TAHITI - GEOCHEMICAL EVOLUTION OF A FRENCH-POLYNESIAN VOLCANO

The island of Tahiti, the largest in French Polynesia, comprises two m ajor volcanoes aligned NW-SE, parallel with the general trend of the S ociety Islands hotspot track. Rocks from this volcanic system are basa lts transitional to tholeiites, alkali basalts, basanites, picrites, a nd evolved lavas, Through K-Ar radiometric dating we have established the age of volcanic activity. The oldest lavas (similar to 1.7 Ma) cro p 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 a late-stage, valley-filling 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 (M g# = Mg/Mg+Fe2+) 42-49), probably derived from parental liquids of com position transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and cli nopyroxene-phyric alkali basalts (Mg# 60-64), while the later valley-f illing lavas are basanitic (Mg# 64-68) and commonly contain peridotiti c xenoliths (olivine+orthopyroxene+clinopyroxene+spinel). Isotopic com positions also change systematically with time to more depleted signat ures. Rare earth element patterns and incompatible element ratios, how ever, show no systematic variation with time. We focused on a particul arly well exposed sequence of shield-building lavas in the Punaruu Val ley, on the western side of Tahiti Nui. Combined K-Ar ages and magneto stratigraphic boundaries allow high-resolution age assignments to this similar to 0.7-km-thick flow section. We identified an early period o f 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 r ates 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 calc ulated the composition of the parent magma for the shield-building sta ge of volcanism, assuming that it was in equilibrium with Fo(89) olivi ne and that the most primitive aphyric lavas were derived from this pa rent by the crystallization of olivine alone. The majority of the shie ld lavas represent 25 to 50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognize a quasi-periodic evolution of lav a compositions within the early, robust period of volcanic activity, w hich we interpret as regular recharge of the magma chamber (approximat ely every 25 +/- 10 kyr). Volcanic evolution on Tahiti is similar to t he classic Hawaiian pattern. as the shield-building stage waned, the l avas became more silica undersaturated and isotopic ratios of the lava s became more MORE-like. We propose that the Society plume is radially zoned due to entrainment of a sheath of viscously coupled, depleted m antle surrounding a central core of deeper mantle material. All parts of the rising plume melt, but the thermal and compositional radial gra dient 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, vis cously coupled sheath. A correlation between Pb/Ce and isotope ratios suggests that the Society plume contains deeply recycled continental m aterial.