Oxygen Isotope Evidence for Chemical Interaction of Ki lauea Historical Magmas with Basement Rocks

Abstract

Kīlauea historical summit lavas have a wide range in matrix δ18OVSMOW values (4·9–5·6‰) with lower values in rocks erupted following a major summit collapse or eruptive hiatus. In contrast, δ18O values for olivines in most of these lavas are nearly constant (5·1 ± 0·1‰). The disequilibrium between matrix and olivine δ18O values in many samples indicates that the lower matrix values were acquired by the magma after olivine growth, probably just before or during eruption. Both Mauna Loa and Kīlauea basement rocks are the likely sources of the contamination, based on O, Pb and Sr isotope data. However, the extent of crustal contamination of Kīlauea historical magmas is probably minor (< 12%, depending on the assumed contaminant) and it is superimposed on a longer-term, cyclic geochemical variation that reflects source heterogeneity. Kīlauea's heterogeneous source, which is well represented by the historical summit lavas, probably has magma δ18O values within the normal mid-ocean ridge basalt mantle range (5·4–5·8‰) based on the new olivine δ18O values.