The nature and origin of smectites in the Kerguelen-Heard Archipelagoes of the southern Indian Ocean

Abstract

The clay phases of the Quaternary volcanics, soils and sediments from the northern part of the Kerguelen-Heard Plateau, southern Indian Ocean, are essentially well-crystallized smectites. The origin of these smectites was studied based on variations in their mineralogy and their different stages of development in the different environments. This study indicates: (1) Two major groups of smectites occur in the hydrothermal alteration products of the Kerguelen Island volcanic formations Mg s.s. and Fe-Mg saponites of the basalts, and Al-Fe and Al beidellites in the other formations. In the soils formed on the volcanic substratum, the well-crystallized smectites are mainly of the Al beidellite type and, secondarily, of the Al-Fe beidellite type. (2) In the Morbihan Gulf sediments, the well-crystallized smectites are dominant and of the Al-Fe and Al beidellite type: these smectites are inherited from the volcanic formations and soils of the Kerguelen Islands. (3) In the Quaternary sediments of the Kerguelen-Heard Plateau, the well-crystallized and relatively abundant smectites are of the Mg-rich, Al nontronite type. The absence of related links with the Kerguelen Island smectites, the substantial dissolution of the siliceous tests, the degree of silica saturation of the water context and the occurrence of glauconite, the first stage of formation being a rich-Mg ferriferous smectite, supports the hypothesis of an authigenous origin for these smectites rather than originating by inheritance from the volcanic islands.