Preliminary Results from Petrography and Crystal Size Distribution (CSD) Analysis of Lavas from Nelson Island, South Shetland Islands (Antarctica)

Geological fieldwork was conducted on Nelson Island (South Shetland Islands, West Antarctica) during January and February 2023, with logistical support provided by the Czech Antarctic Research Center and in-kind assistance from TÜBİTAK MAM Polar Research Institute. This study presents the petrography and preliminary crystal size distribution (CSD) results of the volcanic rocks from the Nelson Island-Stansburry Peninsula.

Nelson Island is situated in the northwestern region of the Antarctic Peninsula, within the South Shetland Islands. The dominant lithologies in the study area are volcanic and intrusive rocks, with a minor presence of sedimentary rocks distributed throughout the ice-free areas. The volcanic rocks are primarily basalt/basaltic andesite lavas, accompanied by associated pyroclastic rocks. Petrographic analyses reveal that the lavas are predominantly composed of plagioclase and orthopyroxene crystals, with occasional olivine. Plagioclases occur as phenocrysts and microlites, with phenocrysts exhibiting labradorite composition (An_50–70). The basaltic lavas display phaneritic to hemicrystalline-porphyritic textures, with pilotaxitic to intersertal groundmass. Disequilibrium textures, such as sieve-textured plagioclase, multiple generations of plagioclase, and embayed pyroxenes and plagioclases, are prominent.

To understand magma chamber processes, the crystal size distributions (CSD) of two basalt samples from Nelson Island were analyzed. A total of 400 crystals were measured, yielding average results indicating a relatively short magma residence time of 10.6 years and a population density (​n₀) of 19.7 mm⁻⁴. These results produce a concave-up trend in the population density versus size diagram. Petrographic and CSD data collectively highlight dynamic disequilibrium crystallization conditions, likely influenced by variations in temperature and pressure within the magmatic system. Future petrological and geothermobarometric analyses will refine these interpretations, providing deeper insights into the evolution and magma chamber processes of Eocene magmatism in the South Shetland Islands.