The OIB paradox

Abstract

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 ∆Nb, a parameter that expresses the deviation from a reference line (∆Nb=0) separating parallel Icelandic and N-MORB arrays on a logarithmic plot of Nb/Y vs. Zr/Y (Fitton et al., Earth and Planetary Science Letters, v. 153, p. 197-208). 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 ∆Nb<0 while all Icelandic basalts have ∆Nb>0. E-MORB with ∆Nb>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 ∆Nb, separated at ∆Nb=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 ∆Nb>0. HIMU OIB (OIB with high 206 Pb/ 204 Pb and therefore a high-(U/Pb) source) has higher ∆Nb on average than does EM (enriched mantle) OIB, consistent with the presence of recycled continental crust (which has ∆Nb<0) in the EM source. Although EM OIBs tend to have the lowest values, most still have ∆Nb>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 ∆Nb) carried in the upper mantle

Multistage Cretaceous magmatism in the northern coastal region of Ellesmere Island and its relation to the formation of Alpha Ridge – evidence from aeromagnetic, geochemical and geochronological data

“…2). OIB is geochemically very similar to continental rift basalts (e.g., Fitton, 2007). Therefore, the mafic Cretaceous igneous rocks of the Yelverton Bay area are interpreted to have formed in a continental rift zone.…”

Olivine and melt inclusion chemical constraints on the source of intracontinental basalts from the eastern North China Craton: Discrimination of contributions from the subducted Pacific slab

“…mantle. ''Blobs" of subducted material dispersed in a depleted peridotite matrix is said to be common in the upper asthenospheric mantle (see the review by Fitton, 2007). We therefore posit that there are present in the NCC asthenospheric mantle dispersed domains of garnetbearing pyroxenites resulting from reactions between recycled ancient (>1.0 Ga) materials (rich in marine sediments) and peridotite (Fig.…”

Asthenosphere–lithosphere interaction triggered by a slab window during ridge subduction: Trace element and Sr–Nd–Hf–Os isotopic evidence from Late Carboniferous tholeiites in the western Junggar area (NW China)

“…However, the Hatu basalts are not flood basalts related to oceanic plateau as discussed above. Oceanic plateau basalts are generally thought to be derived from a mantle source involving mixing between a depleted mantle plume component and a HIMU (high μ) component with a small contribution of an enriched component (Condie et al, 2002;Fitton, 2007;Fitton et al, 1997) (Fig. 6a, b).…”