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

通过对西南印度洋脊大西洋Ⅱ断裂带东斜坡地幔和地壳岩石的岩石学和地球化学研究，探讨了超低速扩张脊洋壳岩石圈的结构和演化。根据岩石学检查，主要，微量和稀土元素和Sr-Nd同位素分析的全岩和矿物，它的结论是辉长岩是非常相似的ODP孔735 B钻在亚特兰蒂斯银行，一个阶地上的Atlantis II断裂带的东横脊的顶部恢复。橄榄岩样品，我们检查，也非常类似于周围疏浚这一地区，是残留地幔橄榄岩与低程度的肥沃地幔橄榄岩的签名。通过对西南印度洋海岭、大西洋中脊和东太平洋海隆辉长岩的对比，发现扩张较慢的海岭辉长岩和某些组分矿物的Na/Ca和/或Na值高于扩张较快的海岭，支持扩张速率与扩张海岭下地幔熔融程度之间的密切相关性。在这一地区，原始堆晶至今尚未观察到，这种“缺失堆晶”的问题已被讨论的地壳和/或软流圈岩浆过程下的超低速扩张脊。在研究过程中，我们发现了一个来自于粗粒岩的原始粗粒岩包裹体，填补了原始堆晶岩的“空白”。这一发现提供了证据，原始地幔熔融在段端部的超低速扩张脊。原始岩石的出现为我们提供了一个非常重要的洞察洋中扩张脊下地幔流动系统，因为非常原始的岩石直接指示地幔熔体提取从上涌软流圈流的位置。

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.