南海高镁洋中脊玄武岩的成因及动力学机制研究

Basalt is one of the keys for understanding the operating mechanism of deep earth. High-Mg basalts are usually associated with high mantle potential temperature in source, and thus has particular information concerning the mantle geodynamics. However, some high-Mg mid-ocean ridge basalts (MORBs) are not directly related to mantle plume setting. Investigation of the petrogenesis of these high-Mg MORBs is important for better understanding the mantle composition, thermal state, and magmatism of oceanic ridges. But there is very few information on the high-Mg MORBs due to their rarity. Therefore, the high-Mg MORB remains unclear concerning its petrogenesis, magmatic processes and related geodynamics. The IODP Expedition 349 had drilled high-Mg MORBs at the site of Hole U1431E in the South China Sea, which provides a rarely opportunity for better understanding of the petrogenesis of the high-Mg MORB and related geodynamics. In this study, we will conduct detailed petrographic observation and in situ analyses of mineral compositions, compositional texture, and crystal size distributions of the high-Mg MORBs from the South China Sea, with aims to decipher the magmatic dynamics, mantle composition and thermal state beneath the spreading ridge. This study will provide new insights for understanding the petrogenesis of high-Mg MORB.

玄武岩是了解地球内部运行机制的主要媒介之一。玄武岩的高镁特征常指示源区具较高的地幔潜能温度，因而具有重要的地球动力学意义。然而，地球上存在一类并不直接出露于地幔柱构造背景的高镁洋中脊玄武岩（MORB），深入认识其岩石成因机制对理解地球深部地幔物质组成、热状态、洋中脊岩浆作用过程具有重要科学意义。但由于高镁MORB极少见，研究资料稀少，对其成因、岩浆作用过程及其地球动力学意义并不清楚。南海IODP 349航次的U1431E钻孔所获得的高镁MORB为进行相关研究提供了难得机会。本项目拟利用显微镜、矿物集成分析系统、电子探针、扫描电镜和激光剥蚀等离子质谱对南海高镁MORB及其伴生的MORB开展详细的矿物成分、结构和粒度统计分析工作，以揭示南海高镁MORB的岩浆动力学机制、地幔成分与热状态，阐明其地球动力学意义。本研究将为理解高镁MORB的成因机制以及动力学意义提供新的认识。

深入认识高镁洋中脊玄武岩（MORB）的岩石成因机制对理解地球地幔物质组成、热状态、洋中脊岩浆作用过程具有重要科学意义。本项目对南海高镁MORB及其伴生的MORB开展详细的矿物成分和结构分析，发现南海东部海盆地幔相对西南次海盆地幔温度较高、辉石岩比例较高、富集的再循环组分较多，指示南海下方的地幔柱与滞留板片相互作用而在上地幔发生分叉。东部海盆的地幔高比例的辉石岩导致岩浆成分高铁低硅，使得岩浆析出斜长石相对橄榄石明显变晚，进而在岩浆演化过程中形成橄榄石为主的晶粥体系，橄榄石晶粥被岩浆捕获进而形成了高镁洋中脊玄武岩。综上，南海高镁洋中脊玄武岩是浅部岩浆作用响应地幔柱-滞留板片相互作用的产物。