Downhole variations in grain sizes at DSDP/ODP Hole 504B

The maximum grain sizes of plagioclase and magnetite in the groundmass of the sheeted dike complex drilled at Hole 504B have been measured. Downhole variations through a 440-m-long section show a crude zig-zag pattern consisting of a gradual decrease or increase followed by an abrupt jump. The gradual decrease or increase in grain size extends over many lithologic units, and hence, does not reflect variations in grain size within a single dike. Such a zig-zag pattern is well explained by grain-size variations through multiple dikes. By using the observed inclination of sheeted dikes of 81° ± 2.5°, thickness of the multiple dikes varies from 0.7 to 8.5 m and averages to 4 ± 1 m. The average thickness of individual dikes forming multiple dikes is 0.8 m. We expect such multiple dikes to be formed during rifting events beneath mid-oceanic spreading ridges. If the average expansion at rifting episodes is twice as wide as the average width of the multiple dike units, the full spreading rate of 7.2 cm/yr of Cocos Ridge gives 112 ± 33 yr for a time interval of the rifting. A simple one-dimensional conductive cooling model is applied to solidification of multiple dikes. Numerical simulations show that the grain-size variations observed through the drill hole are more consistent with a model where a new injection of a dike occurs periodically with a constant time interval rather than one where the next dike intrudes just after the solidification of the previous one. Grain-size variations within simple dikes from Iritono, Japan, and those for Makaopuhi lava lake, Hawaii, show that square root of crystallization time is linearly correlated with the logarithm of plagioclase size. By using an empirically derived relationship between these two variables, the variations of plagioclase size through Hole 504B are directly compared with the calculated times for crystallization. Each rifting episode at the Costa Rica Rift lasts for several years, and periodic injection of a new dike occurs into the center of a previously solidified multiple dike at time intervals varying from 1 to 12 months.本研究对504B钻孔（Hole 504B）钻取的席状岩墙杂岩（sheeted dike complex）基质中的斜长石（plagioclase）与磁铁矿（magnetite）最大晶粒尺寸开展了测量工作。穿过440米长剖面的井下粒度变化呈现出近似锯齿状的分布模式：先经历逐渐增大或减小的渐变过程，随后出现突变式跃升。该渐变过程跨越多个岩石地层单元，因此并非反映单条岩墙内部的晶粒尺寸变化。此类锯齿状分布模式可通过多条岩墙组成的复合岩墙的晶粒尺寸变化得到合理解释。结合观测得到的席状岩墙倾角81°±2.5°，可推算出复合岩墙的厚度范围为0.7至8.5米，平均厚度为4±1米，构成复合岩墙的单条岩墙平均厚度为0.8米。我们推测这类复合岩墙形成于洋中脊扩张脊（mid-oceanic spreading ridges）下方的裂谷作用事件中。若裂谷事件的平均扩张宽度为复合岩墙单元平均宽度的两倍，那么科科斯海岭（Cocos Ridge）7.2 cm/yr的全扩张速率可推算出裂谷事件的时间间隔为112±33年。 我们采用一维传导冷却模型（conductive cooling model）模拟复合岩墙的凝固过程。数值模拟（numerical simulations）结果表明，本次钻孔观测到的晶粒尺寸变化，更符合“岩墙以恒定时间间隔周期性侵入”的模型，而非“前一条岩墙完全凝固后即刻侵入下一条”的模型。对日本入富野（Iritono）地区简单岩墙以及夏威夷马卡奥普希熔岩湖（Makaopuhi lava lake）的晶粒尺寸分析显示，结晶时间的平方根与斜长石尺寸的对数呈线性相关关系。借助这两个变量间的经验推导关系，我们可将504B钻孔的斜长石尺寸变化与计算得到的结晶时间直接进行对比。哥斯达黎加裂谷（Costa Rica Rift）的每次裂谷事件持续数年，且在已完全凝固的复合岩墙中心，新岩墙的周期性侵入时间间隔为1至12个月不等。