Mantle Redox Conditions in the North Atlantic Igneous Province

The North Atlantic igneous province (NAIP) has long been viewed as a region of anomalous mantle upwelling related to plume activity, continental rifting, and a heterogeneous mantle source. Prior to continental rifting in the Tertiary, the northern portion of the region was the site of closure of the Iapetus ocean basin. This tectonic event may have contributed to heterogeneities within the upper mantle and altered its oxidation state relative to the ambient mantle. Vanadium has been shown to be a useful indicator of redox conditions due to its multiple valence states (e.g. [1-2]). In mantle minerals, vanadium becomes increasingly incompatible under more oxidizing conditions [3]. Because both scandium and vanadium are moderately incompatible during melting, the Sc/V ratio of primitive basalts can be used to investigate the oxidation state of the mantle [1-3]. We have examined the Sc/V ratios of primitive lavas from the mid-Atlantic ridge (MAR), Iceland, and the East Greenland margin to determine if there are spatial or temporal variations in the oxidation state of the NAIP mantle. The Sc/V ratios for MAR basalts are 0.13-0.20 (GEOROC chemical database); while Icelandic basalts range from 0.10-0.25 with an average of 0.16 (1 σ =0.05). The entire range of Sc/V ratios of the Paleogene East Greenland basalts is 0.07-0.17 with an average of 0.10 (1 σ = 0.05). The Sc/V ratios of Icelandic basalts are similar to MAR basalts, but the East Greenland lavas are distinctly lower than both the MAR and Iceland. The Sc/V ratio also can vary as a function of mean pressure of melting (i.e. spinel versus garnet lherzolite). To test the relative importance of melting systematics, source composition, and oxygen fugacity on the Sc/V systematics for NAIP basalts, we incorporated the oxygen-fugacity-dependent V mineral-melt partitioning data of [3] into the polybaric decompression melting model REEBOX [4]. The best-fit model parameters for the majority of the Iceland and MAR basalts constrain the oxygen fugacity of the mantle for the modern ridge system to be one log unit below the Ni-NiO buffer. To model the entire range of East Greenland lavas requires that the Paleogene mantle source was ∼0.5-0.8 log units more oxidized than the Iceland source. These differences may be attributed to a change in the composition of the Iceland plume or reflect the involvement of metasomatized upper mantle associated with Iapetus subduction in the formation of the East Greenland basalts. [1] Canil, D., 1997, Nature 389, 842-845; [2] Canil, D. 1999, Geochem. Cosmochim. Acta 63, 557-572; [3] Canil and Fedortchouk, 2000, J. Geophys. Res. 105, 26003-26016; [4] Fram and Lesher, 1993, Nature 363, 712-715