EPMA_ikaite and methane-derived authigenic carbonate recovered from Echigo Bank

Ikaite and methane-derived authigenic carbonate (MDAC) recovered from narrow area (<500 m in distance) of an unnamed bank structure (hereafter referred to as Echigo Bank) in the Sea of Japan were compared for understanding the geochemistry of ikaite. Electron probe microanalysis (EPMA) was applied to compare the elemental compositions. Ikaite sample were stored in refrigerator to avoid decomposition. After CT imaging, ikaite sample was broken into several pieces using chisel and hammer (Fig. S1). A relatively large piece (>~2 cm) was freeze dried, and cemented with resin to obtain a chip sample. After polishing and powder sample preparation for stable isotope analysis, thin section (~300 μm for future investigations after repolishing) was prepared from the chip (Fig. S2). Finally, the surface was polished with 1 μm diamond paste and then coated with carbon. Chip of a MDAC (PC1804-Sec.2-(40-50)_B) was obtained by cutting the sample. Similarly, thin section (~100 μm) of the chip was prepared. The elemental compositions of Fe, Ca, Sr, Ba, and Mn in calcite were determined by an electron probe microanalyzer (JXA-8900L of JEOL) at the Department of Earth and Planetary Science, University of Tokyo. Carbonate minerals were used for standards.The mapping measurements were applied only to yellowish calcite grains in the ikaite chip. Because calcite grains in MDAC were indistinguishable through the SEM images of the analyzer, well-polished parts at the calcite-enriched areas were randomly measured until data of high Ca content could be obtained. Such areas were selected through polarized microscope observations of the thin section before the analysis. XY positons of landmarks, point analyses, and mapping margins were recorded for indicating mapping area and measurement points on photo of thin section (Fig. S3 and S4). The yellow grains in ikaite chips contain Mg in low level (Mg content: <3.7 mol%), while the other elements (Fe, Mn, Sr, and Ba) are almost below the detection limit (Table S1). Only the Mg content covaries with the zonation pattern (Fig. S3 and S4). The white matrix parts were unsuitable for measurement. Cation oxide totals of most measurement points is less than 41 wt% (Table S2). This is probably due to the softness of the matrix. Exceptional 4 data (cation oxide total: 53.0 to 55.2 wt%) may be derived from tiny yellow grains in the matrix part. The Mg content of the calcites cementing a MDAC is around 10 mol% (Table S3).