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Volatile Degassing on the East Pacific Rise

Virtually all mid-ocean ridge basalt magmas lose gas as they ascend through the oceanic crust because they are saturated or oversaturated with carbon dioxide. An unknown amount of the CO2 that is lost becomes incorporated into hydrothermal vent community ecosystems through biochemical reactions. The composition of the basalt substrate and its volatile makeup also influence hydrothermal fluid compositions and the locations of vent communities. The purpose of this study is to accurately determine the isotope compositions and concentrations of He, Ar and CO2 trapped within bubbles of basalts, from the 1991-1992 and 2005-2006 eruptions at the RIDGE 2000 Integrated Study Site at 9°50'N on the East Pacific Rise (EPR). Helium, argon and carbon isotopes provide fundamental information that can be used to relate volatile degassing along mid-ocean ridges to stages of magma evolution in the ocean crust. The sampling of these recent eruptions offers a unique opportunity to study magma recharge along the mid-ocean ridge system, to quantify the amount of pre-eruptive and syn-eruptive degassing that occurred, and to evaluate the extent to which mantle carbon may be incorporated into hydrothermal fluids and ultimately into seafloor ecosystems.

Our approach utilizes 3He/4He, 4He/40Ar*, CO2/3He and δ13C systematics through a combination of He–Ar–CO2 analyses of basalt glasses. We address the following questions:

  1. What were the relative roles of pre-and syn-eruptive degassing during the EPR eruptions? How much CO2 was lost prior to versus during the eruptions?
  2. Was there a transition from closed-system to open-system degassing? For example, are there systematic spatial variations in vesicle δ13C and 4He/40Ar*, or temporal trends in lavas dated by the 210Po-210Pb method?
  3. Is there a relationship between gas contents and 210Pb-226Ra disequilibria? If so, can this constrain the mechanism and timing of gas loss?
  4. Do the δ13C and CO2/3He provide evidence for direct volatile injection into the seafloor hydrothermal system?


Mid-ocean Ridges and Upper Mantle Processes

Mantle Plumes and Geodynamics of Earth's Interior