Oxygen Isotope Variation Within a Quaternary, Caldera-forming, Phonolitic Eruptive Sequence; the Diego Hernández Formation, Tenerife, Canary Islands (Spain)

Much of the work done on oxygen isotopes in volcanic rocks has been based on analyses of whole rock or multiple crystal aliquots, inherently discounting the importance of heterogeneity. Recently, due to advances in technology and the recognition of small-scale chemical disequilibria in magmatic systems, it has proven valuable to measure variations among individual grains to examine the evolution of these systems. Here, we present the results of both single and multi-crystal feldspar oxygen isotope analyses (1σ < 0.1 ‰ ) from phonolitic eruptive units of the Diego Hernández Formation (DHF) as part of an on-going study of the evolution of the Las Cañadas caldera complex on Tenerife. δ ¹⁸O (VSMOW) values of feldspar range from 4.7 to 6.7 ‰ (slightly lower than previously reported) and show a positive correlation with average crystal size (mg) in units that display a range of oxygen isotope values. For example, one unit shows a linear variation of feldspar δ ¹⁸O values that range from 4.7 to 6.4 ‰ , and these ratios are positively correlated with average phenocryst size over a range of 0.7 to 3.6 mg. Overall, intra-unit feldspar δ ¹⁸O values vary by as much as ∼2 ‰ and are lower than those predicted by fractionation of a basanitic parental magma. Additionally, δ ¹⁸O values show no coherent relationship with incompatible trace element abundances, an indication that the oxygen isotopic ratios are being controlled by neither fractionation nor recharge of the system by basaltic injections. However, the largest variation in oxygen isotope ratios is found in the most differentiated units, consistent with earlier suggestions that the most highly evolved phonolites may contain a component of hydrothermally-altered wall rock. We propose that oxygen isotope variation in Tenerife phonolites is being controlled by variable amounts of assimilation of hydrothermally-altered syenite country rock (δ ¹⁸O 0.1 - 5.8 ‰ ) and that timescales between assimilation and eruption can be constrained by the oxygen isotope disequilibria shown between larger and smaller feldspar phenocrysts.