Lake Vostok, the largest Antarctic sub-glacial lake (14,000 km2), lies beneath nearly 4 km of ice. Sub-glacial geophysical observations and studies of ice accreting at the lake-glacier interface are the only means available to obtain information on the environment and dynamics of this huge water body formed several million years ago. Accretion ice has been studied using high-resolution synchrotron X-Ray micro-fluorescence. For the first time, liquid brine micro-droplets (3--10 ¿m) are observed, coexisting with large irregular sulfur-rich aggregates (10--800 ¿m) containing gases and a mixture of very fine particles. Most of these objects are sequestered inside large crystals that grew slowly after ice formation. Their structure and composition support the existence of hydrothermal activity at the lake bottom and the occurrence of haline water pulses carrying fine solid debris and eventually biota from a deeper evaporitic reservoir into the lake.