Causes for recent fluid venting at the Cretaceous Henry Seamount, Canary Archipelago

在全球范围内，通过老化海洋地壳的海水热液循环对岩石圈降温、地壳与海洋之间的化学交换以及海洋生态系统具有重要意义。在沉积物覆盖的旧地壳中，有效的流体循环需要基底露头，如海底山，它们在不透水的沉积物层中形成通道。目前发现的与活火山活动无关的喷发海山不多；它们位于温暖且相对年轻的太平洋地壳上。古地壳上热液活动海山的第一个例子可能是加那利群岛耶罗岛附近的亨利海山。在R/V METEOR巡航M66/1期间的一次侦察疏浚活动中，尽管该大厦的年龄可追溯到1.26亿年，但所获得的样本表明，该海底山存在全新世流体渗漏。为了发现、记录和采样渗漏点，2018年R/V METEOR巡航M146期间对亨利海山进行了详细调查。研究发现，海山的大片区域覆盖着贝壳，这证明了相对较近的广泛渗漏。然而，伴随的热流测量没有显示出相应的异常。用重力覆盖法和抓斗法对海底进行的局部取样，除其他外，还发现了许多贝壳，以及出乎意料的新鲜玄武岩灰烬和石钻，这明确表明亨利海山最近有火山活动。在该项目中，我们将采用岩石地球化学方法对玄武岩样品进行调查，确定火山岩和壳的年龄，并评估M146号巡航的水声和电视雪橇调查数据。目的是了解亨利海山推断流体渗流的原因、程度和后果，并总体上提高对很少喷发的海底板内火山的热液系统的了解。

Hydrothermal circulation of seawater through ageing oceanic crust is globally important for cooling of the lithosphere, chemical exchange between crust and ocean, and marine ecosystems. At sediment-covered old crust, efficient fluid circulation requires basement outcrops, such as seamounts, that form pathways through the impermeable sediment blanket. Only few venting seamounts that are not related to active volcanism have been found so far; these are located on warm and relatively young Pacific crust. The probably first example for an hydrothermally active seamount on old crust is Henry Seamount near El Hierro, Canary Islands. A reconnaissance dredging campaign during R/V METEOR cruise M66/1 yielded samples that indicate Holocene fluid seepage at this seamount, even though the edifice was age dated to 126 million years. In order to discover, document and sample the seepage sites, Henry Seamount was investigated in detail during R/V METEOR cruise M146 in 2018. It was found that large areas of the seamount are covered with shells from vesicomyid clams, which documents relatively recent widespread seepage. Accompanying heat flow measurements, however, showed no respective anomalies. Local sampling of the seafloor by gravity corer and grab yielded, among others, numerous shells and unexpectedly fresh basaltic ash and lapilli, which unequivocally indicates recent volcanic activity of Henry Seamount. In the proposed project we will investigate the basaltic samples with petrological-geochemical methods, determine the ages of the volcanic rocks as well as of the shells, and evaluate hydroacoustic and TV-sled survey data from cruise M146. The aim is to understand causes, extent, and consequences of the inferred fluid seepage at Henry Seamount, and generally an improved understanding of hydrothermal systems at submarine intraplate volcanoes that erupt only rarely.