It is a general tendency that epidote, which is a typical greenschist facies mineral, is scarce in the lower oceanic crust, in spite of the widespread occurrence of the other minerals indicative of similar temperature conditions such as chlorite, actinolite, prehnite and serpentine. To find the cause of this, we carried out petrological analyses of lower crustal rocks of the Oman ophiolite sampled by the Oman Drilling Project of the International Continental Scientific Drilling Program (ICDP). Petrographic observations revealed the tendency, as expected, that the amount of epidote formed by static alteration of plagioclase decreases with depth. Because mineral assemblages indicative of a wide range of temperature conditions from amphibolite to subgreenschist facies occur throughout the cores without systematic variations of abundance, the decrease of epidote amount cannot be explained by the difference of temperature condition of alteration. Petrographic observations also revealed that epidote is absent or rare in rocks containing serpentinized olivine in contrast to prehnite showing a close association with serpentinization of olivine. In an exceptional sample containing both epidote and serpentinized olivine, epidote occurs with chlorite that cuts or replaces plagioclase, mantles adjacent olivine and is connected with chlorite + lizardite veins cutting mesh-forming serpentine veins. The distribution and mode of occurrence of epidote suggest decoupling of its formation with the main stage of serpentinization. Serpentine veins cutting olivine to form mesh texture are typically lizardite with magnetite ribbons at vein centres and have compositions of lizardite–cronstedtite solid solution at vein margins or in magnetite-free veins, suggesting a chemical condition with low silica and low oxygen potentials at an early stage of serpentinization. Thermodynamic modelling for olivine and plagioclase alteration at greenschist facies conditions indicates that silica potential for plagioclase alteration to form prehnite + chlorite and epidote + chlorite could be higher than for olivine serpentinization. On the other hand, oxygen potential for the prehnite + chlorite formation is lower than for the epidote + chlorite formation and is comparable with that for olivine serpentinization. From the observations and analyses, it is concluded that epidote formation is inhibited by olivine serpentinization, which maintains a reducing condition for alteration in the lower oceanic crust.

中文翻译：

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下洋壳蛇纹石化与绿帘岩形成之间的不相容性：来自阿曼钻探项目的证据

尽管绿泥石、阳起石、葡萄石和蛇纹石等指示类似温度条件的其他矿物广泛存在，但作为典型绿片岩相矿物的绿帘石在下洋壳中稀少是一种普遍趋势。为了找出其中的原因，我们对国际大陆科学钻探计划（ICDP）阿曼钻探项目采集的阿曼蛇绿岩下地壳岩石进行了岩石学分析。岩相观察揭示了一种趋势，正如预期的那样，由斜长石的静态蚀变形成的绿帘石的数量随着深度的增加而减少。因为指示从角闪岩到亚绿片岩相的各种温度条件的矿物组合出现在整个岩心中，而丰度没有系统变化，绿帘石数量的减少不能用变化的温度条件的差异来解释。岩石学观察还表明，在含有蛇纹石化橄榄石的岩石中，绿帘石不存在或很少见，而葡萄石与橄榄石的蛇纹石化密切相关。在包含绿帘石和蛇纹石化橄榄石的特殊样品中，绿帘石与绿泥石一起出现，它切割或替代斜长石，覆盖相邻的橄榄石，并与绿泥石 + 蛇纹石脉相连，切割形成网状蛇纹石脉。绿帘石的分布和发生方式表明其形成与蛇纹石化的主要阶段脱钩。蛇纹石脉切割橄榄石形成网状结构，典型的是蛇纹石脉，在脉中心有磁铁矿带，在脉缘或无磁铁矿脉中具有锂蛇纹石-斜长石固溶体的成分，表明早期具有低二氧化硅和低氧势的化学条件蛇纹石化阶段。绿片岩相条件下橄榄石和斜长石蚀变的热力学模型表明，斜长石蚀变形成葡萄石 + 绿泥石和绿帘石 + 绿泥石的二氧化硅潜力可能高于橄榄石蛇纹石化。另一方面，葡萄石+绿泥石形成的氧势低于绿帘石+绿泥石形成的氧势，与橄榄石蛇纹石化的氧势相当。从观察和分析来看，