Water-saturated oceanic lavas from the Manus Basin: volatile behaviour during assimilation–fractional crystallisation–degassing (AFCD)

We have analysed volatiles (H2O, He, Ar, CO2) in differentiated (basaltic andesite, dacite) volcanic glasses dredged at a depth of ca. 2000 m in the eastern part of the Manus Basin between 151°20′ and 152°10′ E. These samples have Sr–O–B isotopic ratios that show that they most likely represent lavas evolved from a common magma source. Since these glasses are very fresh, they provide a unique opportunity to study the behaviour of magmatic volatiles during assimilation–fractional crystallisation–degassing (AFCD). The samples are highly vesicular (up to 18%) and the volatiles trapped in vesicles consist predominantly of H2O with minor amounts of CO2, and the concentration of water in the glasses indicates that H2O saturation was attained. Rare gases except helium are atmospheric in origin, and the 3He/4He ratios and the CO2/3He ratios are respectively lower and higher than those typical of Mid-Ocean Ridge Basalt (MORB), and appear to correlate with the degree of differentiation. AFCD allows efficient degassing of mantle-derived volatiles and contribution of crust-derived and atmosphere-derived volatiles. Given the widespread occurrence of differentiated magmatism at arcs, we suggest that AFCD is responsible for large-scale occurrence of 3He-rich crustal fluids and of atmospheric-like rare gases in arc emanations, and that most of the volatiles are lost continuously during fractional crystallisation, rather than catastrophically during eruptions.