Oceanic island basalts have a large range in δ^(18)O values (4.5−7.5‰) compared with the assumed primordial mantle values (5.5–6.0‰) and with mid-ocean ridge basalts (5.7 ± 0.2‰). Some Hawaiian tholeiitic basalts have low ^(18)O values (4.6–5.2‰), which have been interpreted to be either a primary source feature or caused by crustal contamination. This study was undertaken to evaluate the cause of low δ^(18)O values in Hawaiian tholeiitic basalts. We determined the δ^(18)O values of glassy matrix material and coexisting olivines from pristine basalts produced during the current, 14-year-old Puu Oo eruption of Kilauea Volcano. Our results show that the Puu Oo eruption lavas have significant ranges in matrix (0.7‰) and olivine δ18O values (0.5‰) which do not correlate consistently with other geochemical parameters and that many of the lavas are out of oxygen isotopic equilibrium. These features probably reflect partial assimilation of and oxygen exchange with metamophosed Kilauea rocks during the magma's 19 km transit through the volcano's east rift zone. The parental magmas for Puu Oo lavas had a δ^(18)O value of at least 5.2‰ and perhaps as high as 5.6‰. Thus, Puu Oo lavas do not give a clear indication of the δ18O value of Kilauea's mantle source but they do indicate that the oxygen in these otherwise pristine basalts has undergone significant modification by interaction with crustal rocks.
© 1998 Oxford University Press. Received March 31, 1997; Revised typescript accepted December 2, 1997. This manuscript is dedicated to John Snyder. This study of the long-term geochemical changes of the Puu Oo eruption would not have been possible without his continued confidence in this work. We would also like to thank Jocelyn Rayray for her careful picking of the samples for isotope analyses, R. McEwan and J. Tacinelli for oxygen isotope anaylsys of matrix material, T. K. Mayeda for three olivine samples used in this study, J. M. Rhodes for providing whole-rock XRF analyses for a few Puu Oo lavas, F. Goff for sharing a Puu Oo vent sample, D. Muenow for allowing us to use unpublished volatile data for Puu Oo glasses, and J. W. Valley for providing access to his laboratory at the University of Wisconsin in support of J. Eiler's contribution to this study. Reviews of a draft of this manuscript by A. Anderson, A. Grunder, A. Hofmann, P. Larson and M. Thirlwall are gratefully acknowledged. This work was supported by the National Science Foundation (Grants EAR 94-04884 and 96-14247 to M. G.). This is SOEST Contribution 4589.