HR: 09:15h
AN: V51G-06    [Abstracts]
TI: High-Ca Boninites From the Modern Tonga Arc
AU: * Cooper, L B
EM: lcooper@bu.edu
AF: Boston University, Dept. of Earth Sci. 675 Comm. Ave., Boston, MA 02215, United States
AU: Plank, T
EM: tplank@bu.edu
AF: Boston University, Dept. of Earth Sci. 675 Comm. Ave., Boston, MA 02215, United States
AU: Arculus, R J
EM: rarculus@ems.anu.edu.au
AF: Australian National University, Dept. of Earth and Marine Sci., Canberra, ACT 0200, Australia
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: Carnegie Institution of Washington, Dept. of Terrestrial Magnetism 5241 Broad Branch Rd., NW, Washington, DC 20015, United States
AU: Worthington, T J
EM: tw@gpi.uni-kiel.de
AF: University of Kiel, Institute of Geosciences, Kiel, 24118, Germany
AB: High-Ca boninites are volcanic rocks with unusual compositions (SiO₂>53 wt%, Mg#>0.6, CaO/Al₂O₃>0.75) found in forearcs and trenches, continental cratons, and ophiolites. Generation of high-Ca boninites requires a combination of refractory mantle sources, elevated mantle temperatures and the addition of hydrous fluids. To satisfy these conditions, petrogenetic models invoke unusual tectonic settings such as subduction initiation, ridge subduction, or mantle plume interaction. We have discovered high-Ca boninites from an active arc volcano, Volcano A, a submarine volcano in the Tonga arc dredged during the NoToVE cruise in Nov 2004. Multi-beam sonar images of two pristine volcanic cones and glassy samples lacking Mn coatings suggest that these edifices were formed by modern volcanism. The boninites are represented in both the whole rock and melt inclusion populations of a sample dredged from a ridge on the northern flank of the northern cone. Similarities in the major element compositions of the largely aphyric whole rock and the glassy melt inclusions support both as samples of true boninitic liquids (MgO>9 wt%). These liquids are related by coupled crystal fractionation (from Fo₉₂ to Fo₈₅ in olivine hosts) and degassing (from 4 to 1 wt% H₂O in the melt inclusions). Three other dredges from Volc A include whole rocks, glass, and melt inclusions that are related to the boninites by crystal fractionation. Taken together, the samples from Volc A represent a suite of boninites and their differentiates, forming a coherent liquid line of descent with parallel whole rock REE patterns which become more enriched with decreasing Mg#. The REE patterns for Volc A whole rocks are depleted in LREE, however, in contrast to the characteristic U-shaped REE patterns of classic boninites. Volc A is only the second example of boninites being erupted in an active volcanic arc, the first being Bamus volcano in New Britain (Johnson et al., Geol. Rund., 1983). Volc A is not remarkable in its geographical setting, located in the central portion of the Tonga arc, at around 21°S, along-strike with the arc island of Hunga- Ha'apai, and east of the Eastern Lau Spreading Center. Although it contains volcanic rocks with the highest Ba/La (100-130), Si_6.00 (56 wt%) and lowest Ti_6.00 (0.44 wt%) in the Tonga arc, Volc A forms an end- member within a continuous spectrum of compositions, and much of the Tonga arc appears to have boninite- affinities. Of the three conditions that promote boninite production (mantle depletion, high mantle temperatures, and high mantle water contents), we can exclude mantle temperature as the primary cause of Volc A boninites. Olivine-liquid temperatures are 1348±13°C for Volc A liquids, calculated in equilibrium with Fo₉₂ mantle olivine at 2 GPa (using thermometers in Sisson & Grove, CMP, 1993; Sugawara, JGR, 2000). Such temperatures are very similar to those calculated for Central American arc volcanoes (1315-1320°C; Plank et al., this meeting) where magmas with similar water contents erupt, but boninites do not. Volc A olivine-liquid temperatures also contrast with those recorded in the high-Ca boninites found in the Tonga forearc (~1480°C; Falloon and Danyushevsky, JPet, 2000). Thus, we attribute the presence of boninites in the Tonga arc to the remelting of refractory mantle that melted previously during rapid back-arc spreading in the nearby Lau Basin.
DE: 1037 Magma genesis and partial melting (3619)
DE: 3640 Igneous petrology
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting