海洋地殻の構成と火成活動

Abstract

Volumetrically, magmatism at spreading axes occupies more than 60% of present day volcanism, and oceanic crust generated there covers more than two thirds of earth surface. Therefore, studies of the oceanic crust and magmatism at spreading axes yield significant insights to understanding earth evolution. Recent studies on oceanic crust through drilling, submersible surveys, dredging and geophysical exploration indicates that there is considerable variation in lithologic composition and stratigraphic sequences in oceanic crust that is dependent on the spreading rate. Typical ophiolitic stratigraphic sequences are expected to occur at fast-spreading ridges, while complicated and variable stratigraphic successions are encountered at slow-spreading ridges. Mid-Ocean Ridge Basalts (MORBs) are generally regarded as having comparatively uniform compositions. Major element compositions of MORB corrected for low-pressure fractionation show a global systematic variation correlated to axial highs, which can be ascribed to the degree of partial melting. However, the composition of MORB should represent a final product of magmas whose compositions have been greatly modified during ascent through mantle peridotite, by crustal fractionation that produced lower crust (ultramafic to gabbroic cumulates), and by mixing in a melt lens in shallow crust. Therefore, to fully understand MORB petrogenesis it is necessary to study the total magmatic system, which includes surface lavas, dike complexes, plutonic cumulates extracted from melts, and residual mantle. Recent results from the MARK area of the Mid-Atlantic Ridge, the Atlantis bank of the Southwest Indian Ridge and Hess Deep near the East Pacific Rise strongly suggest that source mantle is heterogeneous and that the magmatic system differs significantly depending on the spreading rate.