Abstract Enriched mid-ocean ridge basalts (E-MORBs) near hotspots are commonly ascribed to be products of mantle plume-ridge interaction, but the genesis of E-MORBs far from hotspots remains controversial. Here we report new major and trace element and Sr-Nd-Pb isotopic data for lavas from Holes 559 and 561 of Deep Sea Drilling Project (DSDP) Leg 82 that are located west of the Mid-Atlantic Ridge (MAR) midway between the Oceanographer and Hayes fracture zones (33–35°N). Our data reveal that E-MORBs from Hole 559 have high (87Sr/86Sr)i (0.70292–0.70294) and (206Pb/204Pb)i (19.504–19.692) and low (143Nd/144Nd)i (0.51297–0.51299) values, similar to the mantle focal zone (FOZO) component, that could be partly coming from the New England mantle plume. We propose that the FOZO component could be widely distributed beneath the northern portion of MAR at ~35 Ma. Hole 561 lavas contain both depleted (D-) and E-MORB. The D-MORBs have depleted Sr-Nd-Pb isotope compositions and were derived from the depleted asthenospheric mantle. Hole 561 E-MORBs have radiogenic (87Sr/86Sr)i (0.70337–0.70341), (143Nd/144Nd)i (0.51296–0.51297) and (206Pb/204Pb)i (19.463–19.525) values that are similar to the proposed common component in the mantle plume sources of lavas in all the Azores islands except for the eastern part of Sao Miguel. In addition, Hole 561 E-MORBs also display higher Th/La and Nb/La ratios than the most plume-influenced E-MORBs near the Azores hotspot and even the Azores lavas. These suggest that Hole 561 E-MORBs were derived from partial melting of a depleted mantle metasomatized by deep, low-degree melts of the common Azores plume component. We propose that variable amounts of such metasomatized mantle are widely distributed in the asthenospheric mantle beneath the MAR 33–35°N ridge segment since ~19 Ma and their partial melting is also probably responsible for the generation of the modern MAR 33–35°N E-MORBs.

中文翻译：

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DSDP 孔 559 和 561 MORB 源中 FOZO 和亚速尔群岛羽流成分的新地球化学和 Sr-Nd-Pb 同位素证据

摘要 热点附近的富集洋中脊玄武岩（E-MORBs）通常被认为是地幔柱-脊相互作用的产物，但远离热点的E-MORBs的成因仍存在争议。在这里，我们报告了来自深海钻探项目 (DSDP) 航段 82 的 559 和 561 孔熔岩的新的主要和微量元素以及 Sr-Nd-Pb 同位素数据，它们位于海洋学家之间的中大西洋海脊 (MAR) 以西和 Hayes 断裂带（33-35°N）。我们的数据显示，来自 559 孔的 E-MORB 具有高 (87Sr/86Sr)i (0.70292–0.70294) 和 (206Pb/204Pb)i (19.504–19.692) 和低 (143Nd/144Nd)i (729.5) 值 (0.70292–0.70294)类似于地幔焦点区 (FOZO) 部分，这可能部分来自新英格兰地幔柱。我们建议 FOZO 组件可以在 35 Ma 左右广泛分布在 MAR 北部下方。561 孔熔岩包含耗尽 (D-) 和 E-MORB。D-MORBs 已耗尽 Sr-Nd-Pb 同位素组成，并且来自耗尽的软流圈地幔。孔 561 E-MORB 具有放射成因 (87Sr/86Sr)i (0.70337–0.70341)、(143Nd/144Nd)i (0.51296–0.51297) 和 (206Pb/204Pb)i–19.525 与建议值相似的常见值除圣米格尔岛东部外，所有亚速尔群岛的熔岩地幔柱源中都含有这种成分。此外，与亚速尔群岛热点甚至亚速尔群岛熔岩附近受羽流影响最大的 E-MORB 相比，561 孔 E-MORB 还显示出更高的 Th/La 和 Nb/La 比率。这些表明 561 孔 E-MORB 是由深部交代的耗尽地幔的部分熔化产生的，常见亚速尔群岛羽流成分的低度熔化。我们提出，自~19 Ma以来，不同数量的这种交代地幔广泛分布在MAR 33-35°N脊段下方的软流圈地幔中，它们的部分熔化也可能是现代MAR 33-35°N的产生的原因E-MORB。