Two mantle plumes beneath the east African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts.
Two mantle plumes beneath the east African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts.
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DOI：
10.1016/s0012-821x(00)00012-1
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发表时间：
2000-03
影响因子：
5.3
通讯作者：
N. Rogers;R. Macdonald;J. Fitton;R. George;Martin L. Smith;B. Barreiro
中科院分区：
文献类型：
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作者：
N. Rogers;R. Macdonald;J. Fitton;R. George;Martin L. Smith;B. Barreiro
本文给出了肯尼亚裂谷轴部一套新近纪至现代玄武岩（MgO> 4wt%）的主量元素、微量元素和放射性同位素比值（Sr、Nd和Pb）。样品的组成范围从紫苏辉石规范玄武岩通过碱性玄武岩，是一个更大的数据库，其中组合物延伸到霞石的子集。Zr/Nb（<2-7）和Ce/Y（1-8）之间的负相关关系表明，它们来自于含石榴子石的地幔源区，熔融程度<3%。肯尼亚裂谷轴部玄武岩的143 Nd/144 Nd =0.51300 ~ 0.51255，87 Sr/86 Sr =0.7030 ~ 0.7055，206 Pb/204 Pb = <18 to >20，与OIB的结果基本一致。肯尼亚裂谷贯穿不同时代的基底，镁铁质岩浆的成分反映了下伏岩石圈的各向异性。具体而言，这些玄武岩的裂谷下面的坦桑尼亚克拉通（TC）的一部分，具有较高的Ce/Y和较低的Zr/Nb比爆发通过泛非莫桑比克带（MB）意味着原产地更深或从一个更微量元素富集的源区。通过克拉通再活化边缘（RCM）喷发的样品具有克拉通和移动的带的基性熔岩的特征。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图上定义了一个陡峭的负趋势，在常规Pb同位素图上接近NHRL。TC和RCM样品的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分散和扩展，许多分析图都在NHRL之上。在143 Nd/144 Nd对87 Sr/86 Sr的图上，两组分析都倾向于一个共同的端员，143 Nd/144 Nd = 0.51275，87 Sr/86 Sr = 0.7035。建议这些值反映岩石圈下的肯尼亚地幔的同位素特征，推断是肯尼亚地幔柱。与来自阿法尔的数据比较表明，肯尼亚的地幔柱与阿法尔的地幔柱不同，这意味着东非大裂谷下面至少有两个不同的地幔柱。埃塞俄比亚南部的始新世和渐新世玄武岩与肯尼亚玄武岩比与阿法尔和埃塞俄比亚高原的玄武岩更相似，这表明肯尼亚地幔柱至少活跃了45 Ma。从埃塞俄比亚南部向南迁移的岩浆活动是一致的东北迁移的非洲板块在肯尼亚羽在过去的50我。
Major and trace element and radiogenic isotope ratios (Sr, Nd and Pb) are presented for a suite of Neogene to Recent basalts (MgO>4 wt%) from the axial regions of the Kenya Rift. Samples have compositions ranging from hypersthene-normative 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 have143Nd/144Nd=0.51300 to 0.51255,87Sr/86Sr=0.7030 to 0.7055 and206Pb/204Pb<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 or 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 have143Nd/144Nd=0.5130–0.5127,87Sr/86Sr=0.7030–0.7035 and206Pb/204Pb=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 have143Nd/144Nd=0.5124–0.51275,87Sr/86Sr=0.7035–0.7056 and206Pb/204Pb=17.6–21.2, defining flat-lying arrays on Nd–Sr 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 of143Nd/144Nd against87Sr/86Sr, at143Nd/144Nd∼0.51275 and87Sr/86Sr∼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 the Ethiopian plateau, suggesting that the Kenya plume has been active for at least 45 Ma. Migration of magmatism from southern Ethiopia southwards is consistent with the northeastward migration of the African plate over the Kenya plume over the past 50 My.