HYDROTHERMAL ALTERATION ASSOCIATED WITH RIFT ZONES AT FANGATAUFA ATOLL (FRENCH-POLYNESIA)

The Mururoa and Fangataufa atoll basement consists of superimposed sub marine and subaerial lava flows which have been intruded by late volca nics. The intrusions have developed large hydrothermal alteration halo es throughout the basaltic wall rock. The cuttings of the Natice-1 and Mitre-1 holes, drilled into the submarine volcanic pile at Fangataufa atoll, show a vertical zonation of clay minerals ranging from 270 to 850 m depth. The newly formed clay minerals occurring from top to bott om of the altered pile are: dioctahedral aluminous smectites, saponite , an intimate assemblage of saponite with two random chlorite/saponite mixed layers ansi an intimate assemblage of one random chlorite/sapon ite mixed-layer with one ordered chlorite/saponite mixed layer and one chlorite below 816 m depth. These clay mineral assemblages indicate a general increase in the chloritic component with depth. They are asso ciated throughout the pile with secondary carbonates and quartz. The p artial derivative(18)O and partial derivative(13)C of calcite and part ial derivative(18)O of clay minerals, on the one hand, and the intimat e mixtures of trioctahedral species, on the other, suggest a general c ooling with the evolution of a paleogeothermal gradient from approxima tely 300 degrees C/km during the crystallization of chlorite to 150 de grees C/km for the late calcite precipitation.