Hydroxyl in diopside in diamond-free UHP dolomitic marble from the Kokchetav Massif

The amount of hydroxyl incorporated in diopside in diamond-free UHP marble from the Kokchetav, northern Kazakhstan was examined with micro-Fourier transform IR spectroscopy. The examined diamond-free UHP marble has the assemblage under UHP conditions dolomite + aragonite + diopside + garnet + Ti-clinohumite (+ forsterite) and is categorized as diamond-free dolomitic marble previously described (Ogasawara et al., 2000). This marble was subjected to the physical condition of > 6 GPa and > 1000 C, and extremely low XCO2 condition (< 0.01). Diamond could be unstable in such extremely low XCO2 condition (Ogasawara et al., 2000; 2004). Diopside in dolomitic marble contains no lamella and has no evidence of retrograde alteration. Diopsides in other types of UHP marbles from the same region have K2O-bearing lamellae such as K-feldspar and phengite. Two stage exsolution was confirmed in diopside in calcite marble (K-feldspar at first stage and phengite at second one), implying that OH could survive at lower P than K2O component (Ogasawara et al., 2002). Therefore, diopside in dolomitic marble has been expected to contain significant amount of hydroxyl. Doubly polished thin section of sample no. Y676 with thickness of 150 micrometers was prepared for FTIR analysis. The polished section was kept in acetone and put in a desiccator. Thirty diopside grains in the same thin sections were analyzed. All diopsides exhibit only one major hydroxyl absorption band at 3645 cm-1. Johnson et al. (2002) reported similar absorption band in diopside from marble xenolith from the Cascade Slide. Intensity of hydroxyl bands of the present samples varied because of the random orientation of diopside grains. Amphibole (probably tremolite) band appeared at 3685 cm-1 in almost all diopsides. No amphibole was recognized in diopside under microscope; the intensity of amphibole bands was heterogeneous even in the same diopside grain. This may be caused by submicroscopic amphibole lamellae as described by Skogby (1990). In order to estimate the amount of hydroxyl in diopside, we chose areas in 30 diopside grains where amphibole band did not appear or was weak. Non-polarized spectra of 30 analyzed spots were averaged. Applying the procedures described by Katayama & Nakashima (2003), we obtained that diopside contained 1020 ppm H2O by weight. The source of OH in submicroscopic amphibole could be derived from precursor diopside, not a fluid infiltration origin during retrogression. Microscopic observations and isotopic studies on diamond-free dolomitic marble have shown no evidence for retrograde fluid effect (Ogasawara et al., 2000; Ohta et al., 2003). Therefore, the precursor diopside might contain much higher hydroxyl than 1000 ppm. Diopside in diamond-free dolomitic marble from the Kokchetav could contain at least 1000 ppm OH under UHP conditions. Large part of hydroxyl survived in diopside during the journey to the surface. These results are new evidence for UHP metamorphism in diamond-free carbonate rocks.