Trace Element and Isotopic Evidence for Recycled Lithosphere from Basalts from 48 to 53°E, Southwest Indian Ridge
Trace Element and Isotopic Evidence for Recycled Lithosphere from Basalts from 48 to 53°E, Southwest Indian Ridge
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DOI：
10.1093/petrology/egaa068
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发表时间：
2020-06
影响因子：
3.9
通讯作者：
Jixin Wang;Huaiyang Zhou;V. Salters;H. Dick;J. Standish;Cong Wang
中科院分区：
文献类型：
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作者：
Jixin Wang;Huaiyang Zhou;V. Salters;H. Dick;J. Standish;Cong Wang
西南印度洋中脊（SWIR）大部分地区的地幔来源异质性和岩浆过程已得到广泛研究。但对龙骨无岩浆段（53°E，SWIR）和邻近的岩浆强劲的龙旗段新发现的洋中脊玄武岩知之甚少。龙骨段的新鲜玄武岩玻璃的同位素组成明显比邻近的龙旗玄武岩更丰富，但与平均洋中脊玄武岩（MORB）相比，龙旗玄武岩的微量元素比例比龙骨玄武岩更为极端。它们的地球化学差异只能通过来源差异来解释，而不是通过大致相似来源的熔化程度的变化来解释。龙旗玄武岩受到弧状地幔成分的影响，流体流动元素比流体固定元素富集证明了这一点。然而，由于先前在弧下熔融，与正常的 MORB 源相比，龙旗段的下脊地幔熔体成分有所减少。这种流体交代的、弧下耗尽的地幔被夹带在龙旗部分下方。相比之下，龙骨轴向玄武岩的 Pb 同位素组成以及 Ba、Nb、Ta、K、La、Sr 和 Zr 的轻微富集以及 Pb 和 Ti 浓度的贫化与马达加斯加的埃赫达-贝基利岩脉相似。此外，龙骨 Sr 和 Nd 同位素成分以及 Ce/Pb、La/Nb 和 La/Th 比率可以通过将同位素最贫化的龙旗玄武岩与 Ejeda-Bekily 岩脉范围内的成分混合来模拟。因此，有人提出，龙骨轴向玄武岩与埃赫达-贝基利岩脉类似，源自冈瓦纳大陆下方的次大陆岩石圈太古代地幔，从马达加斯加高原下方拉出。残留的弧下地幔和次大陆岩石圈地幔的再循环可能与冈瓦纳大陆的分裂或莫桑比克洋塌陷期间新元古代岛弧地体的形成和增生有关，并且现在存在于山脊下方。
Mantle source heterogeneity and magmatic processes have been widely studied beneath most parts of the Southwest Indian Ridge (SWIR). But less is known from the newly recovered mid-ocean ridge basalts from the Dragon Bone Amagmatic Segment (53°E, SWIR) and the adjacent magmatically robust Dragon Flag Segment. Fresh basalt glasses from the Dragon Bone Segment are clearly more enriched in isotopic composition than the adjacent Dragon Flag basalts, but the trace element ratios of the Dragon Flag basalts are more extreme compared with average mid-ocean ridge basalts (MORB) than the Dragon Bone basalts. Their geochemical differences can be explained only by source differences rather than by variations in degree of melting of a roughly similar source. The Dragon Flag basalts are influenced by an arc-like mantle component as evidenced by enrichment in fluid-mobile over fluid-immobile elements. However, the sub-ridge mantle at the Dragon Flag Segment is depleted in melt component compared with a normal MORB source owing to previous melting in the subarc. This fluid-metasomatized, subarc depleted mantle is entrained beneath the Dragon Flag Segment. In comparison, for the Dragon Bone axial basalts, their Pb isotopic compositions and their slight enrichment in Ba, Nb, Ta, K, La, Sr and Zr and depletion in Pb and Ti concentrations show resemblance to the Ejeda–Bekily dikes of Madagascar. Also, Dragon Bone Sr and Nd isotopic compositions together with the Ce/Pb, La/Nb and La/Th ratios can be modeled by mixing the most isotopically depleted Dragon Flag basalts with a composition within the range of the Ejeda–Bekily dikes. It is therefore proposed that the Dragon Bone axial basalts, similar to the Ejeda–Bekily dikes, are sourced from subcontinental lithospheric Archean mantle beneath Gondwana, pulled from beneath the Madagascar Plateau. The recycling of the residual subarc mantle and the subcontinental lithospheric mantle could be related to either the breakup of Gondwana or the formation and accretion of Neoproterozoic island arc terranes during the collapse of the Mozambique Ocean, and is now present beneath the ridge.