Search for a deep-mantle component in mafic lavas using a Nb-Y-Zr plot

Concentrations of trace elements with bulk distribution coefficients < 1 have been utilized to discriminate between mafic magmas erupted in different tectonic settings. Herein, the Nb-Y-Zr contents of mafic lavas will be utilized to pinpoint derivation from a deep-mantle source. Magmas derived from the Icelandic plume exhibit Delta Nb > 0, where Delta Nb = log (Nb/Y) + 1.74 - 1.92 log (Zr/Y) (Fitton et al. 1997), whereas melts derived from depleted sections of the mantle and the crust show Delta Nb < 0. Regardless of the nature of the melting event, it was suggested the Delta Nb value of magmas were essentially identical to that of the source material. Theoretical modeling of a garnet lherzolite source suggests very small partial melts may show elevated Delta Nb values, yielding a spurious plumelike signature. For mafic rocks from different areas, with hypothesized hot spot derivation, Delta Nb is calculated to search for a deep-mantle signature (Delta Nb > 0). The signature of the Reunion hot spot, from similar to 65-0 Ma, is seen in the Deccan Traps, India, lavas from the Mascarene Plateau, and Reunion Island. The signature of the Kerguelen plume is detected in rocks from Kerguelen Island and the Ninetyeast and Broken Ridges, Indian Ocean, a few lavas from the Rajmahal province, India, but none in the Bunbury Basalt, Australia. Some rocks from continental flood basalt provinces show a hot spot component, in agreement with conclusions based on He-3/He-4 studies, others show Delta Nb < 0, due to considerable subcontinental contamination. For lavas from Heard Island and Grand Comore, Indian Ocean, the technique pinpoints those showing a plume component. In an area of plate convergence in the Pacific Ocean, lavas with very low (<1 ppm) Nb contents, serve as a critical test for this method. Only rocks close to the Samoan hot spot show Delta Nb > 0, whereas the others show the Nb depletion typical of arc-related material. In conjunction with trace-element and isotopic discriminants, this appears to be a useful tool in identifying a deep-mantle component in mafic lavas from a variety of plate tectonic settings.