Publication Date:
2022-05-25
Description:
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Chemical Geology 259 (2009): 240-252, doi:10.1016/j.chemgeo.2008.11.008.
Description:
In situ sensors capable of real-time measurements and analyses in the deep ocean are
necessary to fulfill the potential created by the development of autonomous, deep-sea
platforms such as autonomous and remotely operated vehicles, and cabled observatories.
Laser Raman spectroscopy (a type of vibrational spectroscopy) is an optical technique
that is capable of in situ molecular identification of minerals in the deep ocean. The goals
of this work are to determine the characteristic spectral bands and relative Raman
scattering strength of hydrothermally- and cold seep-relevant minerals, and to determine
how the quality of the spectra are affected by changes in excitation wavelength and
sampling optics. The information learned from this work will lead to the development of
new, smaller sea-going Raman instruments that are optimized to analyze minerals in the
deep ocean.
Many minerals of interest at seafloor hydrothermal and cold seep sites are Raman
active, such as elemental sulfur, carbonates, sulfates and sulfides. Elemental S8 sulfur is
a strong Raman scatterer with dominant bands at ~219 and 472 Δcm-1. The Raman
spectra of carbonates (such as the polymorphs calcite and aragonite) are dominated by
vibrations within the carbonate ion with a primary band at ~1085 Δcm-1. The positions of minor Raman bands differentiate these polymorphs. Likewise, the Raman spectra of
sulfates (such as anhydrite, gypsum and barite) are dominated by the vibration of the
sulfate ion with a primary band around 1000 Δcm-1 (~1017 for anhydrite, ~1008 for
gypsum, and ~988 for barite). Sulfides (pyrite, marcasite, chalcopyrite, isocubanite,
sphalerite, and wurtzite) are weaker Raman scatters than carbonate and sulfate minerals.
They have distinctive Raman bands in the ~300-500 Δcm-1 region. Raman spectra from
these mineral species are very consistent in band position and normalized band intensity.
High quality Raman spectra are obtained from all of these minerals using both green and
red excitation lasers, and using a variety of sampling optics. The highest quality spectra
(highest signal to noise) were obtained using green excitation (532 nm Nd:YAG laser)
and a sampling optic with a short depth of focus (and thus high power density).
Significant fluorescence was not observed for the minerals analyzed using green
excitation. Spectra were also collected from pieces of active and inactive hydrothermal
chimneys, recovered from the Kilo Moana vent field in 2005 and 11ºN on the East
Pacific Rise in 1988, respectively. Profiles of sample J2-137-1-r1-a show the transition
from the chalcopyrite-rich “inner” wall to the sphalerite-dominated “outer” wall, and
indicate the presence of minor amounts of anhydrite. Spectra collected from sample
A2003-7-1a5 identify Cu-S tarnishes present on the surface of the sample.
Description:
This work was funded by the Cecil H. and Ida M. Green Technology Innovation Fund. Additional support
was provided by the James S. Coles and Cecily C. Selby Endowed Fund in Support of Scientific Staff and
the Penzance Endowed Fund in Support of Assistant Scientists.
Keywords:
Raman spectroscopy
;
Mineralogy
;
Hydrothermal vents
;
Cold seeps
;
Sulfates
;
Sulfides
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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