HR: 16:20h
AN: U14A-02    [Abstracts]
TI: Constraints on Structure and Melting of Heterogeneous Plumes From Laboratory Experiments With Three Components
AU: * Harris, A C
EM: aharris@gso.uri.edu
AF: University of Rhode Island, Graduate School of Oceanography, S. Ferry Rd., Narragansett, RI 02882,
AU: Kincaid, C
EM: kincaid@gso.uri.edu
AF: University of Rhode Island, Graduate School of Oceanography, S. Ferry Rd., Narragansett, RI 02882,
AU: Kelley, K A
EM: kelley@gso.uri.edu
AF: University of Rhode Island, Graduate School of Oceanography, S. Ferry Rd., Narragansett, RI 02882,
AB: Many studies of chemical geodynamics consider the fate of a single, compositionally distinct layer at the base of the mantle, but subducted oceanic lithosphere introduces two distinct lithologies (higher-density eclogite and lower-density harzburgite) into the mantle (a third lithology, intermediate-density lherzolitic peridotite). To address the dynamic complexities of interactions between these materials, we conducted three-dimensional laboratory experiments that use glucose syrup (Rayleigh number: 10⁶\)) to model the mantle and a two-layer subducted lithosphere. The viscosity and density of the syrup are controlled by its water content, which is varied to simulate the distinct physical properties of each of the three lithologies. Experiments were conducted in a 20cc tank, heated from below to create a basal thermal boundary layer (BTBL). The two-layer glucose slab was frozen and placed within the tank, where it sank into the BTBL. These experiments produced heterogeneous upwellings with temporal and spatial variations in both temperature and composition that are much more complex than predicted by classic plume theory. Temperature, composition, and distribution of material in the tank through space and time were recorded during each experiment. We scale these data to mantle-equivalent conditions and address the observational implication for melting such heterogeneous plumes, both within larger (200 - 600 km) plume heads and smaller (<100 km) trailing conduits. Results show length scales of chemical heterogeneity range from <10 km up to 300 km. Thermal heterogeneity was often correlated with composition, where the denser, eclogite analog had higher temperatures than the lighter, harzburgite analog. Distinct domains form within plumes and melting begins at different depths, dependent on the temperature and composition of each domain and the solidus of each composition (e.g. eclogite melts at lower temperatures than harzburgite). The combination of thermo-chemical variation and differences in solidi explains heterogeneous melting behavior. In many cases, whole portions of buoyant plume heads either never reach the dry solidus, or predict very small melt fractions, and primarily serve as pathways for the ascent of subsequent fusible material.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1038 Mantle processes (3621)
DE: 3037 Oceanic hotspots and intraplate volcanism
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
SC: Union [U]
MN: 2007 Fall Meeting