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Tahiti: Geochemical evolution of a French Polynesian volcano

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  • The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW-SE, parallel with the general trend of the Society Islands hotspot track. Rocks form this volcanic system are basalts transitional to tholeiites, alkali basalts, basanites, picrites, and evolved lavas. Through K-Ar radiometric dating we have established the age of volcanic activity. The oldest lavas (~1.7 Ma) crop out in deeply eroded valleys in the center of the NW volcano (Tahiti Nui), while the main exposed shield phase erupted between 1.3 and 0.6 Ma, and the late-stage, valley-filled phase occurred between 0.7 and 0.3 Ma. The SW volcano (Tahiti Iti) was active between 0.9 and 0.3 Ma. There is a clear change in the composition of lavas through time. The earliest lavas are moderately high SiO2, evolved basalts (Mg number (Mg# = Mg/Mg+Fe2+) 42-49), probably derived from parental liquids of composition transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and clinopyroxene-phyric alkali basalts (Mg#60-64), while the later valley-filled lavas are basanitic (Mg#64-68) and commonly contain peridotitic xenoliths (olivine+orthopyroxene+clinopyroxene+spinel). Isotopic compositions also changes systematically with time to more depleted signatures. Rare earth element patterns and incompatible element ratios, however, show no systematic variation with time. We focused on a particularly well exposed sequence of shield building lavas in the Punaruu Valley, on the western side of Tahiti Nui. Combined K-Ar ages and magnetostratigraphic boundaries allow high-resolution age assignments to this ~0.7-km-thick flow section. We identified an early period of intense volcanic activity, from 1.3 to 0.9 Ma, followed by a period of more intermittent activity, from 0.9 to 0.6 Ma. Flow accumulation rates dropped by a factor of 4 at about 0 .9 Ma. This change in rate of magma supply corresponds to a shift in activity to Tahiti Iti. We calculated the composition of the parent magma for the shield-building stage of volcanism, assuming that it was in equilibrium with Fo89 olivine and that the most primitive aphyric lavas were derived from this parent by the crystallization of olivine alone. The majority of the shield lavas represent 25-50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognized a quasi-periodic evolution of lava compositions within the early, robust period of volcanic activity, which we interpret as regular recharge of the magma chamber (approximately every 25±10 kyr). Volcanic evolution on Tahiti is similar to the classic Hawaiian pattern. As the shield-building stage waned, the lavas became more silica undersaturated and isotopic ratios of the lavas became more MORB-like. We propose that the Society plume is radically zoned due to entrainment of a sheath of viscously coupled, depleted mantle surrounding a central core of deeper mantle material. All parts of the rising plume melt, but the thermal compositional radial gradient ensures that greater proportions of melting occur over the plume center than its margins. The changing composition of Tahitian magmas results from lithospheric motion over this zoned plume. Magmas erupted during the main shield-building stage are derived mainly from the hot, incompatible element-enriched central zone of the plume; late-stage magmas are derived from the cooler, incompatible element-depleted viscously coupled sheath. A correlation between Pa/Ce and isotope ratio suggest that the Society plume contains deeply recycled continental material.
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  • Duncan, R.A., Fisk, M.R., White, W.M., and Nielsen, R.L., 1994, Tahiti: Geochemical evolution of a French Polynesian volcano. Journal of Geophysical Research 99(B12) 24,341-23,357.
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  • 99
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  • B12
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  • R.A.D. and M.R.F. were supported by NSF grant EAR85-09012, and W.M.W. by NSF grant EAR87-07484.
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  • 0148-0227

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