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-st udied suites of mid-ocean ridge basalt (MORB) glasses from the Northern Eas t 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 ex amination of the fine details of H2O geochemical behaviour during MORB gene sis. We demonstrate that relative H2O contents [i.e. H2O/(another incompati ble element)] vary systematically with increasing (La/Sm)(N) in Morb glasse s from both the EPR and SEIR. This indicates that H2O behaves like other in compatible (in peridotite mineralogies) elements during MORB petrogenesis, and is primarily controlled by solid-melt partitioning. However, the relati ve H2O contents of MORB glasses from the SEIR are higher than in glasses fr om the EPR at a given (La/SM)(N), demonstrating global variations in the H2 O contents of MORB. Despite regional differences in relative H2O contents, the incompatible behaviour of H2O is similar in both studied regions. The r elative incompatibility of H2O varies systematically with increasing (La/Sm )(N): in depleted MORB, H2O is similar to La whereas in EMORB, H2O is simil ar 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 i n the mantle in the same phase with LREE (clinopyroxene?) at sub-solidus. R egional variations in relative H2O contents in EMORB that have more radioge nic Sr, Nd and Pb isotopes might be explained by differences in the nature of enriched components recycled via subduction processes. However, when EMO RB 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 H2 O contents of NMORB may reflect either initial variations in the Earth's ma ntle or inhomogeneities left after formation of the continental crust.