HR: 0800h
AN: V51B-0527    [Abstracts]
TI: Two-Stage Melting Of Mantle Plumes And The Origin Of Rejuvenescent Volcanism On Oceanic Islands
AU: * White, W M
EM: white@geology.cornell.edu
AF: Cornell University, Dept. of Earth and Atmospheric Sciences, Ithaca, NY 14853 United States
AU: Morgan, J P
EM: jpm@geology.cornell.edu
AF: Cornell University, Dept. of Earth and Atmospheric Sciences, Ithaca, NY 14853 United States
AB: Many mid-plate oceanic volcanoes experience a rejuvenescent, or "post-erosional" phase of volcanism that occurs hundreds of thousands or million years after the main shield-building phase of volcanism has ended. The Hawaiian Islands are the best-documented example, but rejuvenescent volcanism also occurs on the Society Islands, the Marquesas, the Australs, Samoa, and Mauritius. It does not occur on near-ridge islands such as the Galapagos, the Azores, and Iceland. Rejuvenescent lavas have a number of features in common: they are erupted in small volumes, they are highly enriched in incompatible elements, and they are highly alkalic, typically basanitic to nephelenitic. All these features suggest they are quite small degree melts. In addition, rejuvenescent magmas have more depleted isotopic signatures, implying they are melts of more depleted sources, despite their strong incompatible element enrichment. Although isotopic signatures of these lavas are more depleted that those of the corresponding shield stage lavas, they are nevertheless not as depleted as MORB. Furthermore, the isotopic compositions of the rejuvenescent magmas rule out their sources being mixtures of plume material and MORB-source material. Thus geochemical considerations rule out both the lithosphere and the asthenosphere surrounding the plume as the source of rejuvenescent magmas; this implies the plume itself must be the source of rejuvenescent magmas. This conclusion is consistent with geophysical models of plumes. Finite difference numerical models of plume-lithosphere interaction that include both temperature and compositional viscosity dependence reveal that while most melting is concentrated above the hot core of the plume, a melting "tail" extends hundreds of km downstream. In this tail region, lateral spreading of the plume results in a slight rising motion of the plume, and consequently, small extents of melting. The problem thus becomes that of deciphering why melts produced in this tail region are isotopically distinct from those produced in the main melting region. We propose the following model to explain this difference: Mantle plumes are lithologically heterogeneous, consisting of eclogite or pyroxenite "plums" that have a solidus temperature several tens of degrees lower than the more refractory peridotite "pudding" in which they are embedded. Complete isotopic equilibrium is not achieved during melting - either because the plums are large enough ($>$10-100m) or the extraction of plum melts is rapid after their generation. Both the plums and the peridotite are incompatible-element enriched relative to the average depleted upper mantle, but the plums are substantially more enriched. The plums melt entirely in the base of the main melting region and the heat so consumed initially suppresses melting of the peridotite pudding. Plum-derived melts mix as they rise with melts of the peridotite pudding produced higher in the main melting region. This mixture of eclogitic and peridotitic melts form the shield stage magmas. Material in the melting "tail" has had the plums melted out of it in the main melting region. Low degree melting of the plum-free peridotite in the melting tail gives rise to rejuvenescent magmas. Melt production in the tail is more or less continuous, but rejuvenescent volcanism is not. This suggests that some other factor is involved, such as lithospheric loading by adjacent volcanoes, that provides pathways to the surface for small degree tail melts.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 3210 Modeling
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting