Two groups of fluid inclusions in the Yunotani eclogite from the Hida–Gaien Belt: Implications for changes of fluid salinity during exhumation


Late Paleozoic Yunotani eclogites in the Itoigawa–Omi area of the Hida–Gaien Belt were subjected to a blueschist– facies recrystallization and deformation after the peak eclogite–facies metamorphism. We studied fluid inclusions in quartz domains of different metamorphic stages in the retrograde eclogite. Coarse–grained quartz filling pressure shadow of porphyroblastic garnets contains transgranular fluid inclusions (Group–I) with high salinity [6.0–8.0 eq mass% NaCl, with a mean value 7.2 ± 0.6 eq mass% NaCl (n = 12)]. In contrast, transgranular fluid inclusions in later stage quartz–rich band (Group–II) in parallel to a penetrative foliation developed in matrix are characterized by lower salinity [1.1–5.8 eq mass% NaCl, with a mean value 3.4 ± 1.2 eq mass% NaCl (n = 12)]. The occurrences of fluid inclusion trails suggest that each group corresponds to the fluids entrapped prior to the blueschist–facies recrystallization and the fluids entrapped during the blueschist– facies stage, respectively. The Group–II fluid inclusions formed at blueschist–facies conditions support the idea that aqueous fluids in the subduction channel have a salinity similar to seawater (~3.5 eq mass% NaCl). Moreover, the Group–I fluid inclusions indicate a possibility that aqueous fluids at the eclogite–facies depth in subduction channels possess a salinity higher than seawater.

Journal of Mineralogical and Petrological Sciences, v. 114, no. 6, p. 302-307, doi:10.2465/jmps.190729