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Figure F18. Thin section photomicrographs of hyaloclastite breccia from lithologic Unit 21 (Sample 330-U1376A-17R-1, 140–143 cm; Thin Section 262) (plane-polarized light). A. Unaltered basaltic glass shards with curved edges. The homogeneity of the glass suggests that the fragments quenched very rapidly, and the shape of the shards suggests that they are bubble-wall fragments. Voids between glass shards are unfilled in this specimen. The darkening around shard edges is due to numerous minute tubes formed by microorganisms. B. Magnified view of A, showing euhedral olivine microphenocrysts and tiny spinel crystals in basaltic glass. The olivine crystals contain spinel and glassy melt inclusions.

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