Knowledge Management System Of Institute of Oceanology, Chinese Academy of Sciences
|Place of Conferral||中国科学院大学|
|Keyword||南海 IODP 玄武岩 Mg-Fe-Zn 同位素 俯冲流体交代|
本文研究的南海玄武岩形成于扩张晚期，属于拉斑质玄武岩。其中，东次海盆（U1431）玄武岩主要为N-MORB，个别层位出现E-MORB；西南次海盆（U1433和U1434）玄武岩主要为E-MORB。主微量元素特征显示这些玄武岩经历了不同程度的橄榄石、辉石、斜长石等矿物分离结晶作用；微量元素蛛网图显示富集流体活动性元素，如Rb、Ba、U等，以及Pb正异常；同时具有不同于典型地幔的Ce/Pb（7.56 ~ 31.18）、Nb/U（21.53 ~ 55.45）比值；Sr-Nd同位素结果显示具有比N-MORB略微高的87Sr/86Sr值（0.703113 ~ 0.704190），但是与N-MORB相似的143Nd/144Nd值（0.512963 ~ 0.503091）；与全球MORB中橄榄石相比，南海玄武岩中的橄榄石具有高Ni含量和低CaO含量，指示南海橄榄岩地幔源区中存在辉石岩组分。
镁同位素结果（d26Mg）显示，南海玄武岩的d26Mg值从-0.24‰变化至0.00‰，系统地高于典型MORB值（d26Mg = -0.25‰）。由于d26Mg值与指示样品蚀变、部分熔融和岩浆演化的地球化学指标之间不存在相关性，表明南海玄武岩的Mg同位素变化不受蚀变和岩浆过程的影响，而是反映了其岩浆源区特征。结合微量元素特征，本文认为南海玄武岩比MORB重的Mg同位素特征反映了其岩浆源区受到了俯冲板片中滑石/蛇纹石来源的高d26Mg流体的影响。通过对比分析南海周缘地区具有轻Mg同位素特征的新生代玄武岩，本文提出这两类具有不同Mg同位素组成的玄武岩皆是板块俯冲的产物，两者之间的Mg同位素差异受控于俯冲过程中具有不同Mg含量及Mg同位素组成的矿物在不同俯冲深度分解释放流体的影响。在俯冲浅部，滑石/蛇纹石脱水发生分解，产生具有重Mg同位素组成的流体，而白云石可以俯冲至深部地幔，产生具有轻Mg同位素组成的流体/熔体。因此，南海玄武岩的Mg同位素结果支持板块俯冲作用对南海形成演化的影响。
南海玄武岩的Fe同位素组成（d56Fe）为0.08‰ ~ 0.16‰。通过校正分离结晶对样品Fe同位素组成的影响，获得了初始熔体的d56Fe值（0.02‰ ~ 0.10‰），与典型MORB铁同位素值（d56Fe = 0.067‰）类似。基于混合地幔端元（橄榄岩+辉石岩）的微量元素模拟计算表明，南海橄榄岩地幔源区中辉石岩组分比例约10~40%，由此混合源区产生的南海玄武岩理论上应具有比典型MORB重的Fe同位素组成。但是，由于受俯冲过程中蛇纹岩流体（具有低d56Fe值）交代作用的影响，降低了南海玄武岩的d56Fe值，使得其与典型MORB 的d56Fe值相似。进一步地，本人认为辉石岩熔体是在南海扩张过程中以洋脊-地幔柱相互作用的形式参与了南海玄武岩的形成；结合橄榄石高Ni特征，认为辉石岩组分属于由俯冲洋壳形成的榴辉岩/辉石岩熔体交代地幔橄榄岩，形成的二次辉石岩。
进一步利用本次研究获得的洋壳岩石样品，探究了板块俯冲过程中Zn同位素行为。结果显示南海玄武岩Zn同位素组成（d66Zn = 0.22‰ ~ 0.31‰）与典型MORB（d66Zn = 0.27‰）类似，高于上地幔平均值（d56Fe = 0.18‰）。由于d66Zn值与指示样品蚀变、岩浆演化和俯冲流体交代的地球化学指标之间不存在相关性，表明南海玄武岩的Zn同位素组成不受蚀变、分离结晶和板块俯冲作用的影响。南海玄武岩与典型MORB相似的Zn同位素特征主要受地幔熔融过程控制。通过收集、整理形成于俯冲背景下具有不同d66Zn值的火山岩数据，本次研究指出板块俯冲过程中，Zn同位素行为主要受白云石、菱镁矿等富Zn矿物控制。
The South China Sea is the largest marginal sea in China. It has almost undergone a complete geological cycle. Understanding its formation and evolution is of great significance for the improvement of the formation theory of marginal ocean basin. The South China Sea is located in the interaction zone of Eurasian plate, Pacific plate and Indo-Australian plate. Meanwhile, Hainan mantle plume was developed in the northern margin of the South China Sea. Therefore, its formation and evolution was affected by plate subduction, mantle plume and other factors, resulting in the complexity of its mantle source. Because the oceanic crust of the South China Sea is covered by thick sediments, it is difficult to obtain oceanic crust rock samples, which limits the understanding of the characteristics of mantle source in the South China Sea from the perspective of petro-geochemistry. In this study, the oceanic crust basalts obtained during International Ocean Discovery Program (IODP) Expedition 349 in the South China Sea are used to deeply understand the characteristics of the mantle source, and further reveal the influences of plate subduction on the formation and evolution of the South China Sea, through the analysis of whole-rock major and trace elements, Sr-Nd isotopes and emerging Mg-Fe-Zn metal stable isotopes and chemical composition of olivine minerals.
