Figure 6. DNA sequencing and Raman spectroscopy results.(a) Krona plot of C2 showing relative abundances of Phyla, Classes, Orders, Families and Genera. (b) Krona plot of H7.(c) Bar plot showing relative abundance of genera of C2 and H7.(d) Optical microscopy image of blue drop material (sample C2) with visible brown speckles. (e) Raman spectrum of brown speckle inside blue drop material, with prominent peaks at 1008, 1155, and 1513 cm-1 typical of a carotenoid signature.
3.2.4 SEM/EDS analysis of thin sections Figure 7 shows a comprehensive summary of the igneous minerals of the lava rock in the sample H7 thin section. The lava rock is composed of unaltered basalt, as seen from the pristine and angular grain boundaries. The igneous mineralogy consists mainly of clinopyroxene, ilmenite, and plagioclase. Igneous titano-magnetite has exsolved into Ti-poor magnetite and ilmenite. Phosphorus enrichment in tiny crystals in interstitial melt pockets suggests apatite saturation, in agreement with a highly evolved residual melt composition left after high degrees of crystallization.
Figure 7. Igneous mineralogy. (a-c) Backscattered electron (BSE) images of sample H7 thin section, showing clear, angular grains indicating largely unaltered basalt. (c) EDS point spectra taken of spots marked in red, revealing typical basaltic mineralogy: augitic clinopyroxene (2), dendritic titano-magnetite, (oxy-) exsolved into a fine intergrowth of Ti-poor magnetite (white) and ilmenite (slightly less bright) (4), plagioclase (5), magnetite (8), and apatite growth, indicating a highly evolved interstitial melt pocket (10). The spectra can be seen in Supporting Information.
Upon closer inspection of the thin section edges, surfaces potentially exposed to microbial mats, we see a ~10 μm thick secondary mineral crust with a botryoidal, layered appearance in sample C2 (Figure 8a, b). It shows many fractures and damage, and is only sporadically present, suggesting it is fragile and not well-preserved in the sample handling and thin section preparation process. It also appears to have been deposited on top of the igneous rock, rather than leached from it, based on the clear boundary between the two and the independent growth pattern of the precipitate.
Another location in sample C2 with the secondary mineral crust was found and mapped with EDS (Figure 8c). The EDS spectrum in Figure 8i shows an augitic clinopyroxene as an igneous phase of the parent lava rock, which forms a sharp, unaltered boundary to the Ca-free crust (Figure 8e). The EDS spectrum of the crust (Figure 8h) shows a prominent copper enrichment, along with aluminum and silicon, and minor sulfur. The element maps shown in Figures 8f and 8g indicate that the copper enrichment in the crust is associated with uniformly high aluminum intensities. In addition, the crust material was observed to form shrinkage cracks under high vacuum and exposure to the (low-current) electron beam, consistent with a hydrous mineralogy. This resembles the signature of the hydrated copper silicate chrysocolla (Anthony, 1990), supported by the botryoidal, blue appearance. The crust has uniform, percent-level phosphorus concentrations.