Major element variations in ocean-floor basalts

Although ocean-floor basalts form a very convergent group from the point of view of their major element compositions, the variation that they do show can be interpreted in geological terms to a certain extent, though analytical variation must contribute substantially to that observed. Multivariate statistical methods demonstrate a high-level fractional crystallization trend in 94 selected ocean-floor basalts caused by precipitation of calcic plagioclase, forsteritic olivine and augite. After this effect is subtracted, a moderate amount of variation remains which can be interpreted as caused by crystal-liquid processes involving amphibole, though this is less certain. A new average of ocean-floor basalt compositions is presented, and it is suggested that the term ‘oceanic tholeiite’ should not be retained.