The petrogenesis of analcime in the Coppermine Formation on Robert Island, South Shetland Islands, Antarctica

Abstract

: The South Shetland Islands were shaped by island arc volcanism that occurred from the Jurassic to the Quaternary. Robert Island is located in the center of this archipelago, and Coppermine Peninsula, located on the southwestern part of Robert Island, exposures significant rock outcrops of basalts and andesitic-basaltic agglomerates of the Coppermine Formation. The investigated samples were collected during Turkish Antarctic Expedition 2 (TAE-II), in March–April 2018, from the area northeast of Triplet Hill. The volcanic rocks exhibited an amygdaloidal microlithic-porphyritic texture. Subhedral to anhedral phenocrysts of labradorite, augite, and olivine were observed with holocrystalline groundmass composed of plagioclase, pyroxene, and opaques. Petrographic studies revealed that the vesicles were initially coated with clay minerals, and X-ray diffraction studies showed that mostly analcime and carbonates, and less amount of zeolites filled the vesicles. Herein, it was aimed to discuss the analcime formation. Fluid inclusion studies performed on the analcime revealed homogenization temperatures ranging from 83 to 268 °C. The eutectic temperatures of the fluid inclusions suggested that these minerals were formed from NaCl-dominated solutions that contained a limited amount of MgCl 2 and CaCl 2 . The salinity of these inclusions ranged between 0.2–2.9 wt.% NaCl equivalent, and exhibited final ice melting temperatures of –1.7 to –0.1 °C. These salinity values, which are lower than the average salinity values of seawater, suggest that the formation of the analcime and the fibrous zeolites (thomsonite and stilbite) was closely associated with meteoric solutions. Consequently, the salinity values of the fluid inclusions suggest the mixing of meteoric waters form from glaciers and seawater at different rates.