HR: 1330h
AN: V32A-0996    [PDF]
TI: The Loa-Kea trend revisited
AU: * Abouchami, W
EM: wafa@mpch-mainz.mpg.de
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Galer, S J
EM: sjg@mpch-mainz.mpg.de
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Hofmann, A W
EM: hofmann@mpch-mainz.mpg.de
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AB: The existence of two parallel chains of volcanoes in the Hawaiian Islands was first described by Dana [1]. The consistency between the locus of the shields and the age progression along the chain was used to define the so-called "Loa" and "Kea" trends. The two chains do indeed show some systematic geochemical and isotopic differences. New high precision Pb isotope data on several Hawaiian volcanoes along the two chains and detailed Pb isotope stratigraphy on Mauna Kea (HSDP) [2] provide new insights into the length scale of isotopic heterogeneities within the Hawaiian plume. Using these data and published isotope data from submarine landslide blocks from Nuanuu [3], we show that the two trends have systematically different Pb isotopic compositions, showing that they sample two compositionally different sides of the Hawaiian plume. However, the extension of the Loa trends to Oahu and Kauai is inconsistent with the observed Pb isotope systematics. In particular, main shield-stage lavas from these two volcanoes do not have the high 208Pb*/206Pb* ratio typical of Loa trend volcanoes. Although Koolau subaerial lavas Pb isotope data are consistent with Loa-trend compositions, stratigraphically deeper Koolau shield-stage lavas have isotopic characteristics similar to those of Kea trend volcanoes. Similarly, lavas from Kauai, the most remote island on the Loa trend, share more similarities with Kea trend than with Loa trend volcanoes. On the basis of these observations, we suggest that the Loa and Kea trends developed about 2 to 3 Ma ago, following a change in the Pacific plate motion reflected by a bend in the Hawaiian chain near the Molokai Fracture Zone. This inference is consistent with that obtained by geometric relocation of the Hawaiian hotspot track in the Pacific [4] and models calling upon lithospheric flexure to explain the creation of a dual chain of volcanoes [5, 6]. The large-scale left-right asymmetry, evident in the spatial distribution of the volcanoes along the Loa-Kea trend is also present in the temporal evolution of a single volcano (e.g. Koolau and Mauna). This may be explained if the azimuth of the compositional boundary between Loa and Kea type chemistry is at a slight angle to the azimuth of the plate motion. It is quite possible that the change in the direction of plate motion, 2 to 3 Ma ago, also caused a change in the position of Koolau volcano in such a way that its magma supply switched from one side of the plume to the other. [1] J.D. Dana, Geology, Volume 10 of United States Exploring Expedition, during the years 1838-1839, 1840, 1841, 1842, C. Sherman, Philadelphia, 1849. [2] J. Eisele, W. Abouchami, S.J.G. Galer, A.W. Hofmann, Geochem. Geophys. Geosyst., 4(2), 8710, doi:10.1029/2002GC000339, 2003 [3] R. Tanaka, E. Nakamura, Geophys. Monograph Ser., vol. 128, 311-332, 2002 [4] P. Wessel, L. Kroenke, Nature 387, 365-369, 1997. [5] C.F. Hieronymus, D. Bercovici, Nature 397, 604-607, 1999. [6] U. ten Brink, Geology, 19, 397-400, 1991.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting