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.