Chemical and isotopic compositions of minerals from rocks of the Spiess Ridge

The world system of underwater rift-type spreading zones is a long-lived complex evolving system. It includes various provinces differing in age and geodynamics. Igneous activity, which accompanied formation of the oceanic lithosphere, can be used as an indicator of processes that controlled specific characters of different regions. The extreme western termination (Spiess Ridge) of the extensive South-West Indian Ridge in the Atlantic Ocean is referred to the African-Antarctic Ridge (called in the Russian literature) of relatively young age. Proceeding from results of magnetic surveys, the onset of spreading was dated less than 2 Ma. Of particular interest in this connection was to study conditions of formation and character of igneous activity under the newly formed and developing spreading zone. The Spiess Ridge was formed under specific conditions, where the new rift zone originated in the oceanic crust that had been formed earlier in the MAR region. Complex dynamics of the Spiess Ridge area evolution resulted in formation of a new central-type volcanic rise with relative height of about 2 km and minimal depth of 320 m, and a caldera up to 4 km in diameter. The eastern and western slopes of the Spiess Ridge are marked by a series of small secondary volcanic cones. The preliminary study of igneous rocks from this ridge ranked them as tholeiites enriched in lithophile elements. The rise of an enriched plume under the area of the Bouvet Island was a source of this enrichment. The aim of this study was to reconstruct conditions of magma formation under the ridge, to verify potential existence of an intermediate-depth magma chamber, and to estimate its potential size. This information is important for deriving a model for evolution of the oceanic lithosphere in the region of the triple junction.水下裂谷型扩张脊全球系统是一类长期演化的复杂系统，涵盖了不同年龄与地球动力学特征的各类地质构造省。伴随大洋岩石圈（oceanic lithosphere）形成的火成活动，可作为指示不同区域特定演化特征的关键指标。 大西洋海域内大型西南印度洋中脊（South-West Indian Ridge）的最西端（斯皮斯海岭（Spiess Ridge）），被归为俄罗斯文献中所称的相对年轻的非洲-南极海岭（African-Antarctic Ridge）。根据磁法勘探（magnetic surveys）结果，该区域的扩张作用起始年代距今不足2百万年（Ma）。就此而言，探究新生且处于发育阶段的扩张脊之下的火成活动形成条件与活动特征，具有重要的科学意义。 斯皮斯海岭形成于特殊的地质背景：其新生裂谷带起源于大西洋中脊（Mid-Atlantic Ridge, MAR）区域早期形成的洋壳。该区域演化过程中的复杂动力学作用，造就了相对高度约2千米、最小水深达320米的新型中央型火山隆起，以及直径可达4千米的破火山口（caldera）。斯皮斯海岭的东西两侧坡地发育有一系列小型次生火山锥。 对该海岭火成岩的初步研究将其归类为富亲石元素（lithophile elements）拉斑玄武岩（tholeiites），布韦岛（Bouvet Island）下方的富集地幔柱是该元素富集的源区。本研究旨在重建该海岭下方的岩浆形成条件，验证中深岩浆房（intermediate-depth magma chamber）的潜在存在性，并估算其可能的规模。上述信息对于构建该三联点（triple junction）区域的大洋岩石圈演化模型具有重要的科学价值。