Development of an In-Situ Hydrothermal Dissolved Carbon Dioxide Sensor for Hydrothermal Environments
Hydrothermal systems at mid ocean ridges are a unique environment which connect the mantle, lithosphere, ocean, and biosphere in one complex system. Fluid chemistry is one key tool in understanding this environment, and carbon dioxide is the most abundant gas in these fluids. Its relative abundance and the information it can give about the magmatic activity, which is one of the ultimate drivers of these systems, and other processes, make it an attractive target for development of an in-situ sensor. This work has shown that attenuated total reflection (ATR) is a promising technique for in situ carbon dioxide measurements under hydrothermal conditions. This redesign of the hydrothermal carbon dioxide instrument has yielded a sensor that functions well in laboratory tests and has been calibrated between 0 and 250 mmol/L CO2. Mirrored surfaces and rounded faces on the ATR crystal were found to be problematic. An ATR element in the form of a truncated cone may be most appropriate in this aggressive environment and will be used in the next iteration. The high temperature, high pressure seal around the ATR element proved to be the most challenging part of the design, however a graphite seal was tested successfully and will be used in the next instrument. A coating of silicone was an effective method of water exclusion from the optical path length, even at high pressures. Finally, coupling of light into, and out of, the light pipes was far more critical than the output and sensitivity of the source and detector themselves. This work has brought us significantly closer to realizing real time, in situ measurements of sea floor hydrothermal carbon dioxide flux.