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

对西南印度洋海脊大西洋II断裂带东坡的地幔和地壳岩石进行了岩石学和地球化学研究，以探讨超低扩张海脊大洋岩石圈的结构和演化。根据岩相学、全岩和矿物的常量、微量、稀土元素和锶-钕同位素分析，这些辉长岩与在亚特兰蒂斯海岸顶部钻探的ODP735B孔中发现的辉长岩非常相似。我们检验的橄榄岩样品也与在该地区挖掘的样品非常相似，是残留的地幔橄榄岩，具有低可育地幔橄榄岩的特征。对西南印度洋海脊、中大西洋海脊和东太平洋海隆的辉长岩进行了对比，结果表明，较慢扩张海脊的辉长岩和某些成分矿物的Na/Ca和/或Na高于较快扩张海脊的辉长岩，支持了扩张海脊下地幔熔融程度与扩张速率之间的密切关系。在这一地区，直到现在还没有观测到原始的堆积物，这种“丢失的堆积物”问题已经与超低扩张脊之下的地壳和(或)软流圈岩浆作用有关。在这项研究中，我们从榴辉岩中发现了原始沸石包裹体，以填补原始堆积体的“缺口”。这一发现为超低扩张脊段端部的原生地幔熔融提供了证据。原始岩石的存在为我们提供了对大洋中部扩张海脊之下的地幔流动系统的非常重要的认识，因为非常原始的岩石直接指示了从上涌软流圈流动中提取地幔熔体的位置。

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.