海嶺下のマグマチェンバーモデル

Magma reservoir models beneath ocean ridges are reviewed with reference to the spreading rates and the mode of fractional crystallization. Faster spreading rates (5cm/year) cause a large and long-lived magma chamber in which a thick plutonic sequence would be succesively produced away laterally from the chamber. Basalts extruded at fastspreading ridges underwent various degrees of low pressure fractionation causing conspicious chemical zonation in the chambers. On the other hand, small and transient magma chambers may be present beneath slow-spreading ridges. Comparatively less-evolved basalts, where petrological characteristics are governed mostly by the deep-seated processes such as degree of partial melting and polybaric fractionation, could appear at slow-spreading ridges because of absence of a large crustal magma chamber.<BR>Since sheeted dikes in ophiolites intruded vertically and the foliation of mantle tectonites were approximately horizontal in original, the configuration of the magma chamber interlayered between the tectonites and sheeted dike complex, if it existed, can be infered from the structure of a cumulate-gabbro sequence. The appearance of a thick plutonic sequence in the ophiolite where the crystallization order is olivine-plagioclase-clinopyroxene-orthopyroxene suggests that it was generated at a fast-spreading ridge. In such thick plutonic sequence, the fine-scale igneous layering (=time plane) obliquely across the major lithologic boundary of the plutonic sequence may reflect the horizontal chemical zonation of the magma chamber. On the other hand, the appearance of abundant whelritic cumulates crystallized at mantle deths and suffered high temperature deformation suggests that the ophiolite was generated at a slow-spreding ridge.<BR>Spreading rates of ancient ocean ridges at which the ophiolites were generated may be roughly estimated from the considerations on geochemical variation of the basaltic rocks. The occurrence of evolved basalts interpreted by extensive low pressure fractional crystallization suggests fast spreading rates, whereas the appearance of less evolved basalts showing complicated geochemical features such as different incompatible element ratios and crossing REE pattern suggests rather slow spreading rates.<BR>The studies on ophiolites would bring significant informations not only on the dynamic process beneath present-day ocean ridges but also on the tectonic framework and the evolution of ancient spreading center.