Physical, geochemical and magnetic properties of several ODP Leg 192 sites, Ontong Java Plateau

In this manuscript, we present the results of a physical properties investigation carried out on basaltic cores recovered from the four Leg 192 basement sites, focusing on the relationship between physical properties and alteration in basalts. Variations in physical properties in the Leg 192 basement sites closely resemble each other and reflect the amount of alteration and vein formation in the basement basalts. P-wave velocities, magnetic susceptibilities, and densities for the dense massive basalts are higher than those of more altered and heavily veined basalts. Porosity-dependent alteration is observed at Leg 192 basement sites: P-wave velocity displays a general decrease with increasing loss on ignition and potassium content. These trends are consistent with trends documented for typical alteration of oceanic crust and suggest that basalt alteration is largely responsible for the variation of the physical properties exhibited by rocks at Leg 192 basement sites. Our physical property data support the conclusion that only low-temperature seawater-mediated alteration occurred in the lava flows of the Ontong Java Plateau (OJP). This lack of higher-temperature hydrothermal alteration is consistent with the idea that the OJP basement sites are far from their eruptive vents.

本研究稿件报道了对大洋钻探Leg 192航次4个基底站点获取的玄武岩岩芯开展的物理性质调查结果，重点聚焦玄武岩物理性质与蚀变作用之间的关联。Leg 192航次基底站点的物理性质变化特征高度相似，可反映基底玄武岩的蚀变程度与脉体发育规模。致密块状玄武岩的纵波速度、磁化率及密度均高于蚀变程度更强、脉体更发育的玄武岩。Leg 192航次基底站点可见孔隙度相关的蚀变现象：纵波速度随烧失量与钾含量的升高整体呈下降趋势。该变化趋势与典型洋壳蚀变的已有研究结果一致，表明玄武岩蚀变是Leg 192航次基底站点岩石物理性质差异的主要成因。本次获取的物理性质数据支持以下结论：翁通爪哇高原（Ontong Java Plateau, OJP）的熔岩流中仅发生了低温海水介导蚀变。这种缺乏高温热液蚀变的特征，与翁通爪哇高原基底站点远离其喷发口的观点相符。