Water and carbon heterogeneity in MORB mantle sources (Invited)

The abundances of H2O, CO2 and other volatiles in MORB can provide unique information on depths and extents of melting and lateral variations in mantle viscosity and melting temperature, but this information can also be obscured by degassing, fractionation and assimilation processes within the oceanic crust. These confounding effects are minimized at spreading centers within oceanic transforms, where crustal thickness is low or non-existent; and at near-ridge seamounts, where primitive magmas bypass long-lived axial magma reservoirs en route to the surface. We have analyzed a series of submarine MORB glasses collected from eastern Pacific intra-transform ridges and near-ridge seamounts. Volatiles were analyzed by SIMS, and the data are compared with similar data from adjacent normal ridge segments. CO2/Nb ratios decrease systematically with MgO, indicating concurrent degassing and fractional crystallization. Least-degassed CO2/Nb ratios are variable and appear to be related to mantle source depletion or enrichment; depleted sources appear to have low CO2/Nb while enriched sources can have higher CO2/Nb. In the Pacific, ratios of H2O/Ce, F/Nd and Cl/K in uncontaminated magmas show systematic co-variations with other trace element indices of enrichment (Th/La, Nb/La) that are correlated with Sr and Nd isotopes, indicating lateral variability in long-lived mantle source compositions in the upper mantle. The data are consistent with the entire spectrum of Pacific MORB (far from hotspots) being generated by melting of variable mixtures of only two sources, one highly depleted (D-MORB) and one enriched (E-MORB). The proportions of these sources, as expressed in Sr, Nd and Pb isotopes, changes along the EPR axis. This heterogeneity is a feature of the upper mantle that is present in addition to well-known ridge-hotspot interactions, and likely reflects the background convective circulation of the upper mantle.