Incompatible trace elements in OIB and MORB and source enrichment in the sub-oceanic mantle

• Published in: Earth and planetary science letters 1995-07, Vol.133 (3), p.379-395
• Main Authors: Halliday, Alex N., Lee, Der-Chuen, Tommasini, Simone, Davies, Gareth R., Paslick, Cassi R., Godfrey Fitton, J., James, Dodie E.
• Format: Article
• Language: English
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/0012-821X(95)00097-V

The concentrations of incompatible trace elements in ocean island basalts (OIB) from the central Atlantic extend to relatively enriched and fractionated compositions in regions of older oceanic lithosphere. Certain trace element ratios normally considered to be uniform in the mantle, such as Ce Pb , are particularly variable. However, other trace element ratios that are expected to be variable because of differences in bulk distribution coefficient, such as Ce U , are relatively uniform. The Ce Pb ratios in enriched OIB are correlated with unusually high U Pb and low K U . These U Pb ratios would have generated excessively radiogenic Pb if they were long-term (10 9 yr) features of the source such as might result from core formation or recycling of hydrothermally altered ocean floor basalts. However, volcanic centers with high U Pb do have high 206 Pb 204 Pb for their 207 Pb 204 Pb , a feature that is most easily modelled by enrichment in U relative to Pb about 10 8 yr prior to melting, a time similar to the age of the lithosphere. We propose that the source regions of these magmas are enriched by the introduction of small degree partial melts soon after the formation of the oceanic lithosphere. Metasomatism of the uppermost mantle by small degree partial melts produced in equilibrium with a combination of residual upper mantle major silicate phases, together with minor amphibole (≤ 2%), sulfide (≤ 0.2%) and phlogopite (≤ 0.2%) at about the time of formation of the lithosphere, would generate a ‘near-surface fractionated’ (NSF) source with low K U and high U Pb , Δ 206 Pb 204 Pb and Ce Pb , while maintaining Ce U , Nb U , Ba Ce and Ba Nb that are only slightly fractionated relative to other OIB. An important feature of the modelling of NSF mantle is that U is more incompatible than Ba or Rb. This is confirmed by the variability in incompatible trace element ratios with U concentration for enriched OIB. However, this contrasts with the relative incompatibility deduced from UThRa disequilibrium data for MORB and OIB, endorsing the view that the variability in highly incompatible trace element ratios in enriched OIB is dominated by source enrichment effects that are distinct from the fractionation that takes place during the production of the erupted magmas. The Ce U , Ba Ce and U Pb ratios of all OIB, including enriched OIB from regions of old lithosphere, are uniform relative to data for MORB. This appears inconsistent with the degree of isotopic variabili