深部碳循环对典型大洋海山链碱性玄武岩的成因制约

Alkali basalt constitutes the major lithology of the seamounts, and its origin has important implications on how intra-oceanic volcanism is formed. However, the silicate-rich and light rare earth element-rich geochemical characteristics of alkali basalts have not been well constrained. The results of high-pressure experiment in recent years show that mantle melting with CO2 can formed alkali basalt, which, however, has not been tested by geochemical studies on natural volcanic rock samples. Carbon in the mantle was most likely introduced by ancient plate subduction. Thus, the project in application plans to plan to study on the volcanic rocks of the Hawaii and Caroline seamount chains. The melt inclusions of the early-stage fractionated crystals will be analyzed to understand the CO2 compositions of primary melts of alkali basalts; the major and trace elements and Sr-Nd-Pb-Hf isotopes will be compared with Mg isotope to investigate the contribution of CO2 to the origin of alkali basalts; the alkali basalts will be analyzed for carbonated chemical indicators with melt evolution, which will be combined with the mineral and chemical compositions of peridotite-xenoliths to understand the contribution of melt-rock interactions to the origin of alkali basalts; furthermore, the compositional contrasts of basalts between the Hawaii and Caroline seamount chains will analyzed to investigate the influences of mantle potential temperature on the carbonated chemical indicators and the origin of alkali basalts. In summary, this project aims to investigate the role of CO2 in the origin of oceanic alkali basalt.

碱性玄武岩是大洋海山的主要岩石类型，其成因对认识大洋板内岩浆形成机制具有重要科学意义。但是，其贫硅、富轻稀土的特征一直没有得到很好的解释。近年来，高压实验显示CO2参与地幔熔融可形成碱性玄武岩，但尚未得到天然样品的最终验证。地幔的碳最可能来自俯冲板块，本项目选择源区可能含俯冲-再循环洋壳的夏威夷和卡洛琳两个海山链的碱性玄武岩为研究对象，通过早期结晶矿物的熔融包裹体分析，认识母岩浆的CO2等挥发分组成；通过碱性玄武岩Mg同位素组成与主、微量元素及Sr-Nd-Pb-Hf同位素之间的协变关系，揭示CO2对岩浆成因的贡献；分析岩浆演化过程中碳酸盐化指标的变化趋势，并结合橄榄岩捕掳体的化学组成，阐明熔体与亏损地幔相互作用对碱性玄武质岩浆成因的贡献；对比卡洛琳和夏威夷海山链玄武岩化学组成差异，揭示地幔潜能温度变化对碳酸盐化指标和碱性玄武岩成因的影响。综上，以认识CO2对大洋海山碱性玄武岩成因的贡献。

固体地球内部（地幔/地核）是地球上最大的碳储库，大洋板内岩浆作用是认识地球深部碳储库的重要窗口。但是，关于碳酸盐化岩浆与大洋碱性玄武质岩浆的成因关系，以及原始碱性玄武质岩浆在演化过程中与岩石圈地幔相互作用规律的认识，还远为不足。本项目针对北半球太平洋卡洛琳和夏威夷两个来自下地幔的海山链火山岩，以及南海典型火成碳酸岩，开展了详细的火山岩岩石学和地球化学研究，揭示了这些火山岩的源区性质和CO2来源、碳酸盐化岩浆与岩石圈相互作用机制，及其对大洋岩浆的成因约束。取得的主要认识：（1）通过对卡洛琳高原-海山链系统详细的年代学、矿物学和地球化学分析，揭示了卡洛琳洋底高原与海山链形成于同一个地幔柱热点，并且随着时间地幔柱活动逐渐减弱，形成了低程度碳酸盐化熔体。（2）通过大洋板内岩浆演化和源区分析，阐明了碳酸盐化熔体在穿过厚的太平洋岩石圈过程中经历了交代反应和分离结晶二阶段演化过程。（3）在夏威夷海山链复苏期碱性火山岩矿物学和地球化学分析基础上，完成了复苏期碱性火山岩原始CO2含量计算，通过橄榄石元素对比分析，阐明了这些复苏期火山岩来自一个橄榄岩为主的地幔源区。（4）针对南海残留脊和IODP U1431站火成碳酸岩开展了Mg同位素分析，结果显示比正常地幔明显偏轻，结合Sr-Nd-Pb-Hf-Mg耦合关系分析，阐明了源区存在再循环的沉积碳酸盐。成果的主要意义在于：明确了典型大洋/海盆碱性火山岩地幔源区组成性质和来源，及其与俯冲碳酸盐的成因关联，阐明了碳酸盐化岩浆在大洋岩石圈内的演化规律，以及CO2对碱性玄武岩成因的贡献机制，这些成果对认识地幔深部碳储库成因和对地表宜居环境影响有重要意义。