Geochemistry of ODP Hole 111-504B basalts (Table 2)

Fifty samples of basalt recovered during ODP Leg 111 from the dikes (Layer 2C) of Hole 504B (1350.0-1562.3 m below seafloor) were analyzed by X-ray-fluorescence techniques. All of the samples are highly depleted in magmaphile elements relative to other mid-ocean ridge basalts, with TiO2 = 0.75-1.24 wt%, Na2O = 1.59-2.22 wt%, Zr = 38-64 ppm, Nb = 0.3-1.5 ppm, and Y = 20-30 ppm (for samples containing 0%-2% phenocrysts), but have ratios of highly incompatible elements similar to normal Type I mid-ocean ridge basalts (e.g., Zr/Nb > 30). Abundances of compatible elements are similar to those of typical mid-ocean ridge basalts, with MgO = 7.2-9.2 wt%, Fe2O3* = 9.3-12.5 wt%, Ni = 55-164 ppm, and Cr = 26-388 ppm. Approximately 2% of the samples recovered from the top part of Hole 504B are similar to normal Type I or Type II ocean floor basalts. However, all of the analyzed Leg 111 samples from Hole 504B are depleted basalts. Aphyric dike rocks from Leg 111 are virtually identical to the depleted aphyric samples recovered from the pillow lavas and dikes in the upper 1075 m of Hole 504B during DSDP Legs 69, 70, and 83, with the exception of elements readily altered by seawater (Sr, Rb, and K). These elements reach a maximum in both abundance and variability in the pillow lavas of the upper 571.5 m of Hole 504B and decline to more constant values in the dike system sampled on Legs 83 and 111, apparently as a result of a decrease in porosity and increase in alteration temperatures relative to the pillow lavas. Based on compositional similarities to the vast majority of the pillows and flows, the dikes sampled on Leg 111 appear to be the feeder system for the pillow lavas in the upper part of Hole 504B. The incompatible-element-depleted compositions of the Costa Rica Rift Zone basalts are consistent with multistage melting of a normal mid-ocean ridge source.

本研究采用X射线荧光（X-ray fluorescence, XRF）技术，对大洋钻探计划（Ocean Drilling Program, ODP）第111航次从504B钻孔（海底以下1350.0~1562.3 m）的2C层（Layer 2C）岩脉中采集的50件玄武岩样品开展了分析测试。所有样品相较于其他洋中脊玄武岩（mid-ocean ridge basalts, MORB）均表现出显著的亲岩浆元素（magmaphile elements）亏损特征，其TiO₂含量介于0.75~1.24重量百分比（wt%）之间，Na₂O含量为1.59~2.22 wt%，Zr含量为38~64百万分比（ppm），Nb含量为0.3~1.5 ppm，Y含量为20~30 ppm（针对斑晶（phenocrysts）含量0%~2%的样品）；但其高不相容元素（highly incompatible elements）比值与正常I型洋中脊玄武岩（Type I mid-ocean ridge basalts）相近，例如Zr/Nb＞30。相容元素（compatible elements）的丰度则与典型洋中脊玄武岩基本一致，其中MgO含量为7.2~9.2 wt%，全铁（以Fe₂O₃*计）含量为9.3~12.5 wt%，Ni含量为55~164 ppm，Cr含量为26~388 ppm。从504B钻孔上部采集的样品中，约2%的样品与正常I型或II型洋底玄武岩（ocean floor basalts）特征相似，但本次分析的所有504B钻孔第111航次样品均为亏损型玄武岩。第111航次采集的无斑岩脉岩（aphyric dike rocks），与深海钻探计划（Deep Sea Drilling Program, DSDP）第69、70、83航次从504B钻孔上部1075 m范围内的枕状熔岩（pillow lavas）及岩脉中获取的无斑亏损型样品几乎完全一致，仅在易受海水蚀变的元素（Sr、Rb及K）上存在差异。上述易蚀变元素在504B钻孔上部571.5 m范围内的枕状熔岩中，其丰度与变化幅度均达到峰值；而在第83、111航次采样的岩脉体系中，该类元素的丰度逐渐趋于稳定，这一现象显然与枕状熔岩相比，岩脉的孔隙度（porosity）降低、蚀变温度升高有关。基于与绝大多数枕状熔岩及熔岩流的成分相似性，第111航次采样的岩脉应为504B钻孔上部枕状熔岩的补给系统。哥斯达黎加裂谷带（Costa Rica Rift Zone）玄武岩的高不相容元素亏损成分特征，符合正常洋中脊源区的多阶段熔融（multistage melting）模型。