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

Abstract

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 ΔNb > 0, where ΔNb = 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 ΔNb < 0. Regardless of the nature of the melting event, it was suggested the ΔNb 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 ΔNb values, yielding a spurious plumelike signature. For mafic rocks from different areas, with hypothesized hot spot derivation, ΔNb is calculated to search for a deep-mantle signature (ΔNb > 0). The signature of the Réunion hot spot, from ~65–0 Ma, is seen in the Deccan Traps, India, lavas from the Mascarene Plateau, and Réunion Island. The signature of the Kerguélen plume is detected in rocks from Kerguélen 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 3He/4He studies, others show ΔNb < 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 ΔNb > 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.