Oxygen isotope composition of phenocrysts from Tristan da Cunha and Gough Island lavas: variation with fractional crystallization and evidence for assimilation

We have measured the delta(18)O values of the major phenocrysts (olivine, clinopyroxene and plagioclase) present in lavas from Tristan da Cunha and Gough Island. These islands, which result from the same mantle plume, have enriched radiogenic isotope ratios and are, therefore, prime candidates for an oxygen isotope signature that is distinct from that of MORB. Consistent differences between the delta(18)O values of olivine, pyroxene and feldspar in the cough lavas show that the phenocrysts in the mafic Gough Island lavas are in oxygen isotope equilibrium. The olivines in lavas with SiO2 <50 wt% have a mean delta(18)O value of 5.19 parts per thousand, consistent with crystallization from a magma having the same oxygen isotope composition as MORB. Phenocrysts in all the cough lavas show a systematic increase in delta(18)O value as silica content increases, which is consistent with closed-system fractional crystallization. The lack of enrichment in delta(18)O of the Gough magmas suggests that the mantle source contained <2% recycled sediment. In contrast, the Tristan lavas with SiO2 >48 wt% contain phenocrysts which have delta(18)O values that are systematically similar to 0.3 parts per thousand lower than their counterparts from Cough. We suggest that the parental mafic Tristan magmas were contaminated by material from the volcanic edifice that acquired low delta(18)O Values by interaction with water at high temperatures. The highly porphyritic SiO2-poor lavas show a negative correlation between olivine delta(18)O value and whole-rock silica content rather than the expected positive correlation. The minimum delta(18)O value occurs at an SiO2 content of about 45 wt%. Below 45 wt% SiO2, magmas evolved via a combination of assimilation, fractionational crystallization and crystal accumulation; above 45 wt% SiO2, magmas appeared to have evolved via closed-system fractional crystallization.