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

拟进行的项目将描述大西洋中脊（MAR）在~5-8°N范围内的地壳和地幔岩石的地球化学和同位素变化特征。目的是更好地了解海洋岩石圈形成的机制。在区域尺度（100公里）上，脊状岩浆活动可能受到羽状脊相互作用（100公里）的影响，但在更局部尺度（10-1公里）上，由于海洋（巨型）转换带附近或内部复杂的地球动力学条件，脊状岩浆活动也会发生变化。在区域尺度上，我们将确定在~5-8°N海域中洋脊玄武岩（MORB）的地球化学同位素异常沿轴频率。新的同位素数据将限制山脊岩浆活动是否受到附近热点的影响——这可能形成了附近的塞拉利昂隆起和深海海底山链——或者横向非均质地幔的融化是否导致了该地区MORB的地球化学和同位素变化。在更局部的尺度上，在~7-8°N的低潮大转换带（MTZ）附近，MORB化学变化将有助于评估地幔熔融和岩浆演化过程如何响应冷却，这是由于“山脊转换交叉点”（RTI）附近的“冷边效应”。变换域中复杂的地球动力学过程在Doldrums MTZ内产生了四个“变换内脊”（ITR）段。新的水深和地球物理资料表明，外围ITR形成于以岩浆生产速率低为特征的构造活动区。相比之下，尽管形成于局部地幔热极小期，但两个中位热热带的特征是更强烈的地幔熔融和岩浆增生。来自所有四个ITR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中部异常肥沃的地幔融化来调和。此外，比较西RTI地幔岩石（橄榄岩）和伴生玄武岩地球化学同位素组成的难得机会，将为邻近ITR的地幔组成和熔体生成提供补充视角，该ITR可能是在近期MAR轴向东跳跃之后形成的。在脊轴重新定位之前，在脊下融化的地幔可能会向东迁移，并可能在新形成的ITR下再次融化。如果是这样，那么暴露的地幔岩石和伴生玄武岩的地球化学和同位素特征应该显示出先前熔体枯竭的证据。总的来说，我们的研究将在区域尺度上为不同脊段之间的不同地球动力条件和地幔非均质性的响应提供新的约束，在局部尺度上为海洋（巨型）转换带附近或内部的脊岩浆活动提供新的约束。

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.