HR: 0800h
AN: V51A-1466    [Abstracts]
TI: the Geochemical Structure of the Hawaiian Plume
AU: * Huang, S
EM: huangs@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, 77 Mass Ave, Cambridge, MA 02139 United States
AU: Frey, F A
EM: fafrey@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, 77 Mass Ave, Cambridge, MA 02139 United States
AB: The spatial arrangement of modern Hawaiian volcanoes forms two offset trends, the Kea and Loa trends. Lavas from these two volcanic trends have important geochemical differences; e.g., Loa and Kea trend lavas form different trends in ⁸⁷Sr/86Sr and ²⁰⁸Pb*/²⁰⁶Pb* vs ³He/⁴He plots (e.g., Kurz et al., 1995; Lassiter et al., 1996). Abouchami et al. (2005) noted that, compared with Kea trend lavas, Loa trend lavas have relatively higher ²⁰⁸Pb/²⁰⁴Pb at a given ²⁰⁶Pb/²⁰⁴Pb, i.e., Loa trend lavas have higher ²⁰⁸Pb*/²⁰⁶Pb*. Kea and Loa trend lavas also form different trends in plots of ²⁰⁸Pb*/²⁰⁶Pb* vs Hf, Sr and Nd isotopic ratios. An important observation is that in these isotopic ratio plots, Loihi lavas are located at the intersections of the near-linear Loa and Kea trends; implying that the Loihi component (high ³He/⁴He) is a common source component for Loa and Kea trend volcanoes. The other ends of the Loa and Kea trends are defined by Koolau and Mauna Kea lavas, and are designated as the Koolau and Kea components. Loa trend lavas sample the Koolau and Loihi components, and the Kea trend lavas sample the Kea and Loihi components. The Loa-Kea geochemical differences have been inferred to reflect source characteristics. Consequently, different models for the structure of the Hawaiian plume have been proposed, for example, a concentrically zoned plume (Lassiter et al., 1996) and a bilaterally asymmetric plume (Abouchami et al., 2005). Based on the temporal variations of geochemical compositions of shield lavas from several Hawaiian shields, such as Mauna Kea, Koolau and Haleakala, as well as melt inclusion study, Kurz et al. (2004) and Ren et al. (2005) proposed that although the plume is grossly zoned, there are Kea- and Loa-type sources present throughout the plume. In this study, we propose that Loa and Kea volcanoes sample a common, geochemically heterogeneous mantle plume source which contains the Koolau, Kea and Loihi components. These geochemical heterogeneities within the Hawaiian plume are large enough to keep their distinctive geochemical signature throughout the upwelling process, and small enough to be in thermal equilibrium with each other. The Koolau component is eclogite, and the Kea and Loihi components are peridotite. In detail, the Loihi component is wetter than the Kea component; consequently, the solidus temperature increases in the order of Koolau to Loihi to Kea components. Therefore, during upwelling the Koolau component begins to melt at the greatest depth, and the Kea component begins to melt at the shallowest depth. In our model, the Loa trend volcanoes formed at lower temperatures; consequently, the Kea component with the highest solidus temperature does not melt, and Loa trend lavas only sample the Koolau and Loihi components. The Kea trend volcanoes formed at higher temperatures; consequently, the eclogitic Koolau component melts totally. Because of its low abundance, in the pooled Kea trend lavas, its geochemical signature is highly diluted by partial melts of the Kea and Loihi components. Consequently, the Kea trend lavas only reflect varying proportions of the Kea and Loihi components. Therefore, in our model Loa-Kea geochemical differences reflect a temperature difference which reflects their distances to the hot plume center. This leads to the non-conventional inference that Kea volcanoes are closer to the plume center.
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 1749 Volcanology, geochemistry, and petrology
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005