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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.

Crustal Contamination of Kilauea Volcano Magmas Revealed by Oxygen Isotope Analyses of Glass and Olivine fromPuu Oo Eruption Lavas