Shallow recycling of lower continental crust: The Mahoney Seamount at the Southwest Indian Ridge
Shallow recycling of lower continental crust: The Mahoney Seamount at the Southwest Indian Ridge
复制标题
下大陆地壳的浅层再循环：西南印度洋脊的马奥尼海山
DOI：
10.1016/j.epsl.2022.117968
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发表时间：
2023
影响因子：
5.3
通讯作者：
Romer, Rene
中科院分区：
文献类型：
--
作者：
Woelki, Dominic;Salters, Vincent;Beier, Christoph;Dick, Henry;Koepke, Juergen;Romer, Rene
马奥尼海山是最近发现的一座火山建筑物，位于超缓慢扩张的西南印度山脊(SWIR)以北4公里处。西南红外海是世界上最慢的扩张海脊之一，其完全扩张速率为14毫米/年(∼)，岩浆生产力较低。我们报道了马奥尼海山的高孔玄武岩具有非放射性成因的NdHf和放射性成因的锶同位素组成。其明显的低206Pb/204Pb同位素特征与高207Pb/204Pb和208Pb/204Pb相结合，最好的解释是受到下陆壳搁浅强烈影响的地幔熔融。同位素变异性的地理分布有利于较长时间隔离的下大陆地壳浅层再循环的想法，有助于熔体通过离轴断裂系统形成马奥尼海山。马奥尼海山熔岩的同位素组成与EM-1源和DUPAL特征有许多相似之处。以前对SWIR玄武岩的同位素研究表明，古老的次大陆岩石圈地幔(SCLM)或具有洋壳的远洋沉积物的再循环是造成这种丰度同位素特征的原因。远洋沉积物的Lu/Hf和Sm/Nd值会导致143Nd/144Nd值和176Hf/177Hf值的解耦。这种脱钩也在马达加斯加的埃赫达-贝基利堤坝上观察到，但这些堤坝被认为是散布在印度洋的SCLM的样本。然而，马奥尼海山在这些同位素体系中没有表现出脱钩现象，马奥尼海山极下大陆地壳特征的出现受到限制，这意味着富集的同位素特征具有不同的起源。
The Mahoney Seamount is a recently discovered volcanic edifice located 4 km north of the ultra-slow spreading Southwest Indian Ridge (SWIR). The SWIR is one of the slowest spreading ridges worldwide with a full spreading rate of ∼14 mm/year and low magmatic productivity. We report that highly vesicular basalts from the Mahoney Seamount have unradiogenic Nd-Hf together with radiogenic Sr isotopic compositions. Their distinct low206Pb/204Pb isotope signature combined with high207Pb/204Pb and208Pb/204Pb is best explained by melting of a mantle that has been strongly influenced by stranded lower continental crust. The geographic distribution of the isotopic variability favors the idea of shallow recycling of lower continental crust isolated for a longer period contributing to melts forming Mahoney Seamount through off-axis fault systems. The isotopic composition of Mahoney Seamount lavas shares many characteristics with EM-1 sources and the DUPAL signature. Previous isotopic studies of the SWIR basalts proposed recycling of ancient subcontinental lithospheric mantle (SCLM) or pelagic sediments with oceanic crust to be responsible for this enriched isotopic signature. Lu/Hf and Sm/Nd ratios of pelagic sediments would result in decoupled143Nd/144Nd and176Hf/177Hf ratios. This decoupling is also observed in Ejeda-Bekily dikes from Madagascar, but those are believed to sample the SCLM dispersed in the Indian Ocean. However, Mahoney Seamount shows no decoupling in those isotopic systems and the restricted occurrence of the extreme lower continental crustal signature at Mahoney Seamount implies that the enriched isotopic signature has a different origin.