In Situ Raman spectra from the Sea Cliff Hydrothermal Field (Gorda Ridge)
Sheri N White1 (email@example.com), Rachel, M. Dunk1, Peter G Brewer1, Edward T Peltzer III1, Alana D. Sherman1, John J. Freeman2
1MBARI, 7700 Sandholdt Rd., Moss Landing, CA 95039
2Dept. of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130
MBARI’s in situ laser Raman spectrometer (DORISS – Deep Ocean Raman In Situ Spectrometer) was deployed at the SeaCliff Hydrothermal Field on the Gorda Ridge in July 2004. The first In Situ Raman spectra of hydrothermal minerals and high-temperature fluid venting from the seafloor were obtained. These spectra are analyzed and compared to laboratory measurements of samples collected from the site.
Laser Raman spectroscopy is a proven, powerful geochemical technique for analyzing the chemical composition and molecular structure of solids, liquids, and gases. During an expedition to Gorda Ridge on the R/V Western Flyer in July 2004, DORISS was deployed successfully by the ROV Tiburon at hydrothermal vents on the seafloor (~2700 m depth). Data were collected from hydrothermal fluids, chimney minerals (e.g., anhydrite and barite), and bacterial mats using two types of sampling optics – an immersion optic, and a non-contact optic. To collect spectra from opaque mineral samples, a precision underwater positioner (PUP) was used to position the DORISS probe head. PUP is a stand-alone, three degree-of-freedom positioner capable of moving the DORISS probe head with a precision of 0.1 mm (required by the small focal volume of the sampling optic).
Raman spectra were collected of ~300° C vent fluids with both sampling optics. The Raman spectrum of seawater contains bands from the bending (~1640 cm-1) and stretching (3000-3700 cm-1) vibrational modes of the water molecule and a small peak from the S-O stretch of the sulfate ion (~981 cm-1). Compared to ~2° C ambient seawater, vent fluid spectra show changes in the intensity ratios of the water bands due to the elevated temperature, and the sulfate peak is reduced. Additional components of hydrothermal fluid are present in such low concentrations that it is difficult to detect them with the current Raman system. The chimneys in the SeaCliff field are primarily anhydrite, and debris in the area also contains barite. We were able to obtain quality spectra of both anhydrite (primary peak at ~1017 cm-1) and barite (primary peak at ~987 cm-1). In addition, we were able to detect elemental sulfur in the polycrystalline S8 morphology which may have been produced by bacterial mats (peaks at ~218 and 473 cm-1).