Along-Arc Variations in the Pre-Eruptive H₂O Contents of Mariana Arc Magmas Inferred from Fractionation Paths

摘要

New experimental results are presented on the effects of dissolved H₂O during low-pressure (< 2 kbar) crystal fractionation in Mariana Arc magmas. The resulting fractionation paths are used to infer the pre-eruptive H₂O concentrations of Mariana lavas from seamounts and islands. Estimated pre-eruptive H₂O concentrations for Mariana seamounts (2–3 wt %) are relatively constant and similar to three out of eight of the subaerial volcanoes (Alamagan, Pagan and Asuncion). These H₂O concentrations are also similar to measured H₂O concentrations in Mariana back-arc lavas. Lavas from the remaining islands (Guguan, Agrigan, Uracas, Anatahan and possibly Sarigan) have significantly higher H₂O (5–6%) than the other seamounts and islands. These high-H₂O islands are distributed throughout the arc and do not show any relation to variations in subduction parameters such as subduction angle or obliquity. Analysis of the H₂O concentrations of olivine-hosted melt inclusions from seven islands (Guguan, Pagan, Agrigan, Anatahan, Alamagan, Asuncion and Sarigan) and one seamount (Fukujin) match the estimates inferred from the liquid lines of descent. One interpretation of our results is that the flux of volatiles from the Mariana sub-arc mantle is fundamentally heterogeneous. Alternatively, the variations in pre-eruptive H₂O concentrations could reflect variations in the depth and extent of fractionation of Mariana magmas.