ΔNb is a geochemical construct of Fitton et al. (1997) based on two trace-element ratios, Nb/Y and Zr/Y, plotted against one another on a log-log diagram. ΔNb is defined in relation to a diagonal line on the diagram separating Nb-enriched Icelandic tholeiite from Nb-depleted mid-ocean ridge basalt (MORB). Neither crystallization differentiation nor partial melting of a peridotite mantle source can drive compositions below the line, with negative ΔNb, to locations above the line, with positive ΔNb, or vice versa. The parameter ΔNb thus is taken to indicate distinct mantle sources.
Crystallization differentiation along the East Pacific Rise has almost no effect on ΔNb. The low and similar partition coefficients for Y, Zr, and Nb during shallow crystallization differentiation ensure that data points for primitive and evolved basalt as well as rare iron-enriched andesite and dacite plot virtually atop one another on the diagram, with all differentiates retaining negative ΔNb. Only enriched MORB (E-MORB) obtained from both ridge segments and the summits of some near-ridge seamounts, which in the extreme includes alkalic olivine basalt, has positive ΔNb. Normal MORB and E-MORB together produce a curving array that crosses the line on the diagram, and this crossing trend indicates heterogeneity in the source.
At Iceland, on the other hand, tholeiitic ferrobasalt and abundant andesite, dacite, and rhyolite are widely separated from primitive basalt on such a diagram, plotting at successively higher Nb/Y and Zr/Y at positive ΔNb. The most enriched rocks at Iceland are rhyolites with approximately the geochemistry of the inferred common mantle component, FOZO (focal zone). These rocks cannot be related to each other by either crystallization differentiation or partial melting of a peridotite source. Calculations show that mixing of only 1–2% rhyolite or ∼5% evolved basalt with primitive basalt can shift Nb/Y, Zr/Y, and in certain circumstances ΔNb to higher values; it can also shift chondrite-normalized patterns of rare earth elements in primitive basalt from negative to positive slopes, a general attribute of Icelandic lava suites and the progression along Reykjanes Ridge approaching Iceland from the south. Both isotopic ratios and other trace-element concentrations and ratios are consistent with such mixing.
At Iceland, the usual differentiated and enriched mixing component is ferrobasalt with TiO2 contents so high that it cannot be derived from commonly construed mantle peridotite; a Ti phase such as ilmenite, titanomagnetite, or—at high pressure—rutile, must be in the source. Most picrites, on the other hand, have lower amounts of Nb, Y, Zr, and TiO2 than primitive MORB. These attributes have led previous workers to propose a source in recycled ocean crust for both types of basalt, brought to the vicinity of Iceland in a mantle plume.
However, given the generally enriched character of all Icelandic basalt and the association with rhyolite, I suggest instead that they all are linked to a common source and that they represent successive fractional melts of gabbroic cumulates originally produced during differentiation of ancient granitic continental crust. Much later, during the early stages of the current rifting episode in the North Atlantic, the dense gabbro sank into the warming upper mantle by means of convective instability, converting to eclogite. As Greenland separated from Europe, the eclogite experienced decompression partial melting that contributed to the eruption of flood basalt, and some of it is retained in the Iceland melt source to this day. The high ΔNb of the basalt at Iceland is thus less an indication of mantle source heterogeneity than it is of mixing between depleted basalt and petrogenetically unrelated mafic to silicic differentiates with an ultimate origin in ancient continental crust. It is not evidence for the existence of a mantle plume.
