ROCK STRUCTURES AND MINERAL ASSEMBLAGE ASSOCIATED WITH HYDROEXPANSIONIN MIDOCEANIC RIDGE BASALTS

Detailed micromorphological studies show that zeolites, i.e., mesolite , cowlesite, mordenite and ferrierite, that precipitated exclusively i n porous zones, are an important constituent of mid-oceanic ridge basa lts (MORB). Such porous zones may result from hydroexpansive fragmenta tion processes operating during the eruption of the lava under water. Hydroexpansion structures and their secondary mineral assemblages in s ea-floor basalts may be significant in the early alteration history of MORE. However, the original volume of these structures is difficult t o assess owing to overprinting by subsequent alteration processes. The zeolite assemblage of porous zones is typically K-poor and may theref ore be distinguished from the K-rich zeolites of palagonitic origin co mmonly found in MORE.