Isotopically enriched N-MORB: A new geochemical signature of off-axis plume-ridge interaction—A case study at 50°28'E, Southwest Indian Ridge
Isotopically enriched N-MORB: A new geochemical signature of off-axis plume-ridge interaction—A case study at 50°28'E, Southwest Indian Ridge
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同位素富集的 N-MORB：离轴羽流-山脊相互作用的新地球化学特征——西南印度洋中脊 50°28'E 的案例研究
DOI：
10.1002/2016jb013284
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Xi-Guang Deng
中科院分区：
文献类型：
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作者：
Alex;ra Yang Yang;Tai-Ping Zhao;Mei-Fu Zhou;Xi-Guang Deng
地球物理研究已经提出了西南印度洋脊（46°E和52°20′E）和克罗泽热点之间的相互作用，但主要由于缺乏E-MORB（富集的洋中脊玄武岩）而仍然存在争议。从该地区收集的47个新样品，包括15个来自27段，中心位于50°28′E，地壳厚10 km，都是N-MORB（正常MORB），可分为两组：仅在50°28′E的高铝组和广泛分布的主组。前者具有较高的Al_2O_3含量和较低的TiO_2、SiO_2含量，Sr-Nd-Hf-Pb同位素组成稍富。我们建议，他们的主要和微量元素的签名被修改的反应与原始累积在地壳中，而丰富的同位素组成表明Crozet羽材料的贡献。在克罗泽羽流上坡流向海脊的过程中，减压熔融将沿着路径发生，这将耗尽羽流中的不相容元素，但不会显著改变同位素组成。因此，当它们最终到达海脊时，耗尽的残余物将由于在莫尔处的进一步减压而重熔，并在区段27处产生同位素富集的N-MORB。同位素富集的N-MORB在其他地方是已知的，主要是在较慢扩张的山脊上，可能受到具有大的羽流-山脊距离的羽流的影响。特别是，在留尼汪热点向CIR之间的离轴岩浆活动中，Nd同位素组成不变，（La/Sm）N比值降低，与这种地幔柱-山脊相互作用模型完全匹配。因此，除了E-MORB外，同位素富集的N-MORB也可以被认为是离轴羽流-脊相互作用的地球化学特征。
Interaction between the Southwest Indian Ridge (46°E and 52°20′E) and Crozet hotspot has been proposed by geophysical studies but remains controversial mostly due to the lack of E‐MORB (enriched mid‐ocean ridge basalts). Forty‐seven new samples collected from this region, including 15 from segment 27 centered at 50°28′E with a 10 km thick crust, are all N‐MORB (normal MORB) and can be classified into two groups: a high‐Al group only at 50°28′E and a Main group widespread. The former, with higher Al2O3and lower TiO2and SiO2, have slightly enriched Sr‐Nd‐Hf‐Pb isotopic compositions. We propose that their major and trace elemental signatures were modified by reaction with primitive cumulate in the crust, whereas the enriched isotopic compositions indicate the contribution of Crozet plume materials. During upslope flow of the Crozet plume to the ridge, decompression melting would occur along the path, which would deplete the plume in incompatible elements but not significantly change the isotopic compositions. Thus, when they finally reach the ridge, the depleted residue would remelt due to further decompression at MOR and produce isotopically enriched N‐MORB at segment 27. Isotopically enriched N‐MORB are known elsewhere, mostly at slower‐spreading ridges possibly influenced by plumes with large plume‐ridge distances. In particular, the constant Nd isotopic compositions with decreasing (La/Sm)Nratios for off‐axis magmatism between the Réunion hotspot toward the CIR perfectly match such a plume‐ridge interaction model. Therefore, aside from E‐MORB, isotopically enriched N‐MORB can also be considered as the geochemical signature for off‐axis plume‐ridge interaction.