Garnet-quartz intergrowths in graphitic pelites: the role of the fluid phase

Garnets with an unusual inclusion pattern of cylindrical quartz intergrowths have been found to develop exclusively in the presence of graphite. The intergrowths consist of quartz rods, 1–5 µ m in diameter, originating at the sector-zone interfaces in the garnet with the long axes normal to the crystal faces. The lattice orientation and continuity of the quartz suggests that the interphase boundaries between the quartz and garnet are epitaxially related and that new material was added to the tube as the crystal face of the garnet grew. In the presence of a C-O-H fluid, at the temperatures and pressures recorded, ( P = 6.5 kbar, T = 500°C), the amount of CO 2 present restricts the solubility of SiO 2 in the intergranular fluid phase, where the oxygen fugacity ( f o 2 ) is below the Quartz-fayalite-magnetite (QFM) buffer, and within the stability field of graphite. The reduced solubility will lower the concentration of SiO 2 in solution, and hence restrict its ease of transport via the fluid, resulting in an excess of SiO 2 at the site of garnet growth. Under such conditions the SiO 2 is incorporated in the growing garnet in the form of the cylindrical quartz intergrowths.