Oceanic crusts are formed by an integrated process that includes upwelling and partial melting of asthenospheric mantle, generation, segregation, emplacement, and crystallization of primitive basaltic magma, and subsequent metamorphism, alteration, and tectonic deformation. The transition from magmatic to metamorphic processes takes place in magma chambers beneath spreading ridges with decreasing temperature and with incorporation of hydrothermal fluids. It is widely accepted that the hydrothermal circulation of seawater results in heat flow anomalies, metamorphism, and the formation of metalliferous ore deposits. These processes are very important to understanding heat flux and geochemical cycling between oceanic lithosphere and hydrosphere (e.g., Gillis et al., 1993; Kelley et al., 1993; Mével and Cannat, 1991; Cannat et al., 1991; Pflumio, 1991; Schiffman et al., 1991; Manning et al., 1996; Gillis and Roberts, 1999; Talbi et al., 1999). On the other hand, the transition from magmatic to metamorphic/hydrothermal processes is likely to depend on the spreading rate of ridges where the spreading rate has an effect on the thermal regime of the spreading ridges. However, the transition in deep magma chambers has not been described in detail from various settings with different spreading rates. So it is one of the most important subjects to be examined in the long section of oceanic crust drilled in Hole 735B, Atlantis II Fracture Zone, on the Southwest Indian Ridge. Here, we describe a type of high-temperature microscopic vein from olivine gabbros. This type of vein provides a line of evidence for the earliest migration of fluids in the deep magma chambers in the lower oceanic crust of the slow-spreading Southwest Indian Ridge.