【作者】 张国良；

【导师】 曾志刚；

【作者基本信息】 中国科学院研究生院（海洋研究所） ， 海洋地质学， 2010， 博士

【摘要】 本研究对东太平洋海隆13°N附近的玄武岩进行了矿物组成分析、全岩常量和微量元素分析、铀系同位素组成分析、橄榄石和斜长石及其中熔体包裹体成分分析。该研究有利于深入认识快速扩张洋中脊地幔的熔融过程和玄武质岩浆演化特征,并揭示玄武质岩浆动力学过程对岩浆化学和同位素组成的控制机制。该区玄武岩的矿物组成和常量元素特征表明,玄武质岩浆在浅部洋壳内经历了强烈的低压橄榄石+斜长石共结晶,而不存在单斜辉石斑晶。热力学软件COMAGMAT计算表明,岩浆在高压(4-8 kbar)条件下经历了单斜辉石结晶。另外,对65个熔体包裹体的组成分析表明,洋中脊玄武岩结晶深度并不是过去所指出的洋壳内,而是在岩浆侵入洋壳之前就已经历了分异结晶作用,即洋中脊玄武质岩浆过程并不是一个理想的绝热过程。岩浆结晶过程中Ni含量的变化可以通过岩浆房内少量多次的岩浆混合作用解释。洋中脊玄武质岩浆早期结晶产物:高Mg#橄榄石捕获有早期熔体-熔体包裹体。高Mg#橄榄石中熔体包裹体研究证明东太平洋海隆地幔中的确存在不同来源深度和熔融程度的岩浆混合作用。研究对比了洋中脊和洋岛玄武岩230Th-238U不平衡数据,结果表明洋中脊玄武岩过剩230Th受控于地幔熔融条件。我们提出一个密度控制模型以解释玄武岩铀系同位素组成特征。在该模型中,深部来源的岩浆均具有相对浅部岩浆较高的(230Th/238U)、Fe8、La/Sm和岩浆密度。深源岩浆较高的密度会导致其在迁移过程中的上浮力较弱,从而倾向于长时间滞留在岩浆房底部;而浅部来源的岩浆具有较低的岩浆密度和较强的上浮力,从而滞留时间较短。研究对比了2202个东太平洋海隆玄武岩和888个海隆附近海山玄武岩常量元素组成,以揭示研究区地幔熔融条件对岩浆演化的控制作用。结果表明,从东太平洋海隆北段至南段,扩张速率逐渐从8 cm/yr (16°N)变化至15 cm/yr (19°S),其岩浆结晶压力也相应降低。因此,受控于扩张速率的岩浆供应控制了洋壳温度和岩浆结晶深度。更多还原

【Abstract】 The East Pacific Rise (EPR) is located to the east of pacific and is not far from the western coast of America. It is a typical fast spreading ridge, which has produced the major component of the upper part of the Pacific ocean crust. The study of petrology and geochemistry of MORBs from the EPR is critical for understanding the magmatic process and the dynamics for generating new oceanic crust. In this study, we analyzed the mineralogy, whole-rock major and trace elements, U-series isotopic compositions and melt inclusions hosted in olivine- and plagioclase-phenocrysts in MORBs from the EPR 13°N. This study aims to investigate the melting behavior of upper mantle and magma evolution process in fast spreading ridge, and to reveal the control of melting dynamics on chemical and isotopic compositions of basaltic magma.The mineralogical characters and major element compositions show that MORB magmas in this study area have experienced extensive low-pressure crystallization of olivine plus plagioclase, whereas clinopyroxene has not taken part in crystallization. The calculation using COMAGMAT resulted in that these magmas had fractionated clinopyroxene under pressure of 4-8 kbar. The results of analyses for sixty-five initial melt inclusions show that their host magmas have experienced high pressure fractionation of clinopyroxene. The MORB magmas have experienced fractionation not only in the ocean crust but also beneath the ocean crust, and the MORB magmatism is not an ideal adiabatic process. The variations of Ni in these samples can be addressed by multiple magma mixing processes in the magma chamber. High Mg# olivine hosts early-stage melt– melt inclusion, and the study based on these melt inclusions shows that magmas beneath the EPR have experienced mixing with magmas from various melting depths and degrees.The comparison of U-series data between MORBs and OIBs show that the excess 230Th are dominated by mantle melting conditions. We have proposed a density-control model to address the characteristics of U-series isotopic compositions. In this model, the deep-sourced melt has higher (230Th/238U), Fe8, La/Sm and density than that from shallow depth. The high density of the deep-sourced melt leads to its weak buoyancy, thus would tend to reside in the magma chamber for long time; whereas the shallow-sourced melt has a low density and intensive buoyancy, and would have short residence time.Major elements of 2202 basalts from the East Pacific Rise (EPR) and 888 basalts from near-EPR seamounts are used to investigate their differences in magma crystallization pressures and mantle melting conditions. The magma crystallization pressure decreases significantly as spreading rate of the EPR increases from ~80 mm/yr in the north (16°N) to ~160 mm/yr in the south (19°S), while this trend is unobvious in near-EPR seamounts. This suggests that the magma supply controlled by spreading rate dominates the ridge crust temperature and magma crystallization depth.更多还原