The OIB Paradox

Godfrey Fitton

School of GeoSciences, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW

Analyses of Icelandic basalt and global mid-ocean ridge basalt (N-MORB) define distinct parallel arrays on a logarithmic plot of Nb/Y versus Zr/Y [1]. Chemically and isotopically depleted basalt from Iceland therefore represents an intrinsic mantle component that is distinct from the source of N-MORB [2]. Analyses of ocean-island basalt (OIB) from all of the world’s oceans plot at the high-Zr/Y end of the Iceland array and show no evidence for mixing with N-MORB. Basalt from rift valleys and other areas of lithospheric extension (e.g. the Kenya rift; Basin & Range province of the Western USA; Scottish Midland Valley; North Sea basin) plots with OIB, and this observation leads to a paradox. OIB and N-MORB are generated from distinct mantle sources that show only limited evidence for mixing. If the OIB source resides in deep-mantle plumes, then why is it so often tapped, but the N-MORB source never tapped, during lithospheric extension? The occurrence of chemically and isotopically identical basalt in the oceanic and continental sectors of the Cameroon line (West Africa) makes it unlikely that the continental lithospheric mantle could be the source of continental OIB-like basalt [3].

[1] Fitton, J.G., A.D. Saunders, M.J. Norry, B.S. Hardarson and R.N. Taylor, 1997. Thermal and chemical structure of the Iceland plume. Earth Planet. Sci. Letters 153, 197–208.

[2] Fitton, J.G., A.D. Saunders, P.D. Kempton and B.S. Hardarson, 2003. Does depleted mantle form an intrinsic part of the Iceland plume? Geochem. Geophys. Geosyst. 4, 1032, 2002GC000424

[3] Fitton, J.G. and H.M. Dunlop, 1985. The Cameroon line, West Africa, and its bearing on the origin of oceanic and continental alkali basalt. Earth Planet. Sci. Letters 72, 23-38.