Oxygen isotope ratios in phenocrysts from Pitcairn island: constraints on recycled sediment in the "EM1" enriched mantle

Abstract

The Pitcairn hotspot (southeast Pacific Ocean) carries the strongest EM1 signature known in oceanic lavas, both in rocks from Pitcairn Island and from the nearby Pitcairn seamounts. We have measured oxygen isotope ratios by laser fluorination of phenocrysts from Pitcairn Island alkali basalts variably enriched in the EMI mantle end member (defined by radiogenic isotope ratios of ^(143)Nd/^(144)Nd=0.5125, ^(87)Sr/^(86)Sr=0.705, ^(206)Pb/^(204)Pb=17.5). Values of δ^(18)O in olivine phenocrysts average 5.21±0.08‰ (n=8). The average δ^(18)O value of 5.21±0.08‰ for Pitcairn Island olivine phenocrysts is (1) indistinguishable from the average value for olivine from xenoliths of mantle peridotite (5.19±0.14‰; Mattey el al., 1994); (2) similar to the value expected for olivine in the mantle sources of mid-ocean ridge basalt (i.e., 0.5‰ lower than the average for mid-ocean ridge basalt glasses of 5.7±0.2‰ (Ito et al., 1987) which is the expected fractionation between olivine and basaltic liquid based on experimental and natural studies); and (3) equal to the δ^(18)O value of olivine in lunar basalts (Clayton et al., 1971). Pitcairn Island lavas and their phenocrysts are therefore close to oxygen isotopic equilibrium with the best estimates of olivine in the upper and bulk mantle. Plagioclase δ^(18)O values average 6.05±0.15‰ (n=5; 4 from rocks in which olivine was also analyzed), resulting in a plagioclase/olivine fractionation close to the expected magmatic value (0.9-1.0‰). This indicates that olivine and plagioclase were in isotopic equilibrium at magmatic temperatures and therefore suggests that olivine is not exotic to host lavas.