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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 >Geochemistry of a new enriched mantle type locality in the northern hemisphere: Implications for the origin of the EM-I source
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Geochemistry of a new enriched mantle type locality in the northern hemisphere: Implications for the origin of the EM-I source

机译：北半球新的富集地幔类型地区的地球化学：对EM-I来源的暗示

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摘要

Late Cretaceous (66.2 +/- 0.5 Ma amphibole and 66.7 +/- 0.2 Ma phlogopite Ar-40/Ar-39 ages) nephelinitic volcanic rocks from Godzilla Searnount in the eastern North Atlantic (34 degrees N latitude) have trace element and Sr-Nd-Pb-Hf-isotope compositions similar to the Enriched Mantle I (EM-I) endmember, except for their low (207)pb/Pb-204 relative to Pb-206/Pb-204 ratios (Pb-206/Pb-204(in) = 17.7, 207Pb/Pb-204(in) = 15.34) plotting below the Northern Hemisphere Reference Line on the uranogenic Pb isotope diagram. O isotope data on amphibole separates are mantle-like (delta O-18=5.6-5.8%omicron). Age and location of the isolated Godzilla Seamount, however, preclude it firom being derived from the Madeira or Canary hotspots, making a lower-mantle origin unlikely. Therefore we propose derivation from a shallow (lithospheric/asthenospheric) melting anomaly. As observed in mid-ocean-ridge and ocean-island basalts, there is a systematic decrease of Pb-207/Ph-204 ratios (and Delta 7/4) in the individual EM-I endmember type localities towards northern latitudes with Godzilla lying on the extension of this trend. This trend is mirrored in ultra-potassic volcanic rocks such as lamproites and kimberlites, which reflect the composition of enriched subcontinental lithospheric mantle. Therefore, a global pattern in Pb-207/Pb-204 ratios and Delta 7/4 is suggested. The geochemical composition of EM-I endmember type localities, including Godzilla lavas, and the enriched (DUPAL) anomaly in the southern hemisphere could reflect derivation from ancient, metasomatized subcontinental lithospheric mantle. We propose a two-stage model to explain the trace element and isotopic composition of the EM-I mantle endmember localities worldwide: 1) during the early history of the Earth, subcontinental lithosphere was metasomatized by melts from subducted slabs along convergent margins generating high mu (U-238/Pb-204) sources, and 2) as the Earth cooled, hydrous fluids replaced hydrous melts as the main slab component metasomatizing the subcontinental lithospheric mantle (generating EM-I sources with lower mu). In accordance with this model, the global variations in Pb-207/Pb-204 ratios and Delta 7/4 could reflect geographic differences in mu and/or the age at which the transition from stages I to 2 took place in the Archaean lithosphere. The model would require a re-definition of the EM-I endmember to low Pb-206/Pb-204, high (208)pb/Pb-204 (positive Delta 8/4) but variable Pb-207/Pb-204 (positive and negative Delta 7/4). (c) 2007 Elsevier B.V. All rights reserved.
机译：在北大西洋东部（北纬34度）哥斯拉·塞诺特地区晚白垩世（66.2 +/- 0.5 Ma的闪石和66.7 +/- 0.2 Ma的金云母Ar-40 / Ar-39年龄）的云母火山岩含有微量元素，Sr- Nd-Pb-Hf同位素组成类似于富化幔I（EM-1）末端成员，但相对于Pb-206 / Pb-204的比率（Pb-206 / Pb- 204（in）= 17.7，207Pb / Pb-204（in）= 15.34）在成铀Pb同位素图的北半球参考线下方绘制。闪石分离物的O同位素数据为幔样（δO-18 = 5.6-5.8％微米）。但是，孤立的哥斯拉海山的年龄和位置使它无法从马德拉岛或金丝雀热点中获得，因此不太可能出现下地幔。因此，我们建议从浅（岩石圈/软流圈）融化异常中推导。正如在中海脊和海洋岛玄武岩中所观察到的那样，单个的EM-I端成员型地区朝着北纬高地的哥斯拉分布，Pb-207 / Ph-204比值（和Delta 7/4）有系统地降低这种趋势的延续。这种趋势反映在超钾质火山岩中，如斑岩岩和金伯利岩，反映出富集的亚大陆岩石圈地幔的组成。因此，建议使用Pb-207 / Pb-204比率和Delta 7/4的全局模式。包括哥斯拉熔岩在内的EM-1端部类型地区的地球化学组成以及南半球的富集（DUPAL）异常可能反映了源自古代交代的亚大陆岩石圈地幔。我们提出了一个两阶段的模型来解释全世界EM-I地幔末端成员的痕量元素和同位素组成：1）在地球的早期历史中，次大陆岩石圈被俯冲板块的熔体沿会聚边缘的交融交变，从而产生高μ （U-238 / Pb-204）源，以及2）随着地球变冷，含水流体代替含水熔体，成为板状次大陆岩石圈地幔交代的主要板块成分（生成具有较低mu的EM-1源）。根据该模型，Pb-207 / Pb-204比值和Delta 7/4的全球变化可能反映了亩和/或古生界岩石圈从第一阶段过渡到第二阶段的年龄的地理差异。该模型需要将EM-I端部件重新定义为低Pb-206 / Pb-204，高（208）pb / Pb-204（正Delta 8/4），但变量Pb-207 / Pb-204（正负Delta 7/4）。 （c）2007 Elsevier B.V.保留所有权利。

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