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The effect of trace concentrations of Ti on the rate and mechanism of hydrogen diffusion in pure forsterite was investigated experimentally. Forsterite doped with 350–400 ppm Ti (predominantly octahedral Ti 3+ , minor tetrahedral Ti 4+ ) was prepared by diffusing Ti into pure synthetic forsterite at high temperature (1500 °C), very low oxygen fugacity (~QFM-5) at atmospheric pressure. The Ti-doped forsterite was then diffusively hydroxylated in a piston-cylinder apparatus at much lower temperatures (650 –1 000 °C) and higher oxygen fugacities, at 1.5–2.5 GPa, with chemical activities buffered by forsterite-enstatite or forsterite-periclase and partial pressure of H 2 O equal to total pressure. This produced hydrogen concentration-distance profiles of several hundred micrometers in length. Diffusion of hydrogen through the Ti-doped forsterite, even at very high  f  O 2  , does not lead to redox re-equilibration of the high Ti 3+ /STi ratio set during the synthesis of the starting material at extremely reducing conditions—the metastable point defects are partially preserved.

Hydrogen diffusion in Ti-doped forsterite and the preservation of metastable point defects