Incompatibility between serpentinization and epidote formation in the lower oceanic crust: Evidence from the Oman Drilling Project

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2023-02-04 DOI:10.1111/jmg.12713
Toshio Nozaka, Yamato Tateishi
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Abstract

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.

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大洋下地壳蛇纹石化和绿帘石形成之间的不相容性:来自阿曼钻探项目的证据
绿帘石是一种典型的绿片岩相矿物,尽管其他矿物普遍存在,表明温度条件相似,如绿泥石、阳起石、史前岩和蛇纹岩,但绿帘石在下洋地壳中普遍稀少。为了找出原因,我们对国际大陆科学钻探计划(ICDP)阿曼钻探项目采样的阿曼蛇绿岩下地壳岩石进行了岩石学分析。岩石学观察显示,正如预期的那样,斜长石静态蚀变形成的绿帘石数量随着深度的增加而减少。由于指示从角闪岩到亚绿片岩相的广泛温度条件的矿物组合出现在整个岩芯中,而丰度没有系统变化,因此绿帘石数量的减少不能用蚀变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,绿帘石不存在或罕见,而与橄榄石的蛇纹石化密切相关的史前岩则相反。在同时含有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,绿泥石切割或取代斜长石,覆盖相邻的橄榄石,并与绿泥石相连 + 橄榄岩脉切割网状蛇纹。绿帘石的分布和赋存方式表明其形成与蛇纹石化的主要阶段脱钩。切割橄榄石形成网状结构的蛇形矿脉通常是在矿脉中心具有磁铁矿带的丽锌矿,并且在矿脉边缘或无磁铁矿矿脉中具有丽锌矿-铬铁矿固溶体的成分,这表明在蛇纹石化早期存在低硅和低氧势的化学条件。绿片岩相条件下橄榄石和斜长石蚀变的热力学模型表明,二氧化硅有可能使斜长石蚀改形成预热器 + 绿泥石和绿帘石 + 绿泥石可能高于橄榄石蛇纹石化。另一方面,预热器的氧势 + 绿泥石的形成低于绿帘石 + 绿泥石形成,可与橄榄石蛇纹石化相媲美。根据观测和分析,绿帘石的形成受到橄榄石蛇纹石化的抑制,这为下洋地壳的蚀变保持了还原条件。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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