OH and H2O distributions in garnet of diamond-free and diamond-bearing garnet-clinopyroxene rocks from the Kokchetav Massif

Abstract:

Dia-free and Dia-bearing Grt-Cpx rocks occur in the Kumdy-Kol area of the Kokchetav Massif. In order to determine fluid conditions during the Kokchetav metamorphism, water in Grt and Cpx, and their distribution were examined by micro FT-IR method. Structural OH and molecular H₂O in Grt and Cpx both in Dia-free (sample no. XX16) and Dia-bearing Grt-Cpx rocks (nos. 24997 & 25018) were detected. “Wet” Grt and “dry” Grt were recognized in the same sample, respectively.

The mineral assemblage of Grt-Cpx rocks are as in the followings. Dia-free: Grt+Cpx+Cal+Qtz+Kfs+Ttn. Ttn contains exsolved coesite needles in the core. Dia-bearing: Grt+Cpx+Rt+Dia. Large-grained “microdiamond” (ca. 150 μm at average) occurs as an inclusion in Grt and Cpx and as an interstitial phase in their grain boundaries. Rt occurs as intergranular phase in the matrix. Some of Rt grains show lamella texture.

IR absorption bands of Grt and Cpx were determined by the Gaussian/Lorentzian fitting of IR spectra. Molecular H₂O band (3400-3430 cm^-1) and structural OH band (3555-3570 cm^-1) were detected in Grt. Whereas OH band belongs to OH incorporated in Grt structure, H₂O band is assigned to molecular H₂O in submicron fluid inclusions. OH and H₂O contents were estimated based on the assumption that IR coefficient of H₂O is the same as that of OH in Grt (8770 L/mol/cm²; Katayama et al., 2005). The results are as follows:

Dia-free no. XX16 (OH, H₂O): (568, 185) - (981, 870) ppm (wt. H₂O),

Dia-bearing no. 25018 (OH, H₂O): (391, 294) - (1165, 1218) ppm,

Dia-bearing no. 24997 (OH, H₂O): (0, 0) - (528, 778) ppm.

Structural OH contents in Grt in both Grt-Cpx rocks increase with increasing molecular H₂O. OH contents in Grt in Dia-free rock are higher than those in Dia-bearing rocks. Mapping of water distribution in the rock (no. 24997) showed that a Grt grain near a Dia grain (cuboid) contains total water of 1358 ppm, but the other grains apart from the Dia contain very low total water (<40 ppm). Water in Cpx also shows the similar trends. The heterogeneity of water distribution in the Dia-free Grt-Cpx rock was previously reported by Sakamaki & Ogasawara (2014). Our micro FT-IR study clarified the heterogeneous distribution of H₂O-rich fluid both in the Dia-bearing and Dia-free Grt-Cpx rocks, which might have controlled the Dia formation during the Kokchetav UHP metamorphism.