Water in Mid Ocean Ridge Basalts: Some Like it Hot, Some Like it Cold

The presence in the Earth’s mantle of even small amounts of water and other
volatiles has major effects: first, it lowers drastically mantle’s viscosity, thereby facilitating
convection and plate tectonics; second, it lowers the melting temperature
of the rising mantle affecting the formation of the oceanic crust. H2O concentration
in oceanic basalts stays below 0.2 wt% except for basalts sampled near “hot spots”
that contain significantly more H2O than normal MORB, implying that their mantle
plume sources are unusually H2O-rich. Basalts sampled in the Equatorial Atlantic
close to the Romanche transform, a thermal minimum in the Ridge system, have a
H2O content that increases as the ridge is cooled approaching the transform offset.
These basalts are Na-rich, being generated by low degrees of melting of the mantle,
and contain unusually high ratios of light versus heavy rare earth elements implying
the presence of garnet in the melting region. H2O enrichment is due not to an
unusually H2O-rich mantle source, but to a low extent of melting of the upwelling
mantle, confined to a deep wet melting region. Numerical models predict that this
wet melting process takes place mostly in the mantle zone of stability of garnet. This
prediction is verified by the geochemistry of our basalts showing that garnet must
indeed have been present in their mantle source. Thus, oceanic basalts are H2O-rich
not only near “hot spots”, but also at “cold spots”.