PETROGRAPHY AND QUANTIFICATION OF SECONDA RY PHASES IN THE SUBMARINE AND SUBAERIAL LAVA FLOWS OF MURUROA ATOLL - MEASUREMENT OF EXCHANGE SURFACES USING IMAGE TREATMENT SOFTWARE

The quantification of secondary phases using back-scattered electron S EM images of two representative thin sections of altered basaltic lava flows from Mururoa atoll gives details about the phases accessible to late alteration fluids. In the pillow lavas the glass and olivine dis solution surfaces reach 270 mm(2)/g and 250 cm(2)/g respectively. The ratio of olivine/glass dissolution is 1/10. in the subaerial basalts t he increasing percentage of clay minerals from the mesostasis towards the vein (6 to 30%) is associated with an increase of the exchange sur faces developed along the crystallographic faces of the primary microc rystals from 150 to 630 cm(2)/9.