Fig. 2
From: Timing and origin of natural gas accumulation in the Siljan impact structure, Sweden
Appearance and paragenesis of calcite and transects of δ13C and 87Sr/86Sr values. a–d Drill core photographs of fractured core sections with notable gas observations (according to drilling logs), SEM images of secondary minerals on the fracture surfaces (back-scatter electron [BSE] mode, (e–i), polished crystal cross-sections with transects of SIMS analyses indicated (j–n) and δ13C (o–r) and 87Sr/86Sr values (q, r) corresponding to the spot locations in j–n. Details: a Sample VM2:170 m, limestone with open fractures. e Scalenohedral calcite crystals intergrown with cubic pyrite VM2:170 m. f Scalenohedral calcite crystals from a similar limestone fracture at a slightly greater depth, VM2:212 m. j, k polished crystals from e and f, respectively (corresponding δ13C values in o, all 13C-enriched). b Sample CC1:537, with euhedral calcite on a fracture in porphyritic crystalline rock coated by euhedral quartz (c). The calcite shows growth zonation (l) and is 13C-enriched in the outer parts (p). c Sample VM1:442 m, open fracture in heavily fractured crystalline rock section coated by aggregates of euhedral calcite (h). The calcite shows growth zonation (m) and is heavily enriched in 13C and 87Sr in the rim compared to the older growth zones (q). d) Sample VM1:255 m, fractured section of crystalline rock with anhedral calcite coating with pyrite (i). The calcite shows growth zonation (n) but relatively homogeneous 13C-depleted δ13C values and 87Sr/86Sr (r). Errors (2σ) are within the size of the symbols if not visible. Length of scale bars: (a–d) 4 cm, (e, k) 400 µm, (f, h, i, j, n) 500 µm, (g, l, m) 300 µm
