Deep Fractionation of Clinopyroxene in the East Pacific Rise 13°N: Evidence from High MgO MORB and Melt Inclusions
Deep Fractionation of Clinopyroxene in the East Pacific Rise 13°N: Evidence from High MgO MORB and Melt Inclusions
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DOI：
10.1111/j.1755-6724.2009.00030.x
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发表时间：
2009-04
期刊：
影响因子：
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通讯作者：
Guoliang Zhang;Z. Zeng;Xuebo Yin;Xiaoyuan Wang;Daigeng Chen
中科院分区：
文献类型：
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作者：
Guoliang Zhang;Z. Zeng;Xuebo Yin;Xiaoyuan Wang;Daigeng Chen
对东太平洋隆起(EPR)13°N洋脊玄武岩(MORBs)的常量元素和微量元素进行了分析，发现它们具有持续演化的趋势。正的MgO-Al_2O_3关系和负的MgO-Sc关系表现为斜长石和橄榄石的共晶结晶，存在斜长石和橄榄石斑晶，不存在单斜辉石斑晶。而单斜辉石的分馏作用则被氧化镁和氧化钙的正相关所证实。因此，MORB样品被认为表现出“单斜辉石悖论”。用COMAGMAT模拟了含镁最高的MORB样品E13-3B(MgO=9.52%)在不同压力下的恒压结晶过程。观测到的CaO/Al_2O_3比值只能由E13-3B在-4±1kbar压力下通过分离结晶得到，这需要单斜辉石结晶，与岩浆室中橄榄石+斜长石的共晶结晶(压力∼1kbar)不一致。熔融包裹体的初始成分可代表潜在的母质岩浆，通过后捕获结晶(PEC)校正重建了初始成分。初始熔体包裹体的模拟结晶也只在>；4±1kbar处产生了观察到的CaO/Al_2O_3比值，其中单斜辉石参与了结晶。认为MORB岩浆在下地壳、莫霍面过渡带及其下方经历了单斜辉石分馏作用。MORB岩浆经历了4±1kbar的单斜辉石+斜长石+橄榄石结晶到1kbar的以橄榄石+斜长石为主的结晶转变，解释了“单斜辉石悖论”。
Mid‐ocean ridge basalts (MORBs) from East Pacific Rise (EPR) 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO–Al2O3 and negative MgO–Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a “clinopyroxene paradox”. The highest magnesium‐bearing MORB sample E13–3B (MgO = 9.52%) is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13–3B only by fractional crystallization at pressure >4 ±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber (at pressure ∼1 kbar). The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post‐entrapment crystallization (PEC). The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at >4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene+plagioclase+olivine crystallization at >4±1 kbar to mainly olivine+plagioclase crystallization at <1 kbar, which contributes to the explanation of the “clinopyroxene paradox”.