HR: 16:00h
AN: V54A-01    [Abstracts]
TI: Magma Sources Of Rejuvenescent Volcanism On Oceanic Islands
AU: * White, W M
EM: white@geology.cornell.edu
AF: Cornell University, Dept. of Earth & Atmospheric Sciences Snee Hall, Ithaca, NY 14853 United States
AB: Rejuvenescent volcanism occurs on many oceanic islands, including the Society Islands, the Australs, Samoa, and Mauritius as well as the Hawaiian Islands. Rejuvenescent lavas have a number of features in common, including highly alkalic composition, small volumes, and strong incompatible element enrichment, all of which suggest they are quite small degree melts. However, despite the incompatible element enrichment, rejuvenescent magmas have isotopic signatures that show they are melts of sources that are more depleted than the source of corresponding shield stage magmas, although not as depleted as those of MORB. The observations have led to models that propose that the source of rejuvenescent magmas are mixtures of the mantle plume source and depleted upper mantle (MORB source), with the latter either oceanic lithosphere and asthenosphere entrained by the plume. These models, however, fail in two respects. First, in every case examined including the Hawaiian volcanoes, a detailed examination in multi-isotopic space shows that the sources or rejuvenescent magmas cannot be mixtures of the respective shield stage source and a MORB-like source. A second problem is whether any melting occurs outside of the mantle plume. In general geodynamic modeling studies have concluded that neither the overlying lithosphere nor surrounding asthenosphere is sufficiently heated by the plume to melt. Thus a combination of geochemical and geophysical considerations rule out either lithosphere or asthenosphere surrounding the plume as significant contributors to plume volcanism. In that case, the mantle plume itself must be the source of rejuvenescent magmas. While some of the differences between shield and rejuvenescent stage magmatism can be explained by the latter being much smaller degree melts, the isotopic differences require distinct sources with distinct lithologies be present within the plume. This lead to the following model: 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. Both the plums and the peridotite are incompatible-element enriched relative to the average depleted upper mantle, but the plums are substantially more enriched. Isotopic equilibrium between lithologies 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. As the plume rises into the shallow mantle, melting is concentrated above the hot core of the plume. However, as the plume bends due to deflection of the lithosphere, a melting "tail" extends hundreds of km downstream forms. 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 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: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8415 Intra-plate processes (1033, 3615)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005