LOW-TEMPERATURE ALTERATION OF BASALTS IN OCEANIC DOMAIN AND BEHAVIOR OF RARE-EARTH ELEMENTS (SEADMA 1 CRUISE, MID-ATLANTIC RIDGE, 20-DEGREES-24-DEGREES-N)

''Zero age'' tholeiitic pillow basalts were dredged from the axial zon e of the Mid-Atlantic Ridge in the vicinity of 20 degrees and 24 degre es N during SEADMA 1 cruise. These basalts underwent three types of al teration : (i) a deuteric alteration, during which protruding purple T i - rich diopside cristallized in primary voids; (ii) a low temperatur e alteration, which is responsible for the formation of black bands an d halos, where celadonite nontronite, and iron oxy-hydroxides replaced the mesostasis and filled primary voids. These minerals formed at low temperature from mixed fluids (sea water / hydrothermal fluids). This alteration process is characterized by an increase of K2O, Rb/Ba, H2O +, Fe(2)O(3)t, the oxidation ratio and delta(18)O in black halos and b lack bands with respect to the adjacent fresher part of the samples. T he water / rock ratio is very low (0.2 in mass), (iii) a very discrete low temperature alteration, responsible for the cristallization of sa ponite, and then calcite. This process is thought to be mainly due to seawater circulation. Black bands and black halos are either richer or poorer in light rare earth elements (La and Ce) than their fresher eq uivalent. It is shown that the mobilization of light REE is related to alteration, and that LREE are trapped in secondary minerals, e.g. iro n oxy-hydroxides. The LREE mobilization is not linked to the formation of black halos and is due to another alteration process, in which hyd rothermal fluids are probably involved.