Regionally distinctive sources of depleted MORB: Evidence from trace elements and H2O

The sources of depleted mid-ocean ridge basalts (N-MORB) worldwide are regionally distinctive in that they share incompatible element ratios with spatially associated, enriched mid-ocean ridge basalts (E-MORB). The ratio of H2OCe is uniform for N-MORB and E-MORB within a region, suggesting that the order of incompatibility during the evolution of MORB sources is La > H2O ≈ Ce > Nd. However, there are significant regional variations in H2OCe. N-MORB and E-MORB from the American-Antarctic Ridge (AAR), Southwest Indian Ridge (SWIR), southern Mid-Atlantic Ridge (SMAR), Pacific-Nazca Ridge 27–34° S (PNR), East Pacific Rise 10–12° N (EPR), Explorer Ridge, Mid-Cayman Rise Spreading Center (MCR) and Galapagos Spreading Center (GSC), as well as basalts from Loihi Seamount, have H2OCe ratios that average about 155–213 (±40 for each region). N-MORB through E-MORB from the Mid-Atlantic Ridge north of about 22° N (NMAR) have higher H2OCe ratios, averaging 240–280 (±50 for each region). There are no correlations of H2OCe with spreading rate or extent or depth of melting, indicating that variations in H2OCe are not related to MORB melting but are a characteristic of the source. tios are also regionally variable but interpretation is complicated by a slight dependence of KNb on source enrichment. H2OCe is not correlated with KNb, 3He4He or Pb isotopic parameters but may be correlated with high 87Sr86Sr at a given 143Nd144Nd. Data from other regions are needed before a correlation between H2OCe and 87Sr86Sr can be established. or correlation of H2OCe with 3He4He makes it unlikely that H2OCe variations are related to variations in juvenile H2O in the source. It is more likely that H2O in MORB is derived from recycled, subducted, altered oceanic crust. High H2OCe in MORB from the NMAR might be related to a period of rapid subduction in the past that resulted in depressed isotherms and less dehydration in the slab. nstancy of H2OCe within regions despite differences between regions indicates that N-MORB and E-MORB sources may share a common heritage. This constraint on the evolution of the depleted mantle is not easily reconciled with most conventional models of mantle evolution. A model in which the sources of N-MORB have been influenced by inputs of regionally distinctive plume material that has been previously depleted by small extents of melting could account for the trace element variations but is physically implausible. It is possible that high H2OCe may be a regional characteristic of the mantle that is unrelated to plumes.