Volatiles in Alkalic Basalts form the North Arch Volcanic Field, Hawaii: Extensive Degassing of Deep Submarine-erupted Alkalic Series Lavas

Abstract

The North Arch volcanic field is a submarine suite of alkali basaltic to nephelinitic lavas of the seafloor north of Oahu at water depths of 3900–4380 m. Glasses from these lavas were analyzed for H2O, CO2, Cl, S, Fe3+/∑Fe, and noble gases to investigate the role of volatiles in the generation, evolution, and degassing of these alkalic series lavas. In contrast to the systemic negative correlation between concentrations of SiO2 and nonvolatile incompatible elements (e.g. P2O5), the behavior of the volatile components is much more irregular. Concentrations of H2O in glasses vary by a factor of two ( 0.69–1.42 wt %) and show a poor correlation with melt composition, whereas concentrations of dissolved CO2 in glasses (260–800 p.p.m) increase with increasing alkalinity of the glasses. The H2O2 concentrations in the glasses are in equilibrium with an H2O–CO2 vapor at the depth of eruption (400 bar pressure). Samples collected directly from vent structures are highly vesicular, suggesting that these samples were gas rich upon eruption. Estimated bulk volatile contents of the two most vesicular vent samples are high (1.9±0.1 wt % H2O and 5.4±0.4 CO2) and are interpreted to have formed by closed system degassing. Estimated bulk volatile contents in four other vesicular vent samples are lower 1.3±0.2 wt % H2O and 2.0±0.4 wt % CO2), and these samples are interpreted to have lost some gas during eruption. Glass samples from inflated, flat lava flows are nonvesicular and interpreted to have lost essentially all exsolved gas during eruption and flow. Forward degassing models can predict the observed range in dissolved H2O and CO2 contents, calculated vapor compositions, and vesicularity as a function of SiO2. The models involve open to closed system degassing of an H2O–CO2 vapor phase from melts initially having H2O/P2O5=3 and CO2H2O=1–4 mass. Cl concentrations (400–1360 p.p.m) in glasses correlate with concentrations of nonvolatile, incompatible elements. Concentrations of noble gases measured on bulk glass samples are low compared with mid-oceanic ridge basalt (MORB). The low concentrations result mainly from extensive vapor exsolution from the magma. The helium isotopic ratios for gases released from vesicles are similar to MORB values [6.8–8.5 times the air ratio (RA)], whereas those released from glasses are lower than MORB values as a result of in situ decay of U and Th. The S contents (0.11–0.22 wt %) of most of the alkali olivine basaltic and basanitic glasses are sufficient to saturate the silicate melt with immiscible Fe–S–O liquid at the T and P of eruption and quenching. However, two vesicular samples appear to hae lost some dissolved S owing to eruptive degassing. Magmatic oxygen fugacitites estimated from Fe3+/∑Fe range from ΔFMQ=-0.8 to +0.7, with the nephelinitic glasses being more oxidizing than the less alkalic glasses. We infer that the mantle source region for the North Arch magmas was homogeneous with respect to Fe3+/∑Fe and that melting occurred in the absence of graphite or CH4- rich fluid. The effect of variable partial melting on magmatic oxygen fugacity may be a common feature of Hawaiian volcanism. These complex data pint to a simple result, namely that parental magma compositions can be derived by variable extents of melting of a homogeneous source followed by olivine crystallization and degassing at 400 bar. If the parental liquids are produced by 1.6–9.0% partial melting (±20% relative), then mantle volatile contents are estimated to be 525±75 p.p.m. H2O, 1300±800 p.p.m. CO2 and 30± p.p.m. Cl.