We report chemical compositions of authigenic alteration minerals in subglacially erupted hyaloclastite breccias from James Ross Island, Antarctica, which provide a geochemical proxy for paleoenvironment. Analyses of zeolites (phillipsite and chabazite) from several lava-fed deltas on the island were compared with published compositions of zeolites from known freshwater and marine environments. Using values of (Na + K)/Ca = 3.0 (for phillipsites) and 1.0 (for chabazites), the data successfully distinguish between marine and freshwater alteration. However, our study also suggests that volcanic breccias may not be sealed to fluids, even when the pore spaces are completely filled. The James Ross Island data show that, at higher elevations in lava-fed deltas, the zeolites preserve their original freshwater-influenced (i.e., glacial) compositions, whereas at lower elevations, (Na + K)/Ca ratios are generally higher and indicate a later marine influence. We suggest that water may penetrate several kilometers into the volcanic pile by movement along fractures and/or grain boundaries.