CRUSTAL CONTAMINATION OF KILAUEA VOLCANO MAGMAS REVEALED BY OXYGEN-ISOTOPE ANALYSES OF GLASS AND OLIVINE FROM PUN OO ERUPTION LAVAS

Oceanic island basalts have a large range in delta(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 O-18 values (4.6-5.2%), which have been interpreted t o be either a primary source feature or caused by crustal contaminatio n. This study, was undertaken to evaluate the cause of low delta(18)O values in Hawaiian tholeiitic basalts. We determined the delta(18)O va lues of glassy matrix material and coexisting olivines from pristine b asalts produced during the current, 14-year-old Puu Oo eruption of Kil auea Volcano. Our results show that the Puu Oo eruption lavas have sig nificant ranges in matrix (0.7%) and olivine delta(18)O values (0.5%) which do not correlate consistently with other geochemical Parameters and that many of the lavas are out of oxygen isotopic equilibrium. The se features probably reflect partial assimilation of and oxygen exchan ge with metamophosed Kilauea rocks during the magma's 19 km transit th rough the volcano's east rift tone. The parental magmas for Puu Oo lav as had a delta(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 delta(18)O value of Kilauea's mantle source but they do indicate that the oxygen in these otherwise pristine basalts has undergone significant modifica tion by interaction lentil crustal rocks.