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

Oceanic island basalts have a large range in dO values (4·5– INTRODUCTION 7·5‰) compared with the assumed primordial mantle values A 1996 conference on ‘shallow level processes in ocean (5·5–6·0‰) and with mid-ocean ridge basalts (5·7 ± 0·2‰). island magmatism’ drew attention to the controversy that Some Hawaiian tholeiitic basalts have low O values (4·6–5·2‰), is developing on the possible extent of crustal conwhich have been interpreted to be either a primary source feature or tamination for oceanic island basalts (Bohrson et al., caused by crustal contamination. This study was undertaken to 1997). This controversy is important because most studies evaluate the cause of low dO values in Hawaiian tholeiitic of oceanic island basalts have assumed that these lavas basalts. We determined the dO values of glassy matrix material provide a direct indication of the geochemical and isoand coexisting olivines from pristine basalts produced during the topic composition of mantle (e.g. Zindler & Hart, 1986). current, 14-year-old Puu Oo eruption of Kilauea Volcano. Our This assumption has been questioned in several recent results show that the Puu Oo eruption lavas have significant ranges studies (e.g. Eiler et al., 1996a; Thirlwall et al., 1997) and in matrix (0·7‰) and olivine dO values (0·5‰) which do not it has been suggested that even the most primitive basalts correlate consistently with other geochemical parameters and that from some oceanic islands were contaminated by crustal many of the lavas are out of oxygen isotopic equilibrium. These materials (e.g. Hemond et al., 1993). Resolution of this features probably reflect partial assimilation of and oxygen exchange controversy will have fundamental implications for our with metamophosed Kilauea rocks during the magma’s 19 km transit understanding of the magmatic history of oceanic island through the volcano’s east rift zone. The parental magmas for Puu basalts and for the composition of the mantle. Oxygen Oo lavas had a dO value of at least 5·2‰ and perhaps as high isotopes are a potentially powerful tool to help resolve this as 5·6‰. Thus, Puu Oo lavas do not give a clear indication of debate because dO values of magma can be substantially the dO value of Kilauea’s mantle source but they do indicate that modified by assimilation of rocks that have interacted the oxygen in these otherwise pristine basalts has undergone significant with the hydrosphere (e.g. Taylor, 1974). modification by interaction with crustal rocks. Many oceanic island basalts have dO† values somewhat below the assumed mantle range, based on studies of