The D/H ratios and water contents in fresh submarine basalts from the Mid-Atlantic Ridge, the East Pacific Rise, and Hawaii indicate that the primary D/H ratios of many submarine lavas have been altered by processes including (1) outgassing, (2) addition of seawater at magmatic temperature, and (3) low-temperature hydration of glass. Decreases in δD and H²O⁺ from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between δD and H₂O⁺ in basalts from the Galapagos Rise and the FAMOUS Area are attributed to outgassing of CH₄ and H₂. A good correlation between δD values and H₂O is observed in a suite of submarine tholeiites dredged from the Kilauea East Rift Zone where seawater (added directly to the magma), affected only the isotopic compositions of hydrogen and argon. Analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having δD values as low as −100.

δD values vary with H₂O contents of subaerial transitional basalts from Molokai, Hawaii, and subaerial alkali basalts from the Society Islands, indicating that the primary δD values were similar to those of submarine lavas.

Extrapolations to possible unaltered δD values and H₂O contents indicate that the primary δD values of most thoteiite and alkali basalts are near −80 ± 5: the weight percentages of water are variable, 0.15–0.35 for MOR tholeiites, about 0.25 for Hawaiian tholeiites, and up to 1.1 for alkali basalts. The primary δD values of −80 for most basalts are comparable to those measured for deep-seated phlogopites. These results indicate that hydrogen, in marked contrast to other elements such as Sr, Nd, Pb, and O, has a uniform isotopic composition in the mantle. This uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the Earth.