Hi Anthony,

Beware of the precision and accuracy of Nb and Ta analyses. These elements are often close to the detection limit in mafic rocks, and what you see could be due to analytical error.

Otherwise, you might want to look at references such as:

Green, T. H. (1995) Significance of Nb/Ta as an indicator of geochemical processes in the crust-mantle system. Chemical Geology 120(3–4):347-359.

Baier, J., Audétat, A., and Keppler, H. (2008) The origin of the negative niobium tantalum anomaly in subduction zone magmas. Earth and Planetary Science Letters 267(1-2):290-300.

Good luck!

Antoine

My comment goes the same way as the comment by Antoine Gaté. However, the OIB suites display the largest contents of Th, U, K, Ta, Nb... and you should not meet problem with their limit of detection. Nb and Ta very hardly fractionate. They do not fractionate in alkali basalt suites: there are data for instance from basalts to trachytes of french Polynesia. Another exemple is provided by the alkali suites of Chaîne des Puys. In tholeiitic basalts (i.e. MORBs), where concentrations are much lower, we did not detect Nb/Ta fractionation: you may find a lot of data under my name including in DSDP reports. Yaoling Niu reports Nb/Ta fractionation for very "depleted" MORBs. Find enclosed some references dealing with Nb, Ta, Hf, Zr and rare earths related to different settings.

As far as analytical chemistry is concerned, we measured Nb by X ray fluorescence and Ta through Thermal Neutron activation:

Jaffrezic, H., J. L. Joron, et al. (1977). "Trace elements determination in rocks powder. A study of the precision for a given analytical procedure. Instrumental epithermal neutron activation." Journal of Radioanalytical Chemistry 39: 185-188.

Hope these few lines and references may help you

Henri

Article Application du diagramme de Coryell-Masuda "élargi" à l'étud...

Article Local versus Regional Mantle Heterogeneities: Evidence from ...

Article Unusually large Nb-Ta depletions in North Chile ridge basalt...

Article Quantification of Nb, Ta, Ti and V anomalies in magmas assoc...

Article Rare Earth and non Rare Earth magmaphile Elements: Fractiona...

Article Analysis of Trace Elements in Basalts by Shipboard X-Ray Flu...

Article Precise Quantitative Determination of Niobium at Low Level C...

thank you somuch. i really do appreciate your contributions