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

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.