Variable oceanic lithosphere-forming processes in the vicinity of the Doldrums mega-transform zone, Mid Atlantic Ridge

拟议的项目将描述大西洋中脊（MAR）~5-8°N的一个断面沿着地壳和地幔岩石的地球化学和同位素变异性。其目的是更好地了解海洋岩石圈形成的机制。在区域尺度上（100 s km），洋脊岩浆活动可能受到地幔柱-洋脊相互作用（100 s km）的影响，但在更局部的尺度上（10-1 km），洋脊岩浆活动也会因大洋（巨型）转换带附近或内部复杂的地球动力学条件而变化。在区域尺度上，我们将确定5-8°N的MAR洋中脊玄武岩（MORB）中地球化学同位素异常的沿轴频率。新的同位素数据将限制，如果脊岩浆活动的影响，附近的热点-这可能已经形成了附近的塞拉利昂隆起和Bathymetrist海山链-或如果熔化的横向异质地幔是负责该地区的MORB的地球化学和同位素变异。在更局部的尺度上，在7-8°N处接近低潮巨型转换区（MTZ）的MORB化学变化将允许评估地幔熔融和岩浆演化过程如何响应于由于“脊转换交叉点”（RTI）附近的“冷边效应”而导致的冷却。复杂的地球动力学过程中的变换域生成四个“内变换岭”（ITR）段内的低潮MTZ。新的测深和地球物理数据表明，外围ITR形成的构造活动区的特点是低岩浆生产率。相比之下，更强烈的地幔熔融和岩浆吸积的特点，两个中位数ITR，尽管是在当地的地幔热极小形成。来自ITR所有四个区段的MORB的首次同位素数据将表明，这种明显的冲突是否可以通过在Doldrums MTZ中间部分熔化异常肥沃的地幔来调和。此外，难得的机会，比较地幔岩石（橄榄岩）和相关的玄武岩从西部RTI的地球化学同位素组成将提供一个互补的角度地幔成分和熔融生成在邻近的ITR，这可能是最近的MAR轴向东跳后形成的。在洋脊轴迁移之前已经在洋脊下熔融的地幔将从那时起向东迁移，并可能在新形成的ITR下第二次熔融。如果是这样的话，暴露的地幔岩和相关的玄武岩的地球化学和同位素特征应该显示出以前的熔体消耗的证据。总体而言，因此，我们的研究将提供新的限制如何脊岩浆反应不同的地球动力学条件和地幔异质性之间的不同脊段在区域尺度上，和附近或在海洋（兆）转换区在局部范围内。

The proposed project will characterize the geochemical and isotopic variability of crustal and mantle rocks along a transect of the Mid Atlantic Ridge (MAR) from ~5-8°N. The aim is to better understand the mechanisms of oceanic lithosphere formation. On a regional scale (100s km) ridge magmatism may be influenced by plume-ridge interaction (100s km), but also varies on a more local scale (10-1 km) in response to the complex geodynamic conditions near or within oceanic (mega-) transform zones. On a regional scale, we will determine the along-axis frequency of geochemical-isotopic anomalies in Mid-Ocean Ridge Basalts (MORB) of the MAR from ~5-8°N. The new isotope data will constrain if ridge magmatism is influenced by nearby hotspots –which may have formed the nearby Sierra Leone Rise and Bathymetrist seamount chain– or if melting of laterally heterogeneous mantle is responsible for the geochemical and isotopic variability of MORB in the area. On a more local scale, changes in MORB chemistry approaching the Doldrums Mega-Transform Zone (MTZ) at ~7-8°N will allow evaluating how mantle melting and magma evolution processes respond to cooling due to the ‘cold-edge effect’ near ‘Ridge Transform Intersections’ (RTI). The complex geodynamic processes within the transform domain generated four ‘Intra-Transform Ridge’ (ITR) segments within the Doldrums MTZ. New bathymetric and geophysical data suggest that the peripheral ITR form in tectonically active areas characterized by low magma production rates. In contrast, more intense mantle melting and magmatic accretion characterize the two median ITR, despite being formed over a local mantle thermal minimum. The first-ever isotopic data of MORB from all four ITR segments will show if this apparent conflict can be reconciled by melting of anomalously fertile mantle in the median part of the Doldrums MTZ. Moreover, the rare opportunity to compare the geochemical-isotopic composition of mantle rocks (peridotites) and associated basalts from the western RTI will provide a complementary perspective on mantle composition and melt generation at the adjacent ITR, which probably formed after a recent eastward jump of the MAR axis. Mantle that has melted under the ridge before the ridge axis relocation would have migrated eastwards since, and may melt a second time under the newly formed ITR. If so, the geochemical and isotopic characteristics of the exposed mantle rocks and the associated basalts should show evidence for previous melt depletion. Overall, therefore, our study will provide novel constraints on how ridge magmatism responds to the different geodynamic conditions and mantle heterogeneity between different ridge segments on a regional scale, and near or within oceanic (mega-) transform zones on a local scale.