Petrology and geochemistry of the oceanic crust observed at the Atlantis Bank, Southwest Indian Ridge

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Petrological and geochemical researches on the mantle and crustal rocks sampled from the eastern slope of the Atlantic II Fracture Zone, Southwest Indian Ridge, were conducted for discussion on the architecture and evolution of oceanic lithosphere at ultraslow spreading ridges. Based on petrographic examinations, major, trace and rare-earth elements and Sr-Nd isotopic analyses of the whole-rocks and minerals, it is concluded that the gabbroic rocks are very similar to those recovered from ODP Hole 735B drilled at top of the Atlantis Bank, a terrace on the eastern transverse ridge of the Atlantis II Fracture Zone. Peridotite samples, we examined, are also very similar to those dredged around this area and are residual mantle peridotites with a signature of low degree of fertile mantle peridotite. Comparison of gabbroic rocks from Southwest Indian Ridge, Mid-Atlantic Ridge, and East Pacific Rise was done, and it is shown that gabbroic rocks and some constituent minerals from the slower spreading ridges are higher in Na/Ca and/or Na than those from the faster ones, supporting that the intimate correlation between the spreading rate and degree of mantle melting beneath spreading ridges. In this area, primitive cumulates have not been observed until now, and this "missing cumulate" issue has been discussed in relation to crustal and/or asthenospheric magmatic processes beneath ultraslow-spreading ridges. During this study, we found a primitive troctolite inclusion from dolerites to fill the "gap" of primitive cumulates. This finding provides evidence for primary mantle melt at the segment end on ultraslow spreading ridges. Occurrence of primitive rocks provides us a very important insight into mantle flow system beneath mid-ocean spreading ridges, because very primitive rocks directly indicate a location of mantle melt extraction from upwelling asthenospheric flow.