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Preface
Solid earth sciences
Session convener-recommended article JpGU Meeting 2015
201701201701
Interplay of irreversible reactions and deformation: a case of hydrofracturing in the rodingite-serpentinite system
Nishiyama T, Yoshida-Shiosaki C, Mori Y, Shigeno M
rodingite, singular value decomposition method, hydrofracturing, perovskite, reaction zone, serpentinite, irreversible reactions
Compositional cross section of diopside across the reaction zone.
(a) Compositional variation of Fe-rich diopside in the ‘two-diopside zone’.
(b) Compositional variation of Fe-poor diopside in the ‘two-diopside zone’.
The interplay of irreversible reactions and deformation during regional metamorphism was analyzed in the reaction zones between rodingite and serpentinite. Rodingites are leucocratic rocks found commonly in serpentinites and are considered metasomatic products of Ca-rich fluid. Rodingites occur ubiquitously in serpentinite from the Nomo metamorphic rocks, western Kyushu, a Cretaceous accretionary complex of greenschist to epidote–amphibolite facies condition. We used the singular value decomposition (SVD) method to analyze rodingitization reactions based on mineral compositions of a rodingite sample and a possible protolith (clinopyroxene gabbro). The resultant reaction implied that the rodingite was formed due to the addition of considerable amounts of CaO and H2O to the protolith, whereas other components such as SiO2, AF (Al2O3 + Fe2O3), and FM (FeO + MgO) were conserved. The Nomo rodingites are associated with reaction zones between serpentinite. A careful study on the compositional variations of diopside and chlorite in the rodingite, serpentinite, and the reaction zones between them showed a disequilibrium crystallization of diopside in the reaction zone. Diopside shows a serrated variation in composition across the reaction zone together with disequilibrium signature within grains (composite grain consisting of Fe-rich and Fe-poor parts). These features clearly indicate that diopside crystallized in a relatively short period compared to the metamorphism by irreversible reactions. Another conspicuous feature is that perovskite occurs in some reaction zones, whereas titanite is common in rodingites; this indicates that the reaction zone is poorer in SiO2. The tremolite veins are typically derived from the reaction zone into serpentinite; showing this is a good example of hydrofracturing associated with the formation of the reaction zone. All the reactions obtained by the SVD method that contribute to the formation of the reaction zones consumed CaO and evolved H2O, strongly suggesting that the hydrofracturing is caused by a rapid increment of fluid pressure during the progression of the reactions. The reactions consumed considerable amounts of SiO2 in the reaction zone, which is consistent with the occurrence of perovskite, and the conservation of AF and FM components.
