On the basis of measurements of bulk rock and phenocryst chemical compositions, phenocryst content, and estimation of water content and magma temperature, the magma viscosity of 44 dikes from Oki‐Dozen and Tango, southwest Japan, and Ocros, Peruvian Andes, was estimated to examine a relationship between viscosity and dike width. The results show that magma viscosity increases with dike width: mafic magmas with viscosities of 101–102 Pa s form dikes l m wide, while felsic magmas with viscosities of 106–107 Pa s form dikes 100 m wide. This indicates that magma viscosity as well as magma driving pressure and host rock stiffness are important factors in explaining the variation of dike width. Because magma viscosity is a measure of internal friction, the magmatic force required to widen the fracture during hydrofracturing is proportional to viscosity. On the basis of the range of chemical composition of the studied samples, dike widths formed by highly viscous felsic magma should be of the order of 100 m maximum. Furthermore, the observed basaltic dike width variations from various tectonic settings indicate that the effective factor on driving pressure of dike magma may be not magma pressure but compressive stress. On the other hand, dikes derived from low‐viscosity basaltic magma and exceeding 100 m width, may be formed by extremely high magma pressure associated with flood basalt volcanism. This is supported by (1) the observation that a rifting event with decreased compressive stress was not always associated with the volcanism and (2) the inferred effusion rates for this type of volcanism are anomalously high, suggesting increased magma pressure. In addition, the active magma generation and the decreased host rock stiffness by locally anomalous thermal conditions like a hotspot phenomenon may assist flood basalt volcanism or emplacement of anomalously wide mafic dikes.

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In the Mariana forearc, horst and graben structures are well developed in the outer forearc basement, which is composed of both island arc and oceanic crust-mantle rocks. A zone of dome-shaped diapiric seamounts, which are composed mainly of serpentinized peridotites, formed on the basement in the outer forearc regions. Serpentine minerals in peridotites from both diapiric seamounts and basement are mostly chrysotile and/or lizardite. Antigorite, however, is rarely found in peridotites recovered from Conical, Big Blue, Celestial, and South Chamorro Seamounts. Antigorite-bearing peridotites always contain secondary iron-rich olivine and metamorphic clinopyroxene, and antigorite seems to coexist stably with them. Iron-rich secondary olivine (Fo 86-90 ) occurs as overgrowth on the rim or along the cleavage traces of primary olivine (Fo 90-92 ). The assemblage shows high-temperature conditions of serpentinization at ~450-550 °C, whereas chrysotile-and/ or lizardite-bearing assemblages occur at ~200-300 °C. In antigorite-bearing samples, chrysotile and/or lizardite veins both predating and postdating antigorite formation are recognized. This may refl ect a complex process of tectonic cycling of shallow mantle wedge serpentinized peridotites to depth and then back again to the surface.

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