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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Two mantle plumes beneath the East African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts
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Two mantle plumes beneath the East African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts

机译：东非裂谷系统下有两个地幔柱：肯尼亚裂谷玄武岩中的Sr，Nd和Pb同位素证据

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摘要

Major and trace element and radiogenic isotope ratios (Sr, Nd and Pb) are presented for a suite of Neogene to Recent basalts (MgO > wt%) from the axial regions of the Kenya Rift. Samples have compositions ranging from hypersthenenormative basalt through alkali basalt to basanite and are a subset of a larger database in which compositions extend to nephelinite. A broadly negative correlation between Zr/Nb (<2-7) and Ce/Y (1-8) indicates derivation from a garnet-bearing mantle source region as a result of <3% melting. Isotope ratios in basalts from the axial regions of the Kenya Rift have ~(143)Nd/~(144)Nd = 0.51300 to 0.51255, ~(87)Sr/~(86)Sr = 0.7030 to 0.7055 and ~(206)Pb/~(204)Pb < 18 to > 20, broadly similar to values from OIB. The Kenya Rift cuts through basement of different ages and aspects of the composition of mafic magmas reflect the anisotropy of the underlying lithosphere. Specifically, those basalts from that part of the rift underlain by the Tanzanian craton (TC) have higher Ce/Y and lower Zr/Nb ratios than those erupted through the Panafrican Mozambique belt (MB) implying an origin either at greater depth of from a more trace element-enriched source region. Samples erupted through the zone of reactivated craton margin (RCM) share the characteristics of mafic lavas from both the craton and the mobile belt. Mb samples have ~(143)Nd/~(144)Nd = 0.5130 - 0.5127, ~(87)Sr/~(86)Sr = 0.7030 - 0.7035 and ~(206)Pb/~(204)Pb = 18.3 - 19.8, defining a steep negative trend on the Nd-Sr diagram and plotting close to the NHRL on conventional Pb isotope diagrams. By contrast TC and RCM samples have ~(143)Nd/~(144)Nd = 0.5124 - 0.51275, ~(87)Sr/~(86)Sr = 0.7035 - 0.7056 and ~(206)Pb/~(204)Pb = 17.6 - 21.2, defining flat-lying arrays on Nd-Sr plots plots and a much greater scatter and spread on Pb isotope diagrams, with many analyses plotting above the NHRL. Both groups of analyses trend towards a common end member on a plot of ~(143)Nd/~(144)Nd against ~(87)Sr/~(86)Sr, at ~(143)Nd/~(144)Nd ～0.51275 and ~(87)Sr/~(86)Sr ～0.7035. These values are suggested to reflect the isotopic characteristics of the sub-lithospheric Kenyan mantle, inferred to be the Kenya mantle plume. Comparison with data from Afar suggest that the Kenya plume is distinct from the Afar plume, implying that the east African Rift is underlain by at least two distinct mantle plumes. Eocene and Oligocene basalts from southern Ethiopia bear a closer resemblance to the Kenyan basalts than to those from Afar and
机译：列出了肯尼亚裂谷轴向区域的一套新近纪玄武岩（MgO> wt％）的主要元素和微量元素以及放射性同位素比（Sr，Nd和Pb）。样品的成分范围从超高规范玄武岩到碱玄武岩再到玄武岩，并且是更大数据库的子集，其中成分扩展到霞石。 Zr / Nb（<2-7）和Ce / Y（1-8）之间存在广泛的负相关性，表明由于<3％的熔化而从石榴石地幔源区衍生而来。肯尼亚大裂谷轴向区域玄武岩的同位素比为〜（143）Nd /〜（144）Nd = 0.51300至0.51255，〜（87）Sr /〜（86）Sr = 0.7030至0.7055和〜（206）Pb /〜（204）Pb <18至> 20，大致类似于OIB的值。肯尼亚大裂谷贯穿不同年龄的地下室，镁铁质岩浆成分的各个方面反映了下伏岩石圈的各向异性。具体而言，坦桑尼亚克拉通（TC）所裂陷的那部分玄武岩的Ce / Y比和Zr / Nb比都比通过Panafrican莫桑比克地带（MB）喷出的玄武岩高，这意味着该矿床的深度可能大于更多的痕量元素丰富的源区。通过再活化克拉通边缘（RCM）区域喷出的样品具有克拉通和移动带镁铁质熔岩的特征。 Mb样品的〜（143）Nd /〜（144）Nd = 0.5130-0.5127，〜（87）Sr /〜（86）Sr = 0.7030-0.7035和〜（206）Pb /〜（204）Pb = 18.3-19.8 ，在Nd-Sr图上定义了陡峭的负趋势，并在常规Pb同位素图上绘制了接近NHRL的图。相比之下，TC和RCM样本的〜（143）Nd /〜（144）Nd = 0.5124-0.51275，〜（87）Sr /〜（86）Sr = 0.7035-0.7056和〜（206）Pb /〜（204）Pb = 17.6-21.2，在Nd-Sr图上定义了平坦的阵列，在Pb同位素图上定义了更大的散布和散布，其中许多分析都在NHRL上方绘制。两组分析趋于在〜（143）Nd /〜（144）Nd处〜（143）Nd /〜（144）Nd对〜（87）Sr /〜（86）Sr的图上趋于一个公共末端成员。 〜0.51275和〜（87）Sr /〜（86）Sr〜0.7035。这些值被认为反映了岩石圈以下的肯尼亚地幔的同位素特征，据推测是肯尼亚地幔柱。与来自Afar的数据的比较表明，肯尼亚的羽流与Afar的羽流是不同的，这意味着东非大裂谷至少有两个独特的地幔羽。埃塞俄比亚南部的始新世和渐新世玄武岩比肯尼亚的玄武岩与阿法尔和玄武岩的玄武岩更相似。

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