STABLE CARBON ISOTOPES OF CO2 AT VOLCANOES OF THE CASCADE ARC

The Cascade arc hosts a dozen large volcanoes ranked in the USGS National Volcano Early Warning System as high or very high threat features, where multiple methods of monitoring are desirable, including periodic measurements of gas geochemistry. Several of these volcanoes have no summit fumaroles or only weak low-temperature gas vents that are highly air-contaminated. Obtaining useful gas data for such volcanoes requires that gases be analyzed from springs on or around the flanks. Spring data can be problematic in the study of carbon isotopes because of the large fractionation between δ¹³C-CO₂ and δ¹³C-HCO₃, but if both gas and liquid phases are sampled and pH is measured on site, the problems can often be resolved and the data used to constrain the characteristic δ¹³C value of the volcano. Summit gas vents or springs suitable for monitoring δ¹³C have been found at all but one (Medicine Lake) of these 12 volcanoes. A combination of published and unpublished carbon isotope data obtained from these features shows that only Baker and Newberry have MORB-like δ¹³C values near -7‰. Glacier Peak, Rainier, Adams, St. Helens, Hood, South Sister, Crater Lake, Shasta, and Lassen have δ¹³C values in the -8.5 to -12.5‰ range, lighter than typical mantle values. The 12 volcanoes are scattered over a 1000-km length of arc, and their δ¹³C values show no obvious correlation to variations in basement-rock type or to the predominant Holocene lava composition, which ranges from basalt/andesite to rhyolite. Presumably, the δ¹³C values are mainly imparted from the deep basaltic parent magma, and the unusually light values reflect the influence of subducted sediment rich in organic matter. Crustal controls on δ¹³C seem insignificant apart from small (~1‰) temporal changes previously reported at Baker and St. Helens and linked to Rayleigh-type degassing of recently intruded magma.