The oceanic crust basalts studied here were formed during the late period of seafloor expansion, and belong to tholeiite. Among them, the basalts in the East sub-basin (U1431) are mainly N-MORB, and E-MORB appears in some layers; the basalts in the Southwest sub-basin (U1433 and U1434) are mainly E-MORB. The characteristics of major and trace elements show that these basalts have undergone different degrees of crystallization, e.g., olivine, pyroxene, plagioclase and other minerals. Trace element spider diagrams show that they are enriched in fluid-active elements, such as Rb, Ba, U, etc., and crustal elements, such as Pb; meanwhile, they have different Ce/Pb (7.56 ~ 31.18) and Nb/U (21.53 ~ 55.45) ratios from typical mantle;
Sr-Nd isotope results show that they have a slightly high 87Sr/ 86Sr (0.703113 ~ 0.704190), but 143Nd/144Nd (0.512963 ~ 0.503091) similar to N-MORB; Comparing with olivine chemistry in global MORB, the chemical compositions of olivine in studied basalts are with high Ni contents and low CaO contents, indicate that there are pyroxenitic components in peridotitic mantle source of the South China Sea.
The Mg isotopic compositions show that d26Mg of the South China Sea basalts are from -0.24‰ to 0.00‰, which are higher than that of typical MORB (d26Mg = -0.25‰). Due to the lack of correlation between d26Mg and indicators of alteration and magmatic processes, the heavy Mg isotopes of the studied basalts are inherited from their magma source, which are most likely from subducted talc- and/or serpentine-derived fluids with high d26Mg. Through the comparative analysis of the light Mg isotopes characteristics of basalts in the surroundings of the South China Sea, we proposed that the two types basalts with different Mg isotopic compositions are all the products of plate subduction, and the difference in Mg isotopic compositions between them was controlled by minerals with different Mg contents and Mg isotopes during subduction. Specifically, the shallow subduction was mainly affected by heavy Mg isotopic fluids released from talc and/or serpentine, while the deep mantle was mainly affected by light Mg isotopes fluids/melts released from dolomite.
The Fe isotopic compositions (d56Fe) of the South China Sea basalts are from 0.08‰ to 0.16‰. After evaluating the effects of alteration and fractional crystallization, we got the Fe isotopic compositions of primitive melts (d56Fe = 0.02‰ ~ 0.10‰), which are similar to that of typical MORB (d56Fe = 0.067‰). The simulation calculation of mixed mantle shows that about 10~40% pyroxenitic melts was involved into the peridotite mantle source in the South China Sea, which theoretically should produce basalts with heavier Fe isotopes than typical N-MORB. However, due to the effect of fluid-metasomatism released from serpentinite with low d56Fe, the Fe isotopic compositions of the magma source were further reduced, and similar to that of typical MORB. Pyroxenitic components was involved into the formation of the South China Sea basalts by ridge - plume interaction during the expansion of the South China Sea. Combined with the high Ni contents of olivine, the pyroxenitic components in the peridotitic mantle was the secondary pyroxenite formed by reaction of mantle peridotite and the eclogite or pyroxenite melts originated from subducted oceanic crust.
In this study, the influence of plate subduction on the mantle source of basalt in the South China Sea was further illustrated by Zn isotopes. The results show that Zn isotopic compositions (d66Zn) of the South China Sea basalts are from 0.22‰ to 0.31‰, which are similar to that of typical MORB (d66Zn = 0.27‰), but higher than that of average upper mantle (d66Zn = 0.18‰). Since there is no correaltion between d66Zn and indicators of alteration, fractional crystallization and subducted fluid-metasomatism, it indicates that alteration, fractional crystallization and plate subduction have a limted effect on the variation of Fe isotopes. The MORB-like Zn isotopes of the South China Sea basalts mainly reflect the mantle melting process. By collecting and sorting out the data of volcanic rocks with different d66Zn formed in the background of subduction, this study proposed that the behavior of Zn isotopes during subduction was mainly affected by dolomite and other Zn-rich minerals.
This study provides direct evidence that the mantle source in the South China Sea was affected by plate subduction from the perspective of petro-geochemistry, quantitatively simulates the proportion of heterogeneous materials in the basaltic magma source of the South China Sea, and limits the main factors that affect the behavior of zinc isotopes in plate subduction.
|MOST Discipline Catalogue||理学::海洋科学|
|Table of Contents|
|廖仁强. 南海新生代玄武岩地球化学特征及其对岩浆演化的启示[D]. 中国科学院大学. 中国科学院海洋研究所,2020.|
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