Clay minerals in basalt-hawaiite rocks from mururoa atoll (French polynesia). I. mineralogy

Clay minerals in chilled or brecciated margins (altered glass) and massive inner crystalline parts (mesostasis) of three basalt-hawaiite bodies from Mururoa Atoll (French Polynesia) have been studied in order to compare their chemical and mineralogical compositions. Polyphase assemblages comprise di- and trioctahedral phases, both of which consist of non-expandable layers (chlorite, celadonite) and two types of expandable layers (saponite and Fe-rich smectite or 'nontronite-like' material). The presence of the Fe-rich clays is supported by the presence of the X-ray diffraction 060 peak at 1.51-1.52 Å and of the infrared absorption bands at 875 and 822 cm (Fe-Al-OH and Fe-Fe-OH groups, respectively). The chemical composition of the Fe-rich smectites does not fit with the theoretical nontronite field. The layer charge averages 1 per SiO making these Fe-rich smectites close to 'celadonite-type' clays. This could explain the presence of mixed-layer celadonite-smectite. Plotted in an M/4Si vs. Fe/sum octahedral cations diagram, the chemical compositions of clay minerals in the mesostasis form a continuous field limited by the celadonite high-charge nontronite-like smectite and chlorite end-members. The clay assemblages are different from those formed in hydrothermal systems or low-grade metamorphic conditions which are characterized by the sequence: saponite → randomly ordered chlorite-smectite mixed-layered minerals (MLMs)→ corrensite → chlorite. The systematic presence of Ferich clays either in the altered chilled margins or in the massive inner parts of the basalt-hawaiite bodies (high-charge nontronite-like smectite and mixed-layer nontronite-celadonite) makes the Mururoa seamount a potential terrestrial analogue for Mars surface exploration