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

Figure 2

Photomicrographs of hyaloclastite from different depths within the hydrothermal reservoir at Krafla volcano, NE Iceland. Images in plane polarised light. (a) A surface sample with subangular to subrounded clasts and heterogeneously distributed porosity. Clasts are predominantly glassy and vesicular. Zoomed in areas shown in (b) and (c) marked by the blue and magenta squares, respectively, show that the matrix also comprises angular glassy fragments and few crystal fragments, with some very fine-grained patches. (d) The clast-rich sample from 70 m depth contains rounded to subrounded, glassy, vesicular clasts which are occasionally fractured, giving rise to more angular fragments. Pore space consists of intragrain rounded vesicles and voids surrounding larger clasts. The matrix seen in (e) and (f) marked by the blue and magenta squares, respectively, comprises dense angular shards of glass, occasionally oxidised (black) and a few crystal fragments with a higher proportion of very fine grains. Vesicles in the glass fragments are occasionally infilled. (g) The sample from 76 m depth is texturally similar to that from 70 m, though the largest clast sizes are notably smaller, and pore spaces adjacent to clasts are correspondingly smaller. The vesicular, glassy clasts are subrounded and frequently contain fractures, and a few large lithic clasts are seen. The matrix, shown in (h) and (i) marked by the blue and magenta squares, respectively, comprises angular glassy fragments set in a cement-like fine-grained phase. (j) Subsurface hyaloclastite from 556 m is matrix supported and contains some scoria clasts which appear compacted; overall, the material is darker and denser than the shallower samples. Infilled fractures/veins cut through both the clasts and the fine-grained matrix, as seen in (k) and (l) marked by the blue and magenta squares, respectively. Pore space is limited, and vesicles within the glassy fragments are infilled. (m) The 732 m sample is largely dense with partly vesicular, oxidised (black) subrounded to subangular clasts, with the vesicles infilled. Pore space in the matrix is also infilled, as seen in (n) and (o) marked by the blue and magenta squares, respectively, and small rounded grains can be seen in the cement-like matrix. For identification of phases, see Figures 3 and 4.