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。该系统涵盖诸多形成时代与地球动力学背景均存在差异的地质省。伴随大洋岩石圈形成过程的火成活动，可作为指示不同区域特有演化进程的有效标志。大西洋域内规模宏大的西南印度洋脊，其极端西端——斯皮斯海岭（Spiess Ridge），被归属于俄罗斯文献中所称的相对年轻的非洲-南极脊。基于磁法勘探成果，该区域扩张作用的起始时间距今不足2百万年（Ma）。在此研究背景下，探究新生且处于发育阶段的扩张脊的形成条件与火成活动特征，具备特殊的科学价值。斯皮斯海岭形成于特定地质环境：其新生裂谷带发育于此前在大西洋中脊（Mid-Atlantic Ridge, MAR）区域形成的洋壳之上。斯皮斯海岭区域复杂的演化动力学过程，造就了相对高度约2 km、最小水深达320 m的新型中央型火山隆起，以及直径可达4 km的破火山口。海岭东西两侧坡体发育有一系列小型次级火山锥。对该海岭产出的火成岩的初步研究显示，其属于富集亲石元素的拉斑玄武岩。该元素富集的成因，与布韦岛（Bouvet Island）下方富集地幔柱的抬升活动密切相关。本研究旨在重建该海岭下方的岩浆形成条件，验证中深度岩浆房的潜在存在性，并估算其可能的规模。相关信息对构建三联点区域大洋岩石圈的演化模型具有重要的学术意义。