Publication Date:
2011-01-19
Description:
New experimental results are presented on the effects of dissolved H 2 O during low-pressure (〈 2 kbar) crystal fractionation in Mariana Arc magmas. The resulting fractionation paths are used to infer the pre-eruptive H 2 O concentrations of Mariana lavas from seamounts and islands. Estimated pre-eruptive H 2 O 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 H 2 O concentrations are also similar to measured H 2 O concentrations in Mariana back-arc lavas. Lavas from the remaining islands (Guguan, Agrigan, Uracas, Anatahan and possibly Sarigan) have significantly higher H 2 O (5–6%) than the other seamounts and islands. These high-H 2 O 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 H 2 O 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 H 2 O concentrations could reflect variations in the depth and extent of fractionation of Mariana magmas.
Print ISSN:
0022-3530
Electronic ISSN:
1460-2415
Topics:
Geosciences
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