The iron-titanium oxide minerals of three troctolites from the Freetown layered gabbro have been analysed using an electron microprobe. Comparisons of the analyses of the iron-titanium minerals with the data of Buddington and Lindsley (1964) provide temperature and oxygen fugacity information relating to particular magnetite-ilmenite grains. Magnetite-ilmenite grains occurring in different silicate environments within any one single troctolite rock have equilibrated at differing times in the

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... ling history of that rock and this enables part of that history to be established. The partial cooling histories established for the three specimens can be clearly distinguished, and are strongly influenced by the appropriate buffer reactions.