大西洋中脊玄武岩岩浆源区性质、潜在温度及其指示意义

洋中脊玄武岩（MORB）的微量元素成分和同位素比值具有变化范围大的特点，这些变化很难简单地用地幔部分熔融和结晶分异等岩浆演化过程来解释。传统观点认为洋中脊玄武岩的地球化学成分的多样性是由其下部地幔成分的大尺度不均一性决定的。这种地幔不均一性则是外来物质的加入造成的，如再循环的地壳物质、下大陆岩石圈、交代的岩石圈和外地核等成分加入到上地幔中。在本研究中，我们对大西洋洋中脊的玄武岩展开研究工作，评估了玄武岩源区的温压条件并综合对比了微量元素和同位素比值。靠近地幔柱的洋中脊玄武岩的地球化学和同位素成分具有较大的变化。地幔柱对洋中脊地区的影响范围可以达到1400公里，但并不是每个地幔柱都能够影响其周围1400km范围内的所有洋中脊脊段。未受地幔柱影响的洋中脊玄武岩成分和地幔潜在温度均没有异常表现。我们认为上述现象是由于地幔柱柱头形状不同造成的。地幔柱的流动形状可以分为管状和饼状两种，饼状地幔柱影响其周围的地幔是没有方向性的，而管状地幔柱对其周围地幔的影响在方向上具有选择性。沿着大西洋中脊的玄武岩的元素和同位素比值变化较大，暗示其源区具有较高的不均一性。我们认为该地区地幔不均一性主要是由于上地幔中加入了俯冲板片和拆沉下地壳造成的。另外，地幔柱的活动也不容忽视，它们影响了其周围部分洋脊段的成分变化。

Implications of the melting depth and temperature of the Atlantic mid-ocean ridge basalts

Mid-ocean ridge basalts(MORBs) are characterized by large variations in trace element compositions and isotopic ratios, which are difficult to be interpreted solely by using magmatic process such as partial melting of a peridotitic mantle and subsequently fractional crystallization. Geochemical diversity of MORBs have been attributed to large-scale heterogeneity within the underlying mantle, and the heterogeneity might have been caused by addition of recycled crustal component, subcontinental lithosphere, metasomatized lithosphere and outer core contribution. In this study, we investigated the MORBs along the Mid-Atlantic Ridge(MAR) by estimating the temperature and pressure of partial melting, and comprehensively comparing trace element and isotope ratios. The data for MORBs from areas close to mantle plumes show large variations. Mantle plumes can affect mid-oceanic ridges 1 400 km away, but plume effects did not cover all of the ridge segments, and those segments without plume effects did not have any abnormalities in temperature, trace element or isotope ratios.We ascribed the above phenomena to result from the shapes of the plume flow, which we categorized as "pipelike channels" and "pancake-like channels". The "pancake-like channels" plumes affected the ambient mantle nondirectionally, but the range of the mantle affected by the "pipe-like channels" plumes were selective. Element ratios of MORBs reveal that the mantle source of the MORBs along the MAR is highly heterogeneous. We suggest that most of source heterogeneities of the MORBs may be due to the presence of subducted slab and delaminated lower crust in the source. In addition, the plume that carried materials from the core-mantle boundary may affect some of the segments.