The OIB paradox

Ocean island basalt (OIB) and OIB-like basalt are widespread in oceanic and continental settings and, contrary to popular belief, most occur in situations where mantle plumes cannot provide a plausible explanation. They are readily distinguished from normal mid-ocean ridge basalt (N-MORB) through ΔNb, a parameter that expresses the deviation from a reference line (ΔNb = 0) separating parallel Icelandic and N-MORB arrays on a logarithmic plot of Nb/Y versus Zr/Y. Icelandic basalts provide a useful reference set because (1) they are by definition both enriched mid-ocean ridge basalt (E-MORB) and OIB, and (2) they represent a larger range of mantle melt fractions than do intraplate OIBs. Virtually all N-MORB has ΔNb < 0, whereas all Icelandic basalts have ΔNb > 0. E-MORB with ΔNb > 0 is abundant on other sections of ridge, notably in the south Atlantic and south Indian oceans. E-MORB and N-MORB from this region form strongly bimodal populations in ΔNb, separated at ΔNb = 0, suggesting that mixing between their respective mantle sources is very limited. Most OIBs and basalts from many small seamounts, especially those formed on old lithosphere, also have ΔNb > 0. HIMU OIB (OIB with high 206Pb/204Pb values and therefore a high-µ [U/Pb] source) has higher ΔNb on average than does EM (enriched mantle) OIB, consistent with the presence of recycled continental crust (which has ΔNb < 0) in the EM source. Although EM OIBs tend to have the lowest values, most still have ΔNb > 0, suggesting that a relatively Nb-rich component (probably subducted ocean crust) is present in all OIB sources. The OIB source components seem to be present on all scales, from small streaks or blobs of enriched material (with positive ΔNb) carried in the upper-mantle convective flow and responsible for small ocean islands, some seamounts, and most E-MORB, to large mantle upwellings (plumes), inferred to be present beneath Hawaii, Iceland, Réunion, and Galápagos. It is not possible to identify a point on this continuum at which mantle plumes (if they exist) become involved, and it follows that OIB cannot be a diagnostic feature of plumes. The geochemical similarity of allegedly plume-related OIB and manifestly nonplume OIB is the first part of the OIB paradox. Continental intraplate transitional and alkali basalt in both rift and nonrift (e.g., Cameroon line) settings usually has positive ΔNb and is geochemically indistinguishable from OIB. Continental volcanic rift systems erupt OIB-like basalt, irrespective of whether they are apparently plume-driven (e.g., East Africa, Basin and Range), passive (e.g., Scottish Midland Valley) or somewhere between (e.g., North Sea basin). Magma erupted in passive rifts must have its source in the upper mantle, and yet it is always OIB-like. N-MORB–like magma is only erupted when rifting progresses to continental break-up and the onset of seafloor spreading. Continental OIB-like magma is frequently erupted almost continuously in the same place on a moving lithospheric plate for tens of millions of years, suggesting that its source is coupled in some way to the plate, and yet the Cameroon line (where continental and oceanic basalts are geochemically indistinguishable) suggests that the source is sub-lithospheric. The causes and sources of continental OIB-like magma remain enigmatic and form the second part of the OIB paradox.