Tahiti: Geochemical Evolution of a French Polynesian Volcano
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B12, PAGES 24,341-24,357, DECEMBER 10, 1994 Tahiti: Geochemical evolution of a French Polynesian volcano Robert A. Duncan and Martin R. Fisk Collegeof Oceanicand AtmosphericSciences, Oregon State University, Corvallis William M. White Departmentof GeologicalSciences, Cornell University, Ithaca, New York Roger L. Nielsen Collegeof Oceanicand Atmospheric Sciences, Oregon State University, Corvallis Abstract. The island of Tahiti, the largestin French Polynesia,comprises two major volcanoes alignedNW-SE, parallelwith the generaltrend of the SocietyIslands hotspot track. Rocksfrom this volcanicsystem are basaltstransitional to tholeiites,alkali basalts,basanites, picrites, and evolvedlavas. Through K-Ar radiometricdating we haveestablished the ageof volcanicactivity. The oldestlavas (--1.7 Ma) crop out in deeplyeroded valleys in the centerof the NW volcano (Tahiti Nui), while the main exposedshield phase erupted between 1.3 and 0.6 Ma, and a late- stage,valley-filling phase occurred between 0.7 and0.3 Ma. The SW volcano(Tahiti Iti) was active between0.9 and 0.3 Ma. There is a clear changein the compositionof lavas throughtime. The earliestlavas are moderatelyhigh SiO2,evolved basalts (Mg number(Mg# = Mg/Mg+Fe2+) 42-49), probablyderived from parentalliquids of compositiontransitional between those of tholeiitesand alkali basalts.The main shieldlavas are predominantlymore primitive olivine and clinopyroxene-phyricalkali basalts(Mg# 60-64), while the latervalley-filling lavas are basanitic (Mg# 64-68) andcommonly contain peridotitic xenoliths (olivine+orthopyroxene+ clinopyroxene+spinel). Isotopic compositions also change systematically with time to more depletedsignatures. Rare earthelement patterns and incompatible element ratios, however, show no systematicvariation with time.We focusedon a particularlywell exposedsequence of shield- buildinglavas in the PunaruuValley, on the westernside of Tahiti Nui.
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