Nature and Origin of Volcanogenic Salts Deposits around the Crater of Erebus volcano, Antarctica

Volcanogenic salt deposits (VSD) are widespread around the summit crater of the active 3794-meter high Erebus volcano in Antarctica. The distribution, composition and formation of the salts provide insight into degassing of the phonolite lava lake and the interaction of the gas plume with the snow and rocks around the summit crater. The greatest accumulations of VSD are located within 1 km of the active crater and are found under rocks and in crevices in the lava flows as white to yellow incrustations, massive deposits, efflorescences, and needles. The VSD are most abundant downwind of the crater where individual VSD accumulations of 20g or more are common. The VSD are not associated with fumaroles or fumarolic ice towers but form under the ambient -20 to -60 oC temperatures. The formation of the salts under such extreme temperatures conditions may have application to salt formation at other active planetary volcanoes. Previous XRD analyses of Erebus VSD have positively identified halite, gypsum, alunite, thenardite, sylvite, mirabilite, and calcite, with numerous tentatively identified phases. SEM observations of VSD indicate that the salts are well crystallized on a very fine scale, with crystal size ranging between 1 to 20 microns. One of the most common crystal morphologies consists of well-formed columnar crystals, although blocky, platy and acicular crystals are also observed. Qualitative electron microprobe chemical scans indicate that a wide range of elements are present in the various VSD phases, including Al, Cl, F, Fe, K, Mg, Mn, Na, P, S, and Ti, with multiple salt phases present in each sample. Different crystal morphologies of salt are chemically distinct; the columnar crystals appear to be largely composed of Al, Cl, F, and Fe, platier crystals appear to be composed largely of Na and S, and flat, rectangular crystals are composed of Na and Cl. The Erebus plume, snow, and ice samples around the summit region contain large quantities of Cl, F, and S, with lesser amounts of Al, Ca, Fe, K, Mg, Mn, and Na, some of which are major components of the VSD. This suggests that although some of the elements that form the VSD are derived from the plume, interaction with rock fragments may be an important component of the salt forming process, allowing for the scavenging of other elements. The VSD may form from freeze-thaw cycles breaking up volcanic rocks, exposing fresh glass surfaces, which are then weathered/altered by reaction with the plume, liberating Al, Fe, Mn, Mg, and Ti. These liberated elements from the rock then combine with elements in the plume to form the well-crystallized salt deposits.