Degassing During a Seafloor Eruption along the Mid-Ocean Ridge: Helium, Argon and Carbon Isotopes in Basalts from the East Pacific Rise Integrated Study Site

D.W. Graham

Corresponding author: dgraham@coas.oregonstate.edu
College of Oceanic & Atmospheric Sciences, Oregon State University, Corvallis, OR 97331

Abstract:
Virtually all mid-ocean ridge basalt magmas degas as they ascend through the oceanic crust because they are typically saturated or oversaturated with carbon dioxide. An unknown amount of the degassed CO2 ultimately plays a role in 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. We have just begun a new investigation to accurately determine He, Ar and C isotope compositions, and He, Ar and CO2 concentrations, in the vesicles of basalt glasses from the 1991-1992 and 2005-2006 eruptions at the RIDGE 2000 Integrated Study Site on the East Pacific Rise. Helium, argon and carbon isotopes in the bubbles of basalt glass provide fundamental information that can be used to relate volatile degassing along mid-ocean ridges to magma evolution. The sampling of the recent eruptions that occurred at the ISS offers a unique opportunity to study magma recharge along the mid-ocean ridge, 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. The planned work builds on and extends the previous work of Peter Michael and Ken Rubin, and provides data for R2K synthesis/integration of basalt and vent fluid data.

The 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-²¹⁶Ra disequilibria? If so, can this constrain the mechanism and timing of gas loss?
4. Do the δ¹³C and CO2/³He measurements provide evidence for direct volatile injection into the seafloor hydrothermal system?

Contributions to Integration and Synthesis:
The mechanisms by which ocean ridge basalts degas, and the possible direct injection of volatiles into hydrothermal systems are key questions for the RIDGE 2000 program. Our work also addresses basic questions about the linkages between volatile degassing and mid-ocean ridge volcanic and seismic activity. The results will allow evaluation of how volatile evolution in MORB magmas impacts deep ocean hydrothermal fluxes, and provide basic constraints on the geochemical budget of key elements such as carbon.