Meteoric alteration of Icelandic volcanic glass: long-term changes in the mechanism

Basaltic hyaloclastites from Hengill, Husafell and Vatnajokull in Iceland, were studied. These glasses, ranging from 0.015 to 3.1 Ma in age, and alter ed by meteoric water, are surrounded by alteration layers. In the youngest samples (< 1 Ma), these layers are formed by precipitation of poorly crysta lline smectite, after total breakdown of the glassy network. This anisotrop ic, porous layer was reproduced during laboratory experiments (Crovisier et al., 1990). In older hyaloclastites, zeolites (chabazite, phillipsite, ana lcime) are always observed, sometimes combined with calcite. Zeolites are s ystematically associated with a gel layer. This gel contains 10 % water, is depleted in Na and Ca, and seems amorphous and isotropic. delta(18)O measu rements indicate, for both alteration types, major modifications of the ini tial glass structure. ((C) Academie des sciences / Elsevier, Paris.).