Water content of basalt erupted on the ocean floor

Deep sea pillow basalts dredged from the ocean floor show that vesicularity changes with composition as well as with depth. Alkalic basalts are more vesicular than tholeiitic basalts erupted at the same depth. The vesicularity data, when related to experimentally determined solubility of water in basalt, indicate that K-poor oceanic tholeiites originally contained about 0.25 percent water, Hawaiian tholeiites of intermediate K-content, about 0.5 percent water, and alkali-rich basalts, about 0.9 percent water. Analyses of fresh basalt pillows show a systematic increase of H₂O⁺ as the rocks become more alkalic. K-poor oceanic tholeiites contain 0.06–0.42 percent H₂O⁺, Hawaiian tholeiites, 0.31–0.60 percent H₂O⁺, and alkali rich basalts 0.49–0.98 percent H₂O⁺. The contents of K₂O, P₂O₅, F, and Cl increase directly with an increase in H₂O⁺ content such that at 1.0 weight percent H₂O⁺, K₂O is 1.58 percent, P₂O₅ is 0.55 percent, F is 0.07 percent, and Cl is 0.1 percent. The measured weight percent of deuterium on the rim of one Hawaiian pillow is −6.0 (relative to SMOW); this value, which is similar to other indications of magmatic water, suggests that no appreciable sea water was absorbed by the pillow during or subsequent to eruption on the ocean floor.

Concentrations of volatile constituents in the alkali basalt melts relative to tholeiitic melts can be explained by varying degrees of partial melting of mantle material or by fractional crystallization of a magma batch.