The Volatiles of Mt. Pagan, Northern Mariana Islands

Abstract

The powerful 15 May 198I eruption and frequent subsequent activity of Mt.Pagan have led to the formation of an extensive summit fumarole field. An exhaustive search using a portable gas "sniffer" failed to detect a significant primary magmatic gas component in the fumarole field. Collected gas samples contain mostly ambient air with concentrations of CO2 to a maximum of 5.5% of the dry gas. He, H2, H2S, S02 , and CH4 are below detection limits in all samples collected. The (H20)/(C02) ratio was 13 for one fumarole tested. The carbon isotope composition of the fumarolic CO2 (b13C-C02 = -1.1) shows that this gas is derived from marine carbonates. The physical location of these carbonates has not been determined. The hydrogen isotope compositions of the fumarolic water (D-H20 = -29 to -48) and low concentrations of HCl (approximately 500 mg/L) indicate that the fumarolic condensates are produced mostly from locally derived meteoric water. Seawater is apparently excluded from the central volcanic body. The lack of detectable primary magmatic volatiles in fumarolic emanations may reflect prior degassing of the mantle beneath Mt.Pagan.