Major and rare earth element concentrations of basalts beneath ODP Hole 118-735B (Table 3)

A wide-angle seismic experiment at the Atlantis II Fracture Zone, Southwest Indian Ridge, together with geochemical analyses of dredged basalt glass samples from a site conjugate to Ocean Drilling Program hole 735B has allowed determination of the thickness and the most likely lithological composition of the crust beneath hole 735B. The measured Na, composition of 3.3 +/- 0.1 corresponds to a melt thickness of 3 +/- 1 km, a result consistent with rare earth element inversions which indicate a melt thickness of between 1.5 and 4.5 km. The seismic crustal thickness to the north and south of the Atlantis Platform (on which hole 735B is located) is 4 +/- 1 km, and probably consists largely of magmatic material since the seismic and inferred melt thicknesses agree within experimental uncertainty. Beneath hole 735B itself. the Moho is at a depth of 5 +/- 1 km beneath the seafloor. The seismic model suggests that, on average. about 1 km of upper crust has been unroofed on the Atlantis Platform. However, allowing for the inferred local unroofing of 2 km of upper crust at 735B, the base of the magmatic crust beneath this location is probably about 2 km beneath the seafloor, and is underlain by a 2-3 km thick layer of serpentinised mantle peridotite. The P-wave velocity of 6.9 km/s for the serpentinised peridotite layer corresponds to a 35 +/- 10 vol% serpentine content. The Moho beneath hole 735B probably represents a serpentinisation front.

针对西南印度洋中脊亚特兰蒂斯II断裂带开展的广角地震实验，结合对与大洋钻探计划（Ocean Drilling Program, ODP）735B井共轭站位拖网采集的玄武岩玻璃样品开展的地球化学分析，得以确定735B井下方地壳的厚度及其最可能的岩性组成。测得的钠（Na）成分为3.3±0.1，对应熔体厚度为3±1 km，该结果与稀土元素反演得到的1.5~4.5 km熔体厚度一致。亚特兰蒂斯平台（735B井所在区域）南北两侧的地震地壳厚度为4±1 km，由于地震反演得到的厚度与推断的熔体厚度在实验误差范围内相符，该地壳大概率以岩浆物质为主。就735B井下方而言，莫霍界面（Moho）位于海底以下5±1 km处。地震模型显示，亚特兰蒂斯平台的上地壳平均被剥露了约1 km。不过，若考虑到735B井处推断的2 km上地壳局部剥露量，则该位置下方的岩浆地壳基底大约位于海底以下2 km处，其下伏着一层厚度为2~3 km的蛇纹石化地幔橄榄岩。该蛇纹石化橄榄岩层的纵波速度（P-wave velocity）为6.9 km/s，对应蛇纹石体积占比为35±10%。735B井下方的莫霍界面可能即为蛇纹石化前锋面。