Clay mineral assemblages as indicators of hydrothermalism in the basal part of the CRP-3 core (Victoria Land Basin, Antarctica)

The CRP-3 drilling project collected sediments from 3 to 939 mbsf (metres below sea floor) in the Victoria Land Basin in Antarctica. The upper sequence (down to approximately 790 m bsf) is of Cenozoic age and made up of detrital glaciogenic sediments; the characteristics of clay minerals in this part have been reported elsewhere. Here, the compositional features of clay minerals in the lower sequence such as conglomerates, Devonian sandstones and dolerites are described and genetic processes clarified. Clay minerals in the deepest part of the sequence derive from the alteration of different lithologies that mostly make up the sedimentary basin. Two clay mineral assemblages were characterized through analysis by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). From 790 to 823 mbsf, samples consist of authigenic smectite of variable chemical composition forming imbricated texture of plates or flakes. The smectites probably result from hydrothermal/diagenetic transformation of earlier minerals. The primary smectite cement underwent reorganization during shearing and cataclasis. The lowest part of the sequence (below 823 mbsf) is characterized by an assemblage of kaolinite, mixed-layer illite-smectite, Fe oxyhydroxide, sporadic smectite and poorly crystallized illite. It reflects a stronger alteration process than that recorded in the upper units of core CRP-3, related to hydrothermalism connected with the intrusion of an igneous body. Both assemblages show clear differences in particle morphology, texture and smectite composition to the clay assemblages found in the Cenozoic glaciomarine sediments in the upper sequence. The different phases of alteration appear related to the processes of rifting, exhumation and faulting that characterized this region since the Mesozoic.