P. Dudoignon et al., HYDROTHERMAL ALTERATION ASSOCIATED WITH RIFT ZONES AT FANGATAUFA ATOLL (FRENCH-POLYNESIA), Bulletin of volcanology, 58(8), 1997, pp. 583-596
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.