the volcanics will help to discriminate between these processes.


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