Helium Isotope and Gas Chemistry Variations at Sulphur Banks, Kīlauea Volcano, HI

The dry gas composition at Sulphur Banks, near the summit of Kīlauea, is known to consist largely of carbon dioxide and sulfur dioxide, with some hydrogen sulfide. Due to its accessibility the area has been well-studied over the years, but has not been subject to regular monitoring over the past decade. Following the eruptive changes that occurred in 2018, Sulphur Banks represents a continuing window into the outgassing dynamics at Kīlauea's summit.

A research well drilled in the 1920s at Sulphur Banks developed into a "fumarole," and has long been used as a gas sampling site. We have collected gas samples approximately quarterly since March 2018, in evacuated bottles for chemical compositional analysis and in copper tubes for helium isotope ratio determinations. Temperatures, measured during sampling, did not exceed 95.5°C, the boiling point at the site's elevation (1230 m).

Bulk gas composition was stable over the sampling period, consisting of ~98% CO₂ and ~1% SO₂, with the balance consisting mainly of air. Minor constituents include hydrogen, helium, and carbon monoxide.

Helium isotope ratios have steadily risen from R_c/R_A (air-corrected ³He/⁴He ratios in the sample over the ratio in the atmosphere) values of 13.7 in March 2018 to R_c/R_A of 15.7 in March 2019, more than eight months after the draining of the lava lake and cessation of activity at the summit. Four samples in total have been analyzed to date. The March 2019 R_c/R_A value of 15.7 approaches the highest ratios measured in eruptive gases at Kīlauea volcano. Such high ratios are indicative of a magmatic source, but the cause of this large change is unknown.

Because of the significant ongoing changes in helium isotope compositions, the USGS will continue sampling the well on a quarterly basis. Future additions to this sampling program will include the measurement of H₂S relative to SO₂, via portable MultiGas analyzer or other sampling strategies, to evaluate the contribution of oxidizing and reducing sulfur species at the Kīlauea summit.