Oxygen isotopes in the Azores islands: Crustal assimilation recorded in olivine

Oxygen isotope ratios of olivine have become a widely used tool for the study of magmatic systems, especially in the interpretation of source heterogeneities in mantle plume-derived ocean island basalts. The underlying assumption is that fresh minerals provide a better guide to magma δ¹⁸O than bulk rock analyses and that olivine is also likely to be a major phenocryst phase in primitive magmas. However, distinctions between source compositions and the effects of subsequent magma evolution have not always been thoroughly scrutinized. For the Azores samples investigated here, we can demonstrate that the δ¹⁸O variation (+4.84‰ to +5.25‰ Vienna standard mean ocean water) observed in the olivine phenocryst population is closely linked to evolution in the host magmas during ascent to the surface. We observe a linear, positive correlation between forsterite (Fo) content and δ¹⁸O in all of the individual island lava suites. This forces us to conclude that the low oxygen isotope ratios result from combined assimilation and fractional crystallization processes, the assimilant being hydrothermally (temperature > 250 ^oC) altered, lower oceanic crust. Linear regression of the measured δ¹⁸O olivine values to Fo₈₉ suggests a homogeneous mantle source with δ¹⁸O = +5.2‰ ± 0.1‰.