H2O abundance in depleted to moderately enriched mid-ocean ridge magmas;: Part I:: Incompatible behaviour, implications for mantle storage, and origin of regional variations

The H2O contents and trace-element abundances are presented for two well-studied suites of mid-ocean ridge basalt (MORB) glasses from the Northern East Pacific Rise (EPR, 9-11 degrees N) and the South East Indian Ridge (SEIR, 127-129 degrees E). Exactly the same region of the glass samples has been analysed for these components using microbeam techniques. Our data allow examination of the fine details of H2O geochemical behaviour during MORB genesis. We demonstrate that relative H2O contents [i.e. H2O/(another incompatible element)] vary systematically with increasing (La/Sm)(N) in Morb glasses from both the EPR and SEIR. This indicates that H2O behaves like other incompatible (in peridotite mineralogies) elements during MORB petrogenesis, and is primarily controlled by solid-melt partitioning. However, the relative H2O contents of MORB glasses from the SEIR are higher than in glasses from the EPR at a given (La/SM)(N), demonstrating global variations in the H2O contents of MORB. Despite regional differences in relative H2O contents, the incompatible behaviour of H2O is similar in both studied regions. The relative incompatibility of H2O varies systematically with increasing (La/Sm)(N): in depleted MORB, H2O is similar to La whereas in EMORB, H2O is similar to Ce. Similar patterns of varying relative incompatibility (to REE) are displayed by Zr, Hf, and P. Our data are best explained if H2O is stored in the mantle in the same phase with LREE (clinopyroxene?) at sub-solidus. Regional variations in relative H2O contents in EMORB that have more radiogenic Sr, Nd and Pb isotopes might be explained by differences in the nature of enriched components recycled via subduction processes. However, when EMORB have the same radiogenic isotope compositions as NMORB within a segment, relative H2O contents in EMORB probably reflect local processes that lead to enrichment in incompatible elements. Regional differences in relative H2O contents of NMORB may reflect either initial variations in the Earth's mantle or inhomogeneities left after formation of the continental crust.