Newly formed phyllosilicates in rock matrices and fractures from CRP-3 core (Antarctica): an electron microscopy study G. GIORGETTI 1, *, F. S. AGHIB 2 , K. J. T. LIVI 3 , A.-C. GAILLOT 3 AND T. J. WILSON 4 1 Dipartimento di Scienze della Terra, Universita ` di Siena, Italy, 2 CNR-Istituto per le Dinamiche dei Processi Ambientali,Milano,Italy, 3 Department of Earth and Planetary Sciences, The John Hopkins University, Baltimore, USA,and 4 Department of Geological Sciences, Ohio State University, USA (Received 5 September 2005; revised 12 October 2006) ABSTRACT: A scanning and transmission electron microscopy study has been performed on Oligocene glacio-marine sediments, Devonian sandstones, and Jurassic dolerites recovered during CRP-3 drilling in the Ross Sea (Antarctica). Newly formed clay minerals occur in the rock matrices and as fillings in veins and faults which crosscut the whole sequence. Authigenic clays in sediments consist of beidellite-montmorillonite, berthierine/chlorite intergrowths and illite. Al,K-rich smectites and kaolinite occur in the Devonian sandstones. Saponite, berthierine/chlorite intergrowths, and Fe-hydroxides develop in the altered dolerites. Hence, the composition of the secondary phases depends also on the geochemistry of the rock they grow in. Within each sample, the same authigenic minerals form in the matrix and in the vein/fault. Clays precipitated from fluids, with variable f O 2 values, which circulated in the system during the contemporaneous diagenetic and faulting events. KEYWORDS: Ross Sea, CRP-3 core, authigenic phyllosilicates, chlorite, illite, smectite, SEM, TEM, EELS. Clay minerals in drill-core sequences can be powerful tools for identifying provenance, post- depositional changes during diagenesis, or very low-grade metamorphism. The Cape Roberts Project (CRP) drilled three wells (Cape Roberts Science Team, 1998, 1999, 2000) on the western margin of the West Antarctic Rift System (Victoria Land Basin, Ross Sea, Antarctica). Some authors have investigated clay minerals from the three CRP cores and from previous drillings in the Ross Sea (CIROS, DSDP site 270 and site 274) focusing their attention mainly on smectites. Claridge & Campbell (1989) and Lopez-Galindo et al. (1998) stated that smectites from borehole sedimentary sections of CIROS-1, DSDP site 270, and DSDP site 274 are predominantly of detrital origin with minor amounts of authigenic Fe- and Mg-rich smectites. Based also on previous papers (e.g. Cole & Shaw, 1983), these authors stated that authigenic smectites in the oceans are nontronitic, while detrital smectites are more aluminous. CRP-1 and CRP-2 cores recovered Quaternary- Miocene-Oligocene age sediments, the mineralogy and clay content of which have been studied by Ehrmann (1998) and by Setti et al. (1998, 2000). Similar to the findings in the previous studies, dioctahedral smectites have been interpreted to be of detrital origin and the basement rocks of the Transantarctic Mountains (TAM) have been recog- nized as probable source terrains. Setti et al. (2001) and Wise et al. (2001) investigated smectites in the upper Oligocene sediments and in the basement sandstones and dolerites recovered in CRP-3 core. Clay minerals, particularly smectites, in CRP-3 have * E-mail: giorgettig@unisi.it DOI: 10.1180/claymin.2007.042.1.03 ClayMinerals, (2007) 42, 21–43 # 2007 The Mineralogical Society