Compaction of Hyaloclastite from the Active Geothermal System at Krafla Volcano, Iceland

Figure 4

Colour-coded phase distribution maps of the matrix of hyaloclastites from different depths within the hydrothermal reservoir at Krafla volcano, NE Iceland. Maps, produced by SEM-EDS QEMSCAN® (Quantitative Evaluation of Minerals by SCANning electron microscopy) with 2 µm step size, show distribution of all identified phases with porosity in white (left), smectite and zeolite with other solids in grey and porosity in white (centre), and pore space in black with all solid phases in grey (right). (a–c) The surface sample has a matrix of angular to subangular shards of glass in heterogeneously distributed, fine-grained smectite and zeolite. (d–f) The matrix of the sample from 70 m depth is texturally similar, though it lacks zeolites. Instead, smectite dominates and also infills vesicles within the larger, vesicular glassy clasts. (g–i) The sample from 76 m has a matrix similarly dominated by smectite, with additional calcite infilling pore space in the matrix and in the round intraclast vesicles. The pore space is notably reduced from the surface and 70 m samples. (j–l) The matrix of the sample from 556 m is almost indistinguishable from the larger clasts, which are poorly defined. Limited porosity further hampers their distinction. Zeolite-dominated veins crosscut the sample, and zeolite is additionally distributed heterogeneously throughout the sample. Smectite is also widely distributed and occasionally takes rounded form suggestive of the infill of round vesicles. Minor calcite is present in what would have been pore space, and porosity is limited to fine fractures and very small isolated pores. (m–o) The dense sample from 732 m shows traces of glassy clasts, with a zeolite-, smectite-, and actinolite-bearing matrix. Zeolite occasionally infills round pores within glassy clasts and is additionally distributed within the glassy fragments. Smectite is distributed throughout the matrix, occasionally appearing in patches. Some areas contain patches of calcite, which appears to infill pore space. Remaining pores are generally fine fractures.