VOLATILES IN ALKALIC BASALTS FROM THE NORTH ARCH VOLCANIC FIELD, HAWAII - EXTENSIVE DEGASSING OF DEEP SUBMARINE-ERUPTED ALKALIC SERIES LAVAS

The North Arch volcanic field is a submarine suite of alkali basaltic to nephelinitic lavas on the seafloor north of Oahu at water depths of 3900-4380 m. Glasses from these lavas were analyzed for H2O, CO2, Cl, S, Fe3+/Sigma Fe, and noble gases to investigate the role of volatile s in the generation, evolution, and degassing of these alkalic series lavas. In contrast to the systematic negative correlation between conc entrations of SiO2 and nonvolatile incompatible elements (e.g. P2O5), the behavior of the volatile components is much more irregular Concent rations of H2O in glasses vary by a factor of two (similar to 0.69-1.4 2 wt%) and show a poor correlation with melt composition, whereas conc entrations of dissolved CO2 in glasses (260-800 p.p.m.) increase with increasing alkalinity of the glasses. The H2O and CO2 concentrations i n the glasses are in equilibrium with an H2O-CO2 vapor at the depth of eruption (similar to 400 bar pressure). Samples collected directly fr om 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 degas sing. Estimated bulk volatile contents in four other vesicular vent sa mples are lower (1.3 +/- 0.2 wt% H2O and 2.0 +/- 0.4 wt% CO2), and the se samples are interpreted to have lost some gas during eruption. Glas s samples from inflated, flat lava flows are nonvesicular and interpre ted 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 degas sing of an H2O-CO2 vapor phase from melts initially having H2O/P2O5 = 3 and CO2/H2O = 1-4 by mass. Cl concentrations (400-1360 p.p.m.) in gl asses correlate with concentrations of nonvolatile, incompatible eleme nts. Concentrations of noble gases measured on bulk glass samples are low compared with mid-oceanic ridge basalt (MORB). The low concentrati ons 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 (R-A)], whereas those releas ed from glasses are lower than MORB values as a result of in situ deca y of U and Th. The S contents (0.11-0.22 wt%) of most of the alkali ol ivine basaltic and basanitic glasses are sufficient to saturate the si licate melt with immiscible Fe-S-O liquid at the T and P of eruption a nd quenching. However, two vesicular samples appear to have last some dissolved S owing to eruptive degassing. Magmatic oxygen fugacities es timated from Fe3+/Sigma Fe range from Delta FMQ = -0.8 to +0.7, with t he nephelinitic glasses being more oxidizing than the less alkalic gla sses. We infer that the mantle source region for the North Arch magmas was homogeneous with respect to Fe3+/Sigma Fe and that melting occurr ed In the absence of graphite or CH4-rich fluid. The effect of variabl e partial melting on magmatic oxygen fugacity may be a common feature of Hawaiian volcanism. These complex data point to a simple result, na mely that parental magma compositions can be derived by variable exten ts of melting of a homogeneous source followed by olivine crystallizat ion and degassing at 400 bar. If the parental liquids are produced by 1.6-9.0% partial melting (+/- 20% relative), then mantle volatile cont ents are Estimated to be 525 +/- 75 p.p.m. H2O, 1300 +/- 800 p.p.m. CO 2 and 30 +/- 6 p.p.m. Cl.