Chemical composition of basalts and basaltic glasses from the Sierra Leone Fracture Zone region

Major and trace element (including REE) geochemistry of basalts and chilled basaltic glasses from the MAR axial zone in the vicinity of the Sierra Leone FZ (5-7°10'N) has been studied. Associations of basalts of various compositions with particular ocean-floor geological structural features have been analyzed as well. Three basaltic varieties have been discriminated. Almost ubiquitous are high-Mg basalts (Variety 1) that are derivatives of N-MORB tholeiitic melts and that are produced in the axial zone of spreading. Variety 2 is alkaline basalts widespread on the southwestern flank of the MAR crest zone in the Sierra Leone region, likely generated through deep mantle melting under plume impact. Variety 3 is basalts derivative from T- and P-MORB-like tholeiitic melts and originating through addition of deeper mantle material to depleted upper mantle melts. Magma generation parameters, as calculated from chilled glass compositions, are different for depleted tholeiites (44-55 km, 1320-1370°C) and enriched tholeiites (45-78 km, 1330-1450°C). Mantle plume impact is shown to affect not only tholeiitic basalt compositions but also magma generation conditions in the axial spreading zone, resulting in higher Ti and Na concentrations in melts parental to rift-related basalts occurring near the plume. T- and P-MORBs are also developed near areas where mantle plumes are localized. High-Mg basalts are shown to come in several types with distinctive Ti and Na contents. Nearly every single MAR segment (bounded by sinistral strike slips and the Bogdanov Fracture Zone) is featured by its own basalt type suggesting that it has formed above an asthenospheric diapir with its unique magma generation conditions. These conditions are time variable. Likely causes of temporal and spatial instability of the mantle upwelling beneath this portion of the MAR are singular tectonic processes and plume activity. In sulfide-bearing rift morphostructures (so-called "Ore area'' and the Markov Basin), basalts make up highly evolved suites generated through olivine and plagioclase fractionation, which is suggestive of relatively long-lived magma chambers beneath the sulfide-bearing rift morphostructures. Functioning of these chambers is a combined effect of singular geodynamic regime and plume activity. In these chambers melts undergo deep differentiation leading to progressively increasing concentration of sulfide phase, eventually to be supplied to the hydrothermal plumbing system.

本数据集针对塞拉利昂断裂带（Sierra Leone Fracture Zone, FZ）附近（5°~7°10'N）大西洋中脊（Mid-Atlantic Ridge, MAR）轴带产出的玄武岩及淬火玄武岩玻璃开展了常量元素、微量元素及稀土元素（Rare Earth Elements, REE）地球化学研究，同时分析了不同成分玄武岩与特定海底地质构造特征的关联。研究共区分出三类玄武岩：分布最为普遍的是高镁玄武岩（类型1），其属于正常型大洋中脊拉斑玄武岩（Normal Mid-Ocean Ridge Basalt, N-MORB）熔体的衍生产物，形成于扩张轴带区域；类型2为碱性玄武岩，广泛分布于塞拉利昂区域大西洋中脊脊顶带的西南翼，大概率形成于地幔柱作用下的深部地幔熔融过程；类型3为源自过渡型（Transitional MORB, T-MORB）及地幔柱型（Plume-type MORB, P-MORB）大洋中脊拉斑玄武岩熔体的玄武岩，其成因是亏损型上地幔熔体混入了深部地幔物质。基于淬火玻璃成分计算得到的岩浆形成参数显示，亏损型拉斑玄武岩与富集型拉斑玄武岩的形成条件存在显著差异：前者熔融深度为44~55 km，温度区间为1320~1370℃；后者则为45~78 km，1330~1450℃。研究证实，地幔柱作用不仅会改变拉斑玄武岩的成分特征，还会影响扩张轴带的岩浆形成条件，使得裂谷相关玄武岩的母熔体中钛（Ti）、钠（Na）元素含量升高，这类玄武岩多分布于地幔柱附近区域。过渡型及地幔柱型大洋中脊玄武岩同样发育于地幔柱定位区域附近。高镁玄武岩可根据钛、钠元素含量划分为多种亚型。大西洋中脊的每一个分段（以左旋走滑断层及博格达诺夫断裂带（Bogdanov Fracture Zone）为边界）均具有独特的玄武岩类型，表明其形成于具备专属岩浆形成条件的软流圈底辟体之上，且该条件随时间发生变化。该区域大西洋中脊下方地幔上涌的时空不稳定性，可能源于单次构造事件及地幔柱活动。在含硫化物的裂谷地貌构造（即所谓"Ore area"及马尔可夫盆地）中，玄武岩构成了通过橄榄石及斜长石分异作用形成的高度演化岩石组合，这表明含硫化物的裂谷地貌构造下方存在寿命相对较长的岩浆房。这类岩浆房的活动是特殊地球动力学环境与地幔柱活动共同作用的结果，在岩浆房内熔体经历深度分异过程，硫化物相的含量逐渐升高，最终被输送至热液管道系统中